March 10, 2016 • 56 mins
Sexual cannibalism is a rarity in the natural world, and scientists are still attempting to understand the act as practiced by mostly arachnids and insects. Why consume your mate after or during sexual intercourse? Why venture into such a scenario in the first place? Join Robert and Joe as they explore the thoroughly inhuman world of sexual cannibalism.
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Speaker 1 (00:03):
Welcome to Stuff to Blow Your Mind from how Stuff
Works dot com. Hey, what can the stuff to blow
your mind? My name is Robert lamp and my name
is Joe mcformack, and we're gonna get right to it
today because we're going to talk about sexual cannibalism, right
(00:23):
And I do want to just throw out here right
at the top, this is not going to concern human
cannibalism at all. I know that in the past when
we've covered cannibalistic topics, some listeners have actually skipped the
episode thinking that there might be human cannibals. And I'm
just gonna let you know, if you've made it this far,
no human cannibals, will cannibals will occur in this episode.
(00:45):
Is that some people's big thing? It's like, Oh, you're
gonna talk about cannibalism. I can't go on. Well, you know,
I mean some listeners, some of our listeners really want
the dark content. And that's why and and and we're
certainly happy to go there and as well as strip
away some of the the taboo and the mystery surrounding
those dark topics. But I know that not everyone wants
(01:06):
to go down that path with us. So I just
want to let everyone know we're not going down the
human cannibal path. Okay, So we are going to be
talking about animal sexual cannibalism. And this is a topic
that's been touched on on the show before. I Know
You and Julie in the Past is an episode about
cannibalism in the animal world, but today we wanted to
focus specifically on sexual cannibalism because of all of the
(01:30):
interesting evolutionary interplay here. Yea, the sort of trade off
of costs and benefits that would lead a species to
at some rate fairly often enough that people would notice
eat during sex, eat each other during sex or after sex. Yeah.
And it's the interesting thing about this topic is that,
on one hand, it is very simplistic, and I'll get
(01:52):
into the simplistic argument shortly, but then it's also not
as cut and dry as you might think. It's an
area where they are multiple theories as to as do
how this evolved and exactly how it's working, and some
of those are actually conflicting theories as well. So it's, uh,
it's a subject with a lot of meat on it,
a lot of sexy meat on it. Okay, so what's
(02:12):
the basic gist of why sexual cannibalism happens. This might
be kind of obvious, but we should get it on
the table so that we have a place to start.
So let's say you have two spiders and they size
each other up, and they say it's time to mate.
You know, we'll pass our genes on. But some way
through the mating procedure, the female sort of grabs hold
(02:33):
of the mail and sinks her fangs into him and
drains all of his delicious juices. Why did that happen?
And that's and that is the big question, right, why
does this evolve and why does this occur? Certainly not
in every species, it's actually a very rare occurrence in
the natural world. But where it does occur, uh, we
(02:56):
can't help it. Study it well. Actually, we should start
with the simple side. It's clear why this happens from
the female side. A lot of the question is why
the male would participate in this, right, But but why
does the female eat the male? Well? For energy, of course,
I mean that. I mean that basically comes down to
stripping away the human complexity surrounding cannibalism in general, because ultimately,
(03:19):
flesh is energy, flesh is food. Uh, flesh is life.
In addition, and so in addition to having all sorts
of cultural hangups about cannibalism, you know, we don't have
any problem wasting boatloads of food and energy, right, so
we kind of have to put that aside to think
of it. But yeah, in the in the natural world,
you see plenty of cases of just straight up cannibalism,
(03:40):
and it all comes down to a basic economy of
the energy. So a mother's young die, she might consume
those young because what are they now but empty vessels
made up of energy. Yeah, I think this is something
that's hard for us to understand because by and large,
if you are a human being listening to a podcast,
(04:02):
you probably are are lucky. You live a fortunate existence
where you have decent amounts of access to food. You're
you're probably not living constantly at the edge of starvation,
and animals are sort of built with the assumption that
they will be living constantly at the edge of starvation.
If you ever you ever noticed how if you have
a dog at home, if you're out walking the dog
(04:26):
and there's something that smells even remotely like it might
be food, the dog's going to try to put it
ahead and get in the mouth anyway, see what it takes.
And you and I would never think that way. I mean,
it's like smells like it could be food. Who knows
if it would hurt me? Better safe than sorry, I'm
just gonna eat it. Um. But in the animal kingdom,
(04:47):
out in the wild, that sort of strategy might make
a lot more sense, because if you are constantly at
risk of having your life extinguished or your competitive power
diminished by lack of access to energy resources, you'll take
anything you can get. Yeah, exactly. And you know, we
also have to think outside of the sort of human
male female scenario, right and get down to the basic
(05:10):
reality that I feel like is far more obvious when
you start looking at in the world of of insects
and arachnids and a few other species, and that is
that the females are the species and the male is
just this uh essentially, this mutation that's necessary to enable
genetic diversity through sexual reproduction. So he's he only has
(05:33):
this one purpose anyway, He's just this genetic material, heat
seeking missile and aid in reproduction. So why not eat him? Right?
His job is done, and there's there, that's it. Gets
to be more complex question, as we'll explore later. But
you know, it makes sense if you just look at
the pure math of the scenario. Yeah, and to get
(05:54):
even weirder, I mean, you might be able to say
that neither the male nor the female is really the
purpose of the speci sees the phenotype, the body, None
of that is the species. The species is a gene pool.
It's this abstract concept. It's really more of a movement
as opposed to a physical thing, because yeah, it's not
the little creatures that exist for especially in the insect world,
(06:17):
often very slim periods of time. It is that that
continued movement, genetic movement through time. Yeah, but but especially
the males. You might say, like if if there is
a sexual dimorphism in the species, a different in body,
a difference in body plans between male and female, and
it just you look at it and you say, wow,
it really looks like there's more going into the female.
(06:38):
She's bigger, she has more survival capacity. It really sometimes
can look to a human observer like, well, that's the
female is what the species is, and the male is
just sort of this thing that exists to mate. And yeah,
and I think you can make a pretty strong case
for that that the female is the primary member of
the species. Now already we're talking ing I think, in
(07:01):
sort of perhaps unscientific terms like what we're using anthropomorphization,
and we're using our our our sort of human judgment
of how things just seem to look to us, based
on the way we think about relationships between animals, probably
mostly based on the way we think about relationships between humans.
(07:22):
And this is sort of an inherent problem in studying
things like animal mating practices and reproduction and especially especially
sexual cannibalism. It we just can't help but infuse it
with all of the sort of social markers of how
we view relationships as humans. Yeah, and that's true of
not only consumers, readers, outsiders, true, not only of science
(07:46):
communicators like like ourselves, but also scientific researchers themselves. And
there was actually a paper on this, yeah, two thousand
ten paper, Sexual Stereotypes the Case of Sexual Cannibalism. This
was from the UK University of St Andrew schoo of Biology.
They looked at about they looked at a larger sample
and they had to to whittle it down. They end
up looking at around two ten relevant papers dealing with
(08:08):
sexual cannibalism, and they found that females were more likely
to be to be described using active words and males
with reactive words. And then a number of the words
used to describe cannibalistic females were highly loaded, suggesting a
negative stereotype of sexually aggressive females. Uh. And then the
males were more likely to be described as it's making
(08:29):
a sacrifice, while the females were just you know, voracious
widows um, that's funny. Voracious is a word I did
see pop up in the literature, in the scientific literature
on sexual cannibalism a lot. And and there are some
good points to be made about, you know, even though
this is sort of a tendency that's hard to overcome.
I know we're going to do it in this episode twenty.
(08:50):
We're gonna anthropomorphize. It's just how we talk about things,
how we talk about it's how we understand the topics,
how we conceptualize them in our head. There is a
good case to be made. I think that scientists do
their best to avoid this, and one simple reason is
that these kind of socially charged words are vague. Yeah,
you know, they're not they don't. They don't deal in
(09:11):
easily reproducible quantities. So if somebody wants to redo your
experiment somewhere else, can they tell what you mean by voracious?
I mean it seems like it makes more sense to
just talk about numerical conditions. So a spider that attacks
this percentage of prey within this you know, radius of
(09:32):
range is voracious. Yeah. And then I mean, just overall,
it's detrimental to view a species normalcy through the lembs
of our own species abnormality, because I mean, for instance,
we can study ducks without going holy crap. Can you
imagine if humans late eggs like a duck? So it's
(09:52):
a different creature, it's a different species. It is an
inhuman thing. And I guess anytime I study insects, I
always come back, maybe problematically to the line of Dr
Brundle in The Fly talking about uh insect politics and
how there are no insect politicians, um, which which I
(10:12):
think it's a great line, not only within the context
of that film, but also in terms of looking at
any of these scenarios where something through our lens, through
our lens of human normalcy, maybe just really horrific and monstrous,
and we want to cast this insect as a villain
and this as a poor, suffering martyr. But it is
an utterly inhuman system and there's no getting around that.
(10:36):
I'm sorry, I'm not listening to you because I'm imagining
the fly with a duck instead of a fly. So
Seth Brundle gets transformed into part duck and he's got
he's uh, he's got a bill. Well, you know how
the Duck and the Fly came out in the same year, Yeah,
oh wow, Yeah, that's a good year for film, the
year for film. In fact, we just had a how
(10:57):
Stuff Works Now article come out on that said, all
of your Howard the duck hate mail to blow the
mind dot com. But no. But moving on from how
weird it would be if humans were part duck, we
should look at the evolution of sexual cannibalism. And we're
going to talk about some specific cases later in this
episode of species that practice it and what we have
(11:17):
learned from them in recent studies. But where do we
typically see sexual cannibalism in the animal kingdom? Like, how
come you don't usually see a female cat eat a
male cat after sex? Well, I mean there's some biting
I think sometimes, but no, for the most part, we're
looking at insects and arachnids uh, spiders and scorpions being
(11:40):
the primary of research. There's some insects such as h
such as manted mantnanted species, and I believes there are
also some uh. Some evidences suggests that there are some
gastropod and copia pod cases. But in the for the
most part though, we're dealing with, especially with spiders and
scorpions and and and mantis, the species. We're dealing with
(12:02):
highly solitary predatory animals, which I think is key, especially
regarding some of the theories regarding sexual cannibalism. Right, So
if you want to keep in mind that they typically
aren't the most social animal rights that eat each other
after sex, and this might help inform our ideas about
how this evolved. Yeah, these are two anthropomorphizes. These are
(12:24):
lone killers. These are wanderers, right, that are just out
there on the highway eating what they need to eat
to survive. I say, even if you don't want to
infuse it with with human concepts of good and evil,
you could easily say that sexual cannibalism is a chaotic,
neutral trait. Yeah, yeah, I would think so so. But
(12:45):
but so, like we've said, it is rare in the
animal kingdom. You don't see it in most species. And
why is that? Well, on one, on one level, I
think we definitely have to again look to the solitary
predatory nature of these particular species. And then there's also
just cannibalism as a whole. If cannibalism is too essential
to the species, then you it eventually is going to
(13:09):
have an economically detrimental effect, right, So imagine your standard
sexual cannibalistic interaction. A female spider as a web. Male
spider approaches the web, climbs onto the web, mates with
the female, may or may not successfully impregnate her, and
the female eats the eats the male spider kills him,
(13:31):
drains all his fluids, gets energy from it. But the
male spider can mate no more. His days of passing
on his genes are now done. So this interaction works
out pretty well for the female spider. I mean, so
you can see why her genes would encourage such an interaction,
But the question is why would the male do that?
Why why does it uh genetically can I mean again,
(13:54):
we're using anthropomorphic terms like consent, But I would say,
why do its genes consent to this inner action where
it has a strong chance of being eaten? Indeed, that's
one of the big questions here, and one of the
questions that a number of these theories um of sexual
cannibism evolution explore. So at this point we should really
(14:14):
start talking about what those theories are and uh, and
then break into some of the examples that explore how
they work. Yeah. So there are several conflicting or competing
hypotheses about the evolution of sexual cannibalism, and we should
say that more than one can be true in different cases. Right,
There's some overlap here for sure. Yeah, But I think
one thing to keep in mind during these explanations is
(14:37):
that there are different ways that evolution can pay for
a trait. Let's say you have a trait that makes
you there's something about your brain that makes you really
likely to kick people in the face at the first
time you meet them. Um, there there are two ways
that could come through in your genes according to evolutionary theory,
and one one way is that the trade itself is
(14:59):
beneficial overall. People who kick people in the face first
time they meet just happened to have more kids than
people who don't, So it could be beneficial, or it
couldn't be. It could be an unintended side effect of
a very beneficial trait. So it could be that there's
actually not a good there's nothing good that comes from
kicking people in the face when you first meet them.
(15:20):
But you're really good at kicking people in the face
the rest of the time when exactly needed, So you're
much much better at defending yourself against predators. So you
just happened to be kicking crazy and and this is
just sort of and you're so good at defending yourself
for predators, this kicking genes survives even though it's not
particularly helpful in the case of kicking strangers in the
(15:40):
face when you first meet them. So the first of
these several theories we're gonna mention here is adaptive foraging.
So this one's pretty basic. Starving female mates the male
is there, she's hungry, she's gonna need energy. Why not
go ahead and eat him during either during the act
or immediately there after. Eating the male increases her chance
(16:02):
of survival, gives her a meal based power up uh
and as a as a as a sort of side
effect to her lover can't help produce any spawn that
end up rivaling her own for resources. But essentially it
comes down to I'm just I'm adapting to the what
food is available to me, and here is a meal
(16:22):
in front of me right now. Okay, So this is
sort of the the opportunism hypothesis. It's just saying that, like, look,
you know, the meal is worth it essentially right now.
Some of the critics argue that the males in many
of these cases, they actually make pretty poor meals. They're
not really going to make or break the female. But
(16:43):
then there are cases where they say that, hey, the
female is actually able to acquire crucial proteins or lipids
from the male that she eats that she might not
otherwise get from her standard predatory diet. Okay, So I'm
thinking that this is saying that the sexual cannibalism is
it's self an adaptation, It is itself a trait that
(17:03):
is encouraged because overall it's beneficial to the spider gene pool. Yeah,
not to the gene pool. I mean, it's I'm not
taking a species level evolution here. It would be beneficial
to the genes themselves in the animals that do it. Yeah,
the females who eat their male have just one little
power up over those who that do not. Yeah. And
the idea here is that this is very much a
(17:25):
kicking people in the face the first time you meet them. Right,
it's a good thing. Okay. But then there's another hypothesis
that is one I've read about some of the studies
I looked up here. It's the sort of I think
it's turned the aggressive spill over hypothesis. Uh. And this
is more that other idea we were talking about that
there are some traits that are very beneficial, but they
(17:45):
encourage side effects. So one trait, for example, might be
that a that a female is very aggressive in chasing
down prey. It has genes that lead it to be
a total killer. It's just a go getter. It sees moving,
it's like gonna eat that. Uh. And this might be
very beneficial to this creature when it's trying to survive
(18:07):
in the wild. It's it's very good at chasing down things,
killing them and eating them. But these same genes also
sometimes they get applied in the wrong direction. Yeah, like
to anthrommorphize the situation. It's it's like imagining that the
spider or the scorpion in the scenario, it was like
a a female wrestler who, in the act of making love,
(18:30):
cannot help like a muscle memory, just an instincts kick
in and she has to throw a suplex or two
or put somebody in a submission hold. Um, she's just
the female in this scenario is just so aggressive, just
so just amped up and ready to go, that the
predatory nature just kicks in and she ends up consuming
the part or all of the male. Yeah. Now, is
(18:51):
there any evidence for this? I think there's some. Yeah,
it depends. I mean in some species of spider. Yes,
researchers have observed higher sexual cannibals rates among females that
also attacked prey at a faster rate than other females
they've observed. I want to talk about one of these
studies in a bit, Okay, but then another species there's
no correlation. So that's the That's one of the issues too,
(19:15):
is you're dealing with You're not dealing with just one
species that's engaging in sexual cannibalism. You're dealing in multiple species. Uh.
The some are rather diverse from one another that have
in many cases independently evolved this adaptation. Yeah. Now, one
one hypothesis I saw just kind of mentioned on the internet,
(19:35):
but I didn't see in any of the scientific literature
I was looking at was the mistaken identity hypothesis. I
couldn't tell if there's really anything to this the I
saw it mentioned. I mean, the basic idea here is
the female is saying, are you food? Are you my lover? Well,
let me just take a bite out of you. I
can't tell, I guess you're my enemy. I'll just bite
into you. Uh. The only place that I saw this
and this is the person who kicks people in the
(19:57):
face because he's near sighted, right, exactly, enemy or lover,
I think the enemy I'm gonna kick. Yeah. So the
only place I saw this mentioned was in um uh
Kenwyn Blake Subtle's paper The Evolution of Sexual Cannibalism uh,
and he said that there's just no empirical evidence to
support this theory. Uh. For instance, he says that the
(20:20):
the fight up spiders offer a strong argument against mistaken
identity because, particularly among the Salta sids, you have the
highest visual acuity of all aractans. So I think that's
jumping spiders, right, Yes, And he tells us that when
you take into account um cannibalism during and after a copulation,
(20:42):
that would seem to even further eliminate the possibility. So
that the idea here is that, yeah, they're not just
blind dummies in these cases. They they have senses available
to them to determine if this is a member of
their own species or a solid uh outsider prey species. Yeah. Now,
of course, one other hypothesis that we could say competes
(21:06):
with these and and maybe sometimes overlaps with them, is
that it's just it's sexual selection. Yeah, it's it's females
being choosy about which males they mate with or whether
intentionally or not. Um, and this is encouraging pairing with
stronger or fitter or more appropriate males through a tendency
to eat the males. Yeah, it's just it's kind of
(21:29):
h it's not it's not me, it's use scenario. So
I'm going to eat you instead. You're not really uh,
you're not really husband material. Maybe you're more meal material. Right,
So yeah, that the basic hypothesis here entails the notion
that the cannibal female just rejects unacceptable males and eats them,
(21:50):
and it's smaller unfit males are better suited as meals.
Larger ones are the better mates. Though you also see
this reversed in some cases, with females favoring small aller mates. Uh. Again,
it kind of depends on the species, right well, As
we know from so many great evolutionary biologists before, it's
not always the strongest that's the fittest. The bigger doesn't
(22:11):
always necessarily mean better. Fitter can mean a lot of
different things in different environments, and the environment can include
the sexual selection environment, the gene environment in which you're competing. Yeah,
and the mere fact that both varieties survive often indicates
that both are valued forms of that species. Uh. And
there are a number of different mate choice splinter theories
(22:34):
as well, um running the anthropomorphic gamut for all the
reasons you might you know, mate reject or cannibalize your
spider lever so that you have it. Are those are
the basic theories regarding sexual capitalism. All right, So now
I think it's time to take a quick break. But
when we come back, we're going to take a look
at our rogues gallery of cannibals. All right, we're back.
(23:04):
Who do we have? First year? Joe? Well, I thought
it would be interesting because of a specific study I
found to look at the cannibalistic burrowing wolf spider, the
Lycosa Hispanica. And this is from a paper called does
female personality determine mate choice through sexual Cannibalism? It was
published in the journal Ethology in and Uh. It was
(23:28):
carried out by some research group at the Experimental Station
of Arid Zones e Z A C S I C
in Spain. And and here's what's going on. Sometimes you
want to eat your mate, but you cannot wait for
the mating. This is a problem because obviously, so we've
talked about the benefits for the female of sexual cannibalism.
(23:51):
She gets to mate, and she gets to pass on
her genes, and she also gets a meal. And we've
been debating the benefits for the male. But what if
the mail eats She wants to eat the mate so
bad that she eats him before they mate. I mean,
that seems like a maladaptive. It seems like it's gotten
(24:12):
out of control here. Yeah, And what they found is
that this happens a lot. Actually, so sometimes this specific tarantula,
This wolf spider doesn't even wait for sex before eating
the potential mate. So why is that? What determines if
the female wolf spider attacks a potential mate before the mating.
(24:34):
Is it how hungry she is? If so, this would
support the adaptive foraging hypothesis perhaps or is it her
personality and if that's the case, it might support the
aggressive spillover. Is it the fitness of the male? This
might support some form of sexual selection or mate choice.
So the researchers decided to try to isolate these and
(24:56):
figure out which one is the most likely. So they
studied a group of female wolves by to see how
aggressive they were in feeding. And some were insanely aggressive
that they were just going to town immediately on any
prey such as beatles, while others were more cautious or
they used the term docile, which I thought was funny.
(25:17):
Essentially more reticent hunters, they just didn't jump on prey
quite as quickly. And then they took these same females
they had studied and offered the females a random selection
of males for mating to see what would happen. Uh,
And you know what. The female personality actually seemed to
play a big role. So I want to read a
quote from a press release by one of the study authors,
(25:39):
Reuben robin Ada Bueno uh And. The author said, more
cannibalistic females were also more voracious towards their prey and
thus better nourished. This is surprising since they have more
nutritive resources to invest in their offspring before finding the
first males, and then said their priority should be in
(26:00):
suring the firm s fertilization of their eggs instead of
eliminating their potential donors. So this is kind of counterintuitive.
You might expect that the hungry or the female, the
more likely she would eat the male, but that's not
what they found. And the more aggressive females they pounced
on the pre war they also ate more, They were
less hungry, and they were more likely to attack, kill,
(26:24):
and eat the males. So it sounds a little bit
more like aggressive spill over to me. Yeah. So unless
it's an aggressive spill over, it would seem to be
evolutionarily counterintuitive. Is that she she's well nourished, she should
be prioritizing the fertilization of her eggs. Instead, she just
kills and eats her potential mates. But this makes it
(26:45):
look like baseline predation tendencies play a larger role in
whether or not the male gets eaten. Uh. Though you
could possibly also look at this and interpret it as
a form of mate testing, like you know, the weaker
males are the ones who are more were likely to
get eaten. Yeah, alright. The next one we have here
(27:05):
is the raft spider, and this is UH Dolomedes fimbriatis.
And according to a two thousand fifteen study from the
University of Melbourne, these spiders do seem to be testing
the males. This would be mate fitness rather than engaging
in an aggressive spill over. Okay, how does this work? Well,
this is how they they They studied it. They watched
(27:26):
a bunch of spiders mate, of course, yes, they watched. Yeah.
The first trial consisted of eleven solving that eleven of
sixteen females copulated then attacked the males deering or immediately
after population. Uh. Four of these attacks were fatal. Then
they did a second trial where six of eight females
that copulated attacked the males with two fatalities. So this
(27:47):
is interesting. Even in these UH I think one thing
we should point out is that even in these species
that practice sexual cannibalism, it doesn't always happen, right, Yeah,
So in these cases, aggression love did not seem to
play a factor. So again they're they're looking at how
aggressive a hunter does. This female appear to be outside
the context of mating. Rather male size, female age, and
(28:12):
her virginity seem to be more important factors. So females
were marginally more likely to attack smaller males. I don't
know if there's another word for this, but I kept
seeing this pop up over and over again, the virginity
of a spider. It just seems like it would that
seems like such a human word. Uh, Like it's weird
(28:32):
to think of a spider. I don't know, it would
just seem to make more sense to say a spider
who had not yet made it. Yeah, spider virginity, or
certainly virginity of a spider sounds like a wonderful name
for a short horror collection, but actually will explore in
a in another example I'm gonna get to in a bit,
you'll see where spider virginity, for lack of a better word,
(28:54):
does become more of an important consideration in sexual cannibalism. Yeah. Now,
one other inter seeing consideration to take into account is
the male decision making process in sexual cannibalism cases. So,
so a male is approaching a potential mate. He I
want to say he knows. I mean again, it's an insect,
(29:16):
so he probably doesn't really consciously know, but something about
his behavioral programming quote knows that he might get eaten
in some scenarios. So he has to do a kind
of behavioral cost benefit analysis. What what makes a delicious
bachelor insects decide to roll the dice with a hungry
(29:36):
female in the case of something like a praying mantis,
where the female in many cases eats the male during
or after sex. So there's a paper published in Plos
one that explores these decision factors and mate choice in
sexual cannibalism. UH in praying mantises is called low mate
encounter rate increases male risk taking in sexual cannibalistic praying mantis. Um.
(30:03):
So you've got a male of a praying mantis species
like tin Odera sinensis and this is this is a
common praying mantis. So I think it's referred to as
Chinese mantis. But it uh it has to make a
calculation of risk versus reward when approaching a female. Now,
I've read in one source that about sixteen percent of
the time a male of this species copulates with a
(30:25):
female in the wild, he gets slaughtered and eaten. So
those aren't good numbers. I mean, you wouldn't want to
copulate if you've got slaughtered sixteen percent of the time.
Uh So there's a chance of getting eaten. But of
course there's also the positive, a chance of reproducing. So
how does the male decide whether to roll the dice?
And the authors of this study point out two major
(30:46):
factors to consider. Number one is the level of predatory
risk imposed by the females, so some females are more
likely to eat you than other ones. And number two
is the frequency of mating opportunities for males. How many
chances has this male had to copulate with other females.
So to study these things, the researchers artificially controlled the
(31:09):
dating pool. Essentially, specifically, they allowed some males of this
praying man of species to encounter and court more females
than others, and then they controlled the risk of predation
by the females on the males. One thing to notice
that a male, a male can tell when a female
is hungry, and the hungrier she is, the more cautious
(31:29):
they are. However, what they found is that if the
male is sex starved, he will take risks with hungrier females.
And in the wild, it's worth pointing out, as we
said earlier, most of the time you're going to be
on the verge of starvation. Females in the wild are
usually hungry. So what what did they find? They found
sexually frustrated males are and those are again sorry anthropomorphic language.
(31:53):
The males who had had fewer chances to encounter females
in court them the ones that are sexually frustrated. We're
willing to take more risks with more dangerous females in
return for chances to mate. And the males who had
had more time around females, had been exposed to more females,
they were more cautious, approached the females more slowly and
stayed farther away. Um, And the ones who had not
(32:17):
been exposed to females, they basically just said to hell
with it and they ran in there. Okay, So yeah,
it comes down to what kind of risk are they
willing to take to carry out their genetic mission. Yeah,
and then there was also a second experiment where the
researchers tried this with hungry females and well fed females,
and they found that the hungry females plus sexually frustrated
(32:38):
males that that combination did in fact lead to cannibalism. Uh.
They said, quote greater risk taking behavior by males with
low mate encounter rates resulted in high rates of sexual
cannibalism when these males were paired with hungry females. So
it essentially paints a picture of how to make a
male praying mantis suicidal. You take away his access to
(33:01):
two lady friends. Hey, yeah, there you go. Okay, but
we we should look, we should get back to some spiders,
because the classic example of sexual cannibalism in the wild
is going to be the black widow. But but let's
let's look at a relative of the black widow. That's right,
We're gonna look at the red back spiders who are relatives,
and they have males, they're seemingly quite willing to take
(33:22):
the risk of being cannibalized. It's another one of these
species where we see some some rather intense uh sexual
dimorphism here because the uh, the the females are are
far larger while the males are tiny about the size
of a grain of rice. Females live for up to
two years, while males generally only make it four to
(33:43):
eight weeks. Oh man, Yeah, so so that's one example.
Like we were talking about at the beginning, like we
we can't help but overlaying our sort of social values
onto the lives of these insects. But it really does
look there like the female is what's being valued somehow
by nature and the male is just kind of there
to provide some sperm and disappear. Yeah, and then you
(34:04):
know it also comes from our attachment to life, I
think too, where you say, well, one lives longer, one's
got a bigger body, one seems to have more of
a sensory experience of the world, whereas I mean, it
really comes down to they each have their genetic mission,
they both carry it out. Ultimately, Hey, you could say
the male wins because he did it faster, we got
(34:25):
to the finish line mission accomplished, uh, no longer wasting resources.
But during copulation among the red back spiders, the smaller
male spider positions itself above the female's jaws. And this
apparent to male here I am, yeah, here, i am,
I'm right here, And it's apparent the male complicity, complicity
and sexual cannibalism is favored by sexual selection because cannibalized
(34:49):
spiders receive a number of paternity advantages. Oh really, so
they're they're documented cases here that show that it's better
to be cannibalized. So it would seem so genetically better,
not so much for the individual. Yeah, but again then
we're ben we're projecting conscious experience onto these these guys.
So cannibalized males copulate longer and they fertilize more eggs
(35:13):
than those who survive. Okay, so that's one advantage. Another advantage,
females are more likely to reject subsequent suitors after consuming
their first mate. Uh. And these results represented to empirical
evidence for uh what is sometimes referred to as male
copulatory suicide as an adapted behavior. And of course, but
(35:35):
of course that's a highly anthropomorphizing term. So two, back
to our analogy that this is again saying, this strangely
counterintuitive seeming behavior is itself being selected for. It's not
like a byproduct of something, it is itself the trait
that's beneficial. Kicking people in the face is good, yes,
(35:55):
And then there's this Uh. They deposit a sperm plug
and so this is a small part of their copula
tory organ that stays inside the female uh. And then
it prevents any future males from successfully fertilizing her eggs,
meaning that males have to mate with a virgin spider
to maximize their reproductive success. So competition is tough then
(36:19):
for the male redback spider for a couple of different
reasons here. And they only get that one mating opportunity
to carry it out. That's it. And given the whole
sperm plug thing, they have to act fast and they
also might get eaten, so so be it. But they
also have to make sure that they have enough energy
to carry out the deed. Interesting yeah, and this is
so they might not they might fail, they might be
(36:42):
too weary to survive the copulation long and or not survive,
but to perform the copulation correctly, right, And this is
where we get into I think it's helpful to think
of the mail and a weaponized sense as a he is.
He is a warhead, that is that has sent on
a mission to reach a destination. He to have the
appropriate navigational tools and fuel to to reach that destination.
(37:05):
But as it is with you know in rocketry, you
you don't you don't want to supply too much fuel.
You want just enough fuel to get where you're going, right,
And that actually plays into the development of the male
redback spider. According to a two thousand six University of
Toronto study, the speed of their development actually depends on
the density of females in the surroundings. When males can
(37:28):
smell females in abundance, they develop rapidly, settling for a
smaller body size and less fat reserves. All also that
he can get to those virgin females sooner and if
there are if there are plenty around, he doesn't need
the fat preserves in order to survive a long search
for a mate. But likewise, if he can smell um,
(37:51):
you know, he doesn't smell any females in the vicinity,
or they're very few, then they're gonna go ahead and
develop those those fat reserves so that he can actually
survive a longer trip. Oh wow, So it's sexually strategic
storage of energy. Yeah, exactly. And if there are a
lot of males in the area already, they tend to
go smaller on size, but with greater fat stores to
(38:12):
sustain them through. And I love this. The extended courtship
rituals in which the male has to essentially play music
on the female's web, just like strumming like the chords,
for up to eight hours at a time. What Yeah,
And if apparently she doesn't like the song he's playing,
she doesn't necessarily eat him. She may just swat him
(38:33):
off the web. We we are just we are jumping
the anthropomorphism shark here, I am. I am so sorry
to the people who don't want to hear the anthropomorphism
because we have gone off the demon. How is there
any way we could do this topic otherwise? For some reason,
it just demands this biased, inappropriate language. I think the
thing is, if you strip away the anti anthropomorphism, it
(38:56):
ultimately becomes less interesting to talk about. That's interesting to
listen to and harder to convey because instead of saying,
he plays their love song on a spiderweb, I have
to get into a little into far stuffier um biological
language about what's actually going on, and ultimately I'm conveying
the same thing, Like it's either you're easier just to say, hey,
(39:17):
this is gonna be a little anthropomorphic, so just be aware.
Well I didn't mean that as a criticism. I mean
I'm doing it too. Uh yeah, I mean, what what
does it take though, for a spider to become good
at playing the strings? Does he have to practice? I
don't know. It's certainly not extensive practice because he really
has to jump right in there, right. Yeah. So here's
(39:40):
one thing I was wondering. It seems like in every
case we look at of sexual cannibalism, it's the female
eating the male. And this obviously makes sense because the
female is the egg carrying member of the species. So
I mean, it just wouldn't make sense for a male
to eat a female after copulation, because that would destroy
all chances of reproduction happening from the you know, you
(40:02):
would eat the female and then the fertilized eggs would
not survive probably, and then what was the point of copulation?
That just seems like wasted energy resources. But is there
any case where some version of this could happen? There is,
interestingly enough we do find an example of reverse sexual
cannibalism in the mi Carria sociabilist spiders. It essentially boils
(40:25):
down to a form of mate male mate selection, much
like that employed by various female cannibals. So I see
that it depends kind of when you when you discuss
like how unrealistic would male sexual cannibalism be, It kind
of depends on which theory you're throwing at it. So
if you're going with the mate selection, yeah, you could
see where a male could say, Nope, don't want to
(40:48):
breathe with that that female. That breeding with that female
is not going to give me optimal results on my
genetic mission here. Better off that I eat her, okay, okay.
And so in two thousands thirteen, a team of researchers
from Messeric University in the Czech Republic found that young
males from the summer generation of these particular spiders were
(41:10):
the were the most likely cannibals here, and they tended
to cannibalize older females of the previous spring generation. So
male size and aggressions seem to play a role in
all of this. Uh, and female virginity or lack thereof,
did not. So this is another case where we're also
getting a little bit of the aggression spillover model in
(41:33):
there as well. Okay, but for the most part, it
seems to come down to mate selection. Yeah, So it
seems like we've seen a couple of scenarios where those
two hypotheses are not necessarily in conflict. They can sort
of work together, right. You know. One of the things
that this is kind of a side note, but one
of the things that's been interesting to me in looking
at this research is the different examples of uh I
(41:55):
guess males that can continue the mating process after they
have been partially consumed or dismembered. Particularly you see this
with with some of the mantis species out there, right,
some mantids, the males can continue to mate after their
their heads bitten off, and I think this is the
case with some of the spiders too. Write they can
like they can inject a sexual organ into the female
(42:19):
and then they can basically be eaten while those individual
organs are still doing their work. Yeah. I mean, we
already discussed in one case where the female made it
longer with the males who were cannibalized. So it's kind
of like if I'm cool again to anthropomorphize, if I'm
cool with having my head eaten off, I actually am
going to get in there longer doing the thing that
(42:41):
I am designed to do. So it makes sense, right,
it makes so much sense. Yeah, it's like she's gonna
eat me, but I'm gonna be there twice as long. Right,
it all evens out. Now. There was another interesting finding
that I wanted to talk about, bree Fulee, which was
about sexual deception in a cannibalistic mating system. Yeah. So,
(43:07):
so there was a paper published inteen called sexual Deception
in a Cannibalistic Mating System testing the film Fatale hypothesis.
And this was by the scientist Catherine L. Berry in
Biological Sciences, Proceedings of the Royal Society b And this
was a study of the female false garden mantis. Now,
(43:27):
what the false garden mantis does is she has pheromones
that are taken as good faith signals of her fecundity,
of of how likely she is to produce numbers of
healthy offspring. So mantis is that are in better health,
they're in better shape, they're more well fed. They give
(43:48):
off pheromones that advertise to males like, hey, I'm good
to go. We we are going to have lots of
beautiful children together if you come mate with me. But
also within this mantis ecs, there is some rate of
sexual cannibalism where the female will eat the male uh
after or during copulation. So this study was supposed to
(44:10):
examine UH whether or not the females could lie using
pheromones about how likely they were, when what and how
good health they were, you know, uh to to the
males who were coming up to them. So the females
were given different feeding regimens. Essentially some of them got
(44:31):
very good nutrition, others got medium nutrition, some got poor nutrition,
and some got very poor nutrition. And then males were
allowed to uh they were they were given the opportunity
to sort of approach a female that they couldn't see.
They were visually obscured in the terms of the study
and UH and so they were just operating off of
(44:52):
these pheromones that were put out by the females. And
what the study found is that for most of these
female mantis is the the rate at which the males
would approach them seemed to be indicating there was good
faith communication with the pheromones so males were most likely
to approach the good the you know, the ones who
had been on a good feeding schedule, and then a
(45:12):
little less likely to approach the ones on the medium
uh feeding schedule. So so there was a pretty much
correct advertisement system there except for one case, and that
one case that did not fit were the females who
had had the very poor feeding schedule, the ones who
got fed the worst, and those actually attracted more males
(45:35):
than any of the other female feeding regimens, even though
that meant the females were in the least good health
and they were the least likely to produce good numbers
of healthy offspring, but they were the most in need
of a good cannibal meal, right exactly, And so a
previous study had found that these females were more likely
to cannibalize males the ones who had had the least
(45:57):
eat So the hungry of the female is, the morely
she is to eat her mate. Now, studies necessarily or
have not necessarily found the same thing about other insects
and aragnets, but in this one particular species, it seems
that despite the fact that there's basically a correct or
good faith pheromone signaling system. UH, there are some cases
(46:18):
in which individuals will violate it in order to eat
somebody interact, so we get we get deception on top
of everything else that we're dealing with. You. Now, there's
one more paper I'd like to talk about that I
thought was pretty interesting. It's not directly addressing sexual cannibalism
as its main focus, but it had some cool observations
(46:39):
about it. And it's a paper called conditional monogamy Female
Quality predicts Male Faithfulness and it was published in Frontiers
in Zoology in So the premise of this paper is
that males from lots of animal species display polygony, you know,
having lots of wives, mating with more than one female,
(46:59):
you really as many females as he can. But some
animal species display I guess the inverse of that monogamy,
wherein the male only mates with one female in his
entire lifetime. Now, this might be for multiple reasons. The
male might be involved in uh, for for example, parent
parental duties maybe, but there there are some species where
(47:21):
that doesn't really seem to be the case yet there
is still just mating with one female in the lifetime.
And this would include especially to interest to our interests
several species of spiders. Now that this is a quote
from the paper monogamy is associated with curious adaptations like
lifelong associations between males and females. I like, how that's
(47:42):
a curious adaptation, but also extreme sexual size dimorphism, genital damage,
and sexual cannibalism. Okay, so that just lays out the
obvious that sometimes you meet that special person and that's
the only person you want to be with for the
rest of your life. Other times they kill you and
eat you or damage your genitals. Yeah. So, uh so, Now,
(48:02):
if you look at a male spiders sex organs on
one thing you might see is that the male spider
has these two copulatory organs called the pet a palps
that are used for copulation. These are sort of like
miniature arms coming out of the spider's head. Also, it
is likely that these organs will be damaged in mating.
So essentially, the male spider has two detachable penises available
(48:27):
for his mating budget and the females, on the other hand,
can mate multiple times. So the female spider has a
pair of incimination ducts. These are her her mating openings
and these can be in a sense plugged by a
broken off piece of male spider copulatory organs sense, so
(48:47):
he can break off part of his pet a palp
in her incimination duct to prevent other male rivals from
successfully coming along and mating with that same female, like
a stabbing in a Cormack McArthur novel where they snapped
the blade off in the wound. Yeah. Yeah, it sounds
pretty violent in our terms, but but I guess this
(49:07):
is how the spiders work. We we shall not judge now.
The study in particular is focused on the orb web
spider known as the wasps spider or our guy a
pee bruiniqui, and a few mating stats from previous research show.
One is that in the laboratory, females are quote highly
cannibalistic and eighty percent of males are killed during their
(49:28):
first copulation. That's a lot pretty high. Yeah yeah. They
also say genital damage is very common in a bruin bruiniqui,
and it occurs in eighty five percent of copulations into
unused genital openings. So there's a really good chance that
you get your sexual organ broken off inside the female.
And then on top of that, they say the broken
(49:49):
off pieces remain in the female insemination duc to nine
percent of these cases, so that these things, when they
get broken off, are highly effective in plugging the hole
so that other males can't come along and mate with
the same female. So that's one positive reason for detaching
one's penis. Y yeah, among others, I assume. Uh no,
(50:12):
I bet that's the only one that may be. But anyway,
so since the male sex organs tend to get broken off,
he has a choice. He has two of these things,
and he can spend them both on the same female.
This would be the case of monogamy, mating with only
one female in his lifetime, or he can try to
mate with two different females in his lifetime. Okay, so
(50:34):
we see us a form of mate selection here that
he can decide if this is definitely the one so
much so that he wants to double down on this
mate selection, or if he wants to save one keep
one in a chamber for labor later used. Right, It's
like if you're you know, you know, you're you're playing
roulette and you have two chips. Do you put them
on two different numbers or do you put them both
(50:55):
on the same number? All in on Black Widow right.
So the researchers tested interactions between these fighters, and they
found lots of interesting correlations. For example, the size of
the male did not seem to have any impact on
the mating strategy, but the size of the female did so.
When when a male encountered a female for the first time,
the bigger the female, the more likely the male was
(51:18):
to use his entire mating budget on her to give
her both of his sex organs. Also, when a male
mated with two different females, when he decided to spread
it over two different two different numbers on roulette, the
second female he broke off a sex organ with tended
to be heavier than the first, so he would tend
to trade up for a larger female. The larger the
(51:41):
female usually the more fecand well like a lot of
what we've discussed here in this episode, and that that
makes perfect economic sense, of course. Now a few other
interesting things they observed. One of them was that the
rate of sexual cannibalism they saw in their study was
much lower than what had been previously reported that number
we cited earlier. They found that the frequency of cannibalism
(52:02):
on the first copulation. So the first time male and
female meet up was thirty one point four percent, so
that's lower than than previous studies. But there was also
one really funny complication of how this worked out. In
the lab, they write, quote, the monogamous males either copulated
once in one and this is what they called the
(52:23):
group m one um, which are males cannibalized after a
single copulation or twice with the same female. In the
M one group, it cannot be decided whether males would
have followed a bigginess or monogamous tactic if only they
had survived their first copulation. So sometimes the male he
gets one broken off sex organ into the female, but
(52:46):
then she eats him and you can't tell whether he
was planning on using both on her or moving on.
That's right, because she has enacted her a mating choice
exactly he can exercise h Yeah. And and related to
that is that one thing they noticed was I think
this goes along with some of the other stuff we've read.
How long a male mate it during his first copulation
(53:07):
was directly positively correlated to the rate of sexual cannibalism.
So the longer he sticks around, the more time he
spends locking down his copulation with this one female, the
higher the chance that he gets slain and devoured. All right, now,
I want to read one more long ish quote from
their discussion section because I thought it was really interesting
related to what we've been talking about today. They say, quote,
(53:30):
some of the monogamous males were cannibalized after their first copulation,
while others used both of their mating options with the
same female. While the latter can be interpreted as a
clear cut male decision. So if the male uses both
on the female and then gets eaten, sort of makes
sense for him to get eaten. Um Uh. They picked
(53:52):
back up. Interpreting the decisions of single mated males is
more difficult. At least, some of these males may have
been forced into anogamy by aggressive females, while others may
have chosen to sacrifice themselves to their mating partner already
during the first copulation. Whether a male falls victim to
sexual cannibalism during the first copulation is a direct function
(54:14):
of its duration. Any copulation longer than ten seconds, remember
that number will most likely end with the death of
the males. Well, males that jump off before ten seconds
have a relatively high chance of surviving. Indeed, copulation durations
of single mated males show a high variance, which suggests
(54:34):
that some of them chose to copulate for longer in
spite of sexual cannibalism being the likely consequence, while others
attempted to escape but failed. So, I don't know, I
feel like that sort of drives home that when you're
speaking about evolution, it's worth remembering that your body is
sort of of little use to your genes once you've
(54:55):
lost all of your reproductive potential. Yes, the male individual
here is is the delivery system for the genetic material.
But anyway, if you want to read that study, it's
again conditional monogamy, female quality predicts male faithfulness. It's pretty
interesting and had some other observations too. All right, so
there you have it. Uh, we've we've discussed sexual cannibalism,
(55:17):
some of the theories regarding uh, it's the evolutionary development
a handful of species why it seems like it might
not make sense, but but it probably does. And of
course we've engaged in a fair amount of anthromomorphism. But
still I felt like I was holding back in some
of these, So was I Now, if you're a professional
researcher in this field, don't take a page from us,
(55:38):
try to do do the right thing, be boring, but
be accurate, and think like an insect than ever possible.
All Right. In the meantime, if you want to check
out more episodes of Stuff to Blow your Mind, head
on over to our website that's the mothership stuff to
Blow your Mind dot com, where you'll find all the
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(56:00):
Mind on both of those. We are Stuff to Blow
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in touch with us with feedback about this episode or
any interesting facts you've ever learned about sexual cannibalism, you
can email us at blow the Mind at how stuff
works dot com for more on this and thousands of
(56:24):
other topics. Is it how stuff works dot com
Welcome to Stuff to Blow Your Mind from how Stuff
Works dot com. Hey, what can the stuff to blow
your mind? My name is Robert lamp and my name
is Joe mcformack, and we're gonna get right to it
today because we're going to talk about sexual cannibalism, right
(00:23):
And I do want to just throw out here right
at the top, this is not going to concern human
cannibalism at all. I know that in the past when
we've covered cannibalistic topics, some listeners have actually skipped the
episode thinking that there might be human cannibals. And I'm
just gonna let you know, if you've made it this far,
no human cannibals, will cannibals will occur in this episode.
(00:45):
Is that some people's big thing? It's like, Oh, you're
gonna talk about cannibalism. I can't go on. Well, you know,
I mean some listeners, some of our listeners really want
the dark content. And that's why and and and we're
certainly happy to go there and as well as strip
away some of the the taboo and the mystery surrounding
those dark topics. But I know that not everyone wants
(01:06):
to go down that path with us. So I just
want to let everyone know we're not going down the
human cannibal path. Okay, So we are going to be
talking about animal sexual cannibalism. And this is a topic
that's been touched on on the show before. I Know
You and Julie in the Past is an episode about
cannibalism in the animal world, but today we wanted to
focus specifically on sexual cannibalism because of all of the
(01:30):
interesting evolutionary interplay here. Yea, the sort of trade off
of costs and benefits that would lead a species to
at some rate fairly often enough that people would notice
eat during sex, eat each other during sex or after sex. Yeah.
And it's the interesting thing about this topic is that,
on one hand, it is very simplistic, and I'll get
(01:52):
into the simplistic argument shortly, but then it's also not
as cut and dry as you might think. It's an
area where they are multiple theories as to as do
how this evolved and exactly how it's working, and some
of those are actually conflicting theories as well. So it's, uh,
it's a subject with a lot of meat on it,
a lot of sexy meat on it. Okay, so what's
(02:12):
the basic gist of why sexual cannibalism happens. This might
be kind of obvious, but we should get it on
the table so that we have a place to start.
So let's say you have two spiders and they size
each other up, and they say it's time to mate.
You know, we'll pass our genes on. But some way
through the mating procedure, the female sort of grabs hold
(02:33):
of the mail and sinks her fangs into him and
drains all of his delicious juices. Why did that happen?
And that's and that is the big question, right, why
does this evolve and why does this occur? Certainly not
in every species, it's actually a very rare occurrence in
the natural world. But where it does occur, uh, we
(02:56):
can't help it. Study it well. Actually, we should start
with the simple side. It's clear why this happens from
the female side. A lot of the question is why
the male would participate in this, right, But but why
does the female eat the male? Well? For energy, of course,
I mean that. I mean that basically comes down to
stripping away the human complexity surrounding cannibalism in general, because ultimately,
(03:19):
flesh is energy, flesh is food. Uh, flesh is life.
In addition, and so in addition to having all sorts
of cultural hangups about cannibalism, you know, we don't have
any problem wasting boatloads of food and energy, right, so
we kind of have to put that aside to think
of it. But yeah, in the in the natural world,
you see plenty of cases of just straight up cannibalism,
(03:40):
and it all comes down to a basic economy of
the energy. So a mother's young die, she might consume
those young because what are they now but empty vessels
made up of energy. Yeah, I think this is something
that's hard for us to understand because by and large,
if you are a human being listening to a podcast,
(04:02):
you probably are are lucky. You live a fortunate existence
where you have decent amounts of access to food. You're
you're probably not living constantly at the edge of starvation,
and animals are sort of built with the assumption that
they will be living constantly at the edge of starvation.
If you ever you ever noticed how if you have
a dog at home, if you're out walking the dog
(04:26):
and there's something that smells even remotely like it might
be food, the dog's going to try to put it
ahead and get in the mouth anyway, see what it takes.
And you and I would never think that way. I mean,
it's like smells like it could be food. Who knows
if it would hurt me? Better safe than sorry, I'm
just gonna eat it. Um. But in the animal kingdom,
(04:47):
out in the wild, that sort of strategy might make
a lot more sense, because if you are constantly at
risk of having your life extinguished or your competitive power
diminished by lack of access to energy resources, you'll take
anything you can get. Yeah, exactly. And you know, we
also have to think outside of the sort of human
male female scenario, right and get down to the basic
(05:10):
reality that I feel like is far more obvious when
you start looking at in the world of of insects
and arachnids and a few other species, and that is
that the females are the species and the male is
just this uh essentially, this mutation that's necessary to enable
genetic diversity through sexual reproduction. So he's he only has
(05:33):
this one purpose anyway, He's just this genetic material, heat
seeking missile and aid in reproduction. So why not eat him? Right?
His job is done, and there's there, that's it. Gets
to be more complex question, as we'll explore later. But
you know, it makes sense if you just look at
the pure math of the scenario. Yeah, and to get
(05:54):
even weirder, I mean, you might be able to say
that neither the male nor the female is really the
purpose of the speci sees the phenotype, the body, None
of that is the species. The species is a gene pool.
It's this abstract concept. It's really more of a movement
as opposed to a physical thing, because yeah, it's not
the little creatures that exist for especially in the insect world,
(06:17):
often very slim periods of time. It is that that
continued movement, genetic movement through time. Yeah, but but especially
the males. You might say, like if if there is
a sexual dimorphism in the species, a different in body,
a difference in body plans between male and female, and
it just you look at it and you say, wow,
it really looks like there's more going into the female.
(06:38):
She's bigger, she has more survival capacity. It really sometimes
can look to a human observer like, well, that's the
female is what the species is, and the male is
just sort of this thing that exists to mate. And yeah,
and I think you can make a pretty strong case
for that that the female is the primary member of
the species. Now already we're talking ing I think, in
(07:01):
sort of perhaps unscientific terms like what we're using anthropomorphization,
and we're using our our our sort of human judgment
of how things just seem to look to us, based
on the way we think about relationships between animals, probably
mostly based on the way we think about relationships between humans.
(07:22):
And this is sort of an inherent problem in studying
things like animal mating practices and reproduction and especially especially
sexual cannibalism. It we just can't help but infuse it
with all of the sort of social markers of how
we view relationships as humans. Yeah, and that's true of
not only consumers, readers, outsiders, true, not only of science
(07:46):
communicators like like ourselves, but also scientific researchers themselves. And
there was actually a paper on this, yeah, two thousand
ten paper, Sexual Stereotypes the Case of Sexual Cannibalism. This
was from the UK University of St Andrew schoo of Biology.
They looked at about they looked at a larger sample
and they had to to whittle it down. They end
up looking at around two ten relevant papers dealing with
(08:08):
sexual cannibalism, and they found that females were more likely
to be to be described using active words and males
with reactive words. And then a number of the words
used to describe cannibalistic females were highly loaded, suggesting a
negative stereotype of sexually aggressive females. Uh. And then the
males were more likely to be described as it's making
(08:29):
a sacrifice, while the females were just you know, voracious
widows um, that's funny. Voracious is a word I did
see pop up in the literature, in the scientific literature
on sexual cannibalism a lot. And and there are some
good points to be made about, you know, even though
this is sort of a tendency that's hard to overcome.
I know we're going to do it in this episode twenty.
(08:50):
We're gonna anthropomorphize. It's just how we talk about things,
how we talk about it's how we understand the topics,
how we conceptualize them in our head. There is a
good case to be made. I think that scientists do
their best to avoid this, and one simple reason is
that these kind of socially charged words are vague. Yeah,
you know, they're not they don't. They don't deal in
(09:11):
easily reproducible quantities. So if somebody wants to redo your
experiment somewhere else, can they tell what you mean by voracious?
I mean it seems like it makes more sense to
just talk about numerical conditions. So a spider that attacks
this percentage of prey within this you know, radius of
(09:32):
range is voracious. Yeah. And then I mean, just overall,
it's detrimental to view a species normalcy through the lembs
of our own species abnormality, because I mean, for instance,
we can study ducks without going holy crap. Can you
imagine if humans late eggs like a duck? So it's
(09:52):
a different creature, it's a different species. It is an
inhuman thing. And I guess anytime I study insects, I
always come back, maybe problematically to the line of Dr
Brundle in The Fly talking about uh insect politics and
how there are no insect politicians, um, which which I
(10:12):
think it's a great line, not only within the context
of that film, but also in terms of looking at
any of these scenarios where something through our lens, through
our lens of human normalcy, maybe just really horrific and monstrous,
and we want to cast this insect as a villain
and this as a poor, suffering martyr. But it is
an utterly inhuman system and there's no getting around that.
(10:36):
I'm sorry, I'm not listening to you because I'm imagining
the fly with a duck instead of a fly. So
Seth Brundle gets transformed into part duck and he's got
he's uh, he's got a bill. Well, you know how
the Duck and the Fly came out in the same year, Yeah,
oh wow, Yeah, that's a good year for film, the
year for film. In fact, we just had a how
(10:57):
Stuff Works Now article come out on that said, all
of your Howard the duck hate mail to blow the
mind dot com. But no. But moving on from how
weird it would be if humans were part duck, we
should look at the evolution of sexual cannibalism. And we're
going to talk about some specific cases later in this
episode of species that practice it and what we have
(11:17):
learned from them in recent studies. But where do we
typically see sexual cannibalism in the animal kingdom? Like, how
come you don't usually see a female cat eat a
male cat after sex? Well, I mean there's some biting
I think sometimes, but no, for the most part, we're
looking at insects and arachnids uh, spiders and scorpions being
(11:40):
the primary of research. There's some insects such as h
such as manted mantnanted species, and I believes there are
also some uh. Some evidences suggests that there are some
gastropod and copia pod cases. But in the for the
most part though, we're dealing with, especially with spiders and
scorpions and and and mantis, the species. We're dealing with
(12:02):
highly solitary predatory animals, which I think is key, especially
regarding some of the theories regarding sexual cannibalism. Right, So
if you want to keep in mind that they typically
aren't the most social animal rights that eat each other
after sex, and this might help inform our ideas about
how this evolved. Yeah, these are two anthropomorphizes. These are
(12:24):
lone killers. These are wanderers, right, that are just out
there on the highway eating what they need to eat
to survive. I say, even if you don't want to
infuse it with with human concepts of good and evil,
you could easily say that sexual cannibalism is a chaotic,
neutral trait. Yeah, yeah, I would think so so. But
(12:45):
but so, like we've said, it is rare in the
animal kingdom. You don't see it in most species. And
why is that? Well, on one, on one level, I
think we definitely have to again look to the solitary
predatory nature of these particular species. And then there's also
just cannibalism as a whole. If cannibalism is too essential
to the species, then you it eventually is going to
(13:09):
have an economically detrimental effect, right, So imagine your standard
sexual cannibalistic interaction. A female spider as a web. Male
spider approaches the web, climbs onto the web, mates with
the female, may or may not successfully impregnate her, and
the female eats the eats the male spider kills him,
(13:31):
drains all his fluids, gets energy from it. But the
male spider can mate no more. His days of passing
on his genes are now done. So this interaction works
out pretty well for the female spider. I mean, so
you can see why her genes would encourage such an interaction,
But the question is why would the male do that?
Why why does it uh genetically can I mean again,
(13:54):
we're using anthropomorphic terms like consent, But I would say,
why do its genes consent to this inner action where
it has a strong chance of being eaten? Indeed, that's
one of the big questions here, and one of the
questions that a number of these theories um of sexual
cannibism evolution explore. So at this point we should really
(14:14):
start talking about what those theories are and uh, and
then break into some of the examples that explore how
they work. Yeah. So there are several conflicting or competing
hypotheses about the evolution of sexual cannibalism, and we should
say that more than one can be true in different cases. Right,
There's some overlap here for sure. Yeah, But I think
one thing to keep in mind during these explanations is
(14:37):
that there are different ways that evolution can pay for
a trait. Let's say you have a trait that makes
you there's something about your brain that makes you really
likely to kick people in the face at the first
time you meet them. Um, there there are two ways
that could come through in your genes according to evolutionary theory,
and one one way is that the trade itself is
(14:59):
beneficial overall. People who kick people in the face first
time they meet just happened to have more kids than
people who don't, So it could be beneficial, or it
couldn't be. It could be an unintended side effect of
a very beneficial trait. So it could be that there's
actually not a good there's nothing good that comes from
kicking people in the face when you first meet them.
(15:20):
But you're really good at kicking people in the face
the rest of the time when exactly needed, So you're
much much better at defending yourself against predators. So you
just happened to be kicking crazy and and this is
just sort of and you're so good at defending yourself
for predators, this kicking genes survives even though it's not
particularly helpful in the case of kicking strangers in the
(15:40):
face when you first meet them. So the first of
these several theories we're gonna mention here is adaptive foraging.
So this one's pretty basic. Starving female mates the male
is there, she's hungry, she's gonna need energy. Why not
go ahead and eat him during either during the act
or immediately there after. Eating the male increases her chance
(16:02):
of survival, gives her a meal based power up uh
and as a as a as a sort of side
effect to her lover can't help produce any spawn that
end up rivaling her own for resources. But essentially it
comes down to I'm just I'm adapting to the what
food is available to me, and here is a meal
(16:22):
in front of me right now. Okay, So this is
sort of the the opportunism hypothesis. It's just saying that, like, look,
you know, the meal is worth it essentially right now.
Some of the critics argue that the males in many
of these cases, they actually make pretty poor meals. They're
not really going to make or break the female. But
(16:43):
then there are cases where they say that, hey, the
female is actually able to acquire crucial proteins or lipids
from the male that she eats that she might not
otherwise get from her standard predatory diet. Okay, So I'm
thinking that this is saying that the sexual cannibalism is
it's self an adaptation, It is itself a trait that
(17:03):
is encouraged because overall it's beneficial to the spider gene pool. Yeah,
not to the gene pool. I mean, it's I'm not
taking a species level evolution here. It would be beneficial
to the genes themselves in the animals that do it. Yeah,
the females who eat their male have just one little
power up over those who that do not. Yeah. And
the idea here is that this is very much a
(17:25):
kicking people in the face the first time you meet them. Right,
it's a good thing. Okay. But then there's another hypothesis
that is one I've read about some of the studies
I looked up here. It's the sort of I think
it's turned the aggressive spill over hypothesis. Uh. And this
is more that other idea we were talking about that
there are some traits that are very beneficial, but they
(17:45):
encourage side effects. So one trait, for example, might be
that a that a female is very aggressive in chasing
down prey. It has genes that lead it to be
a total killer. It's just a go getter. It sees moving,
it's like gonna eat that. Uh. And this might be
very beneficial to this creature when it's trying to survive
(18:07):
in the wild. It's it's very good at chasing down things,
killing them and eating them. But these same genes also
sometimes they get applied in the wrong direction. Yeah, like
to anthrommorphize the situation. It's it's like imagining that the
spider or the scorpion in the scenario, it was like
a a female wrestler who, in the act of making love,
(18:30):
cannot help like a muscle memory, just an instincts kick
in and she has to throw a suplex or two
or put somebody in a submission hold. Um, she's just
the female in this scenario is just so aggressive, just
so just amped up and ready to go, that the
predatory nature just kicks in and she ends up consuming
the part or all of the male. Yeah. Now, is
(18:51):
there any evidence for this? I think there's some. Yeah,
it depends. I mean in some species of spider. Yes,
researchers have observed higher sexual cannibals rates among females that
also attacked prey at a faster rate than other females
they've observed. I want to talk about one of these
studies in a bit, Okay, but then another species there's
no correlation. So that's the That's one of the issues too,
(19:15):
is you're dealing with You're not dealing with just one
species that's engaging in sexual cannibalism. You're dealing in multiple species. Uh.
The some are rather diverse from one another that have
in many cases independently evolved this adaptation. Yeah. Now, one
one hypothesis I saw just kind of mentioned on the internet,
(19:35):
but I didn't see in any of the scientific literature
I was looking at was the mistaken identity hypothesis. I
couldn't tell if there's really anything to this the I
saw it mentioned. I mean, the basic idea here is
the female is saying, are you food? Are you my lover? Well,
let me just take a bite out of you. I
can't tell, I guess you're my enemy. I'll just bite
into you. Uh. The only place that I saw this
and this is the person who kicks people in the
(19:57):
face because he's near sighted, right, exactly, enemy or lover,
I think the enemy I'm gonna kick. Yeah. So the
only place I saw this mentioned was in um uh
Kenwyn Blake Subtle's paper The Evolution of Sexual Cannibalism uh,
and he said that there's just no empirical evidence to
support this theory. Uh. For instance, he says that the
(20:20):
the fight up spiders offer a strong argument against mistaken
identity because, particularly among the Salta sids, you have the
highest visual acuity of all aractans. So I think that's
jumping spiders, right, Yes, And he tells us that when
you take into account um cannibalism during and after a copulation,
(20:42):
that would seem to even further eliminate the possibility. So
that the idea here is that, yeah, they're not just
blind dummies in these cases. They they have senses available
to them to determine if this is a member of
their own species or a solid uh outsider prey species. Yeah. Now,
of course, one other hypothesis that we could say competes
(21:06):
with these and and maybe sometimes overlaps with them, is
that it's just it's sexual selection. Yeah, it's it's females
being choosy about which males they mate with or whether
intentionally or not. Um, and this is encouraging pairing with
stronger or fitter or more appropriate males through a tendency
to eat the males. Yeah, it's just it's kind of
(21:29):
h it's not it's not me, it's use scenario. So
I'm going to eat you instead. You're not really uh,
you're not really husband material. Maybe you're more meal material. Right,
So yeah, that the basic hypothesis here entails the notion
that the cannibal female just rejects unacceptable males and eats them,
(21:50):
and it's smaller unfit males are better suited as meals.
Larger ones are the better mates. Though you also see
this reversed in some cases, with females favoring small aller mates. Uh. Again,
it kind of depends on the species, right well, As
we know from so many great evolutionary biologists before, it's
not always the strongest that's the fittest. The bigger doesn't
(22:11):
always necessarily mean better. Fitter can mean a lot of
different things in different environments, and the environment can include
the sexual selection environment, the gene environment in which you're competing. Yeah,
and the mere fact that both varieties survive often indicates
that both are valued forms of that species. Uh. And
there are a number of different mate choice splinter theories
(22:34):
as well, um running the anthropomorphic gamut for all the
reasons you might you know, mate reject or cannibalize your
spider lever so that you have it. Are those are
the basic theories regarding sexual capitalism. All right, So now
I think it's time to take a quick break. But
when we come back, we're going to take a look
at our rogues gallery of cannibals. All right, we're back.
(23:04):
Who do we have? First year? Joe? Well, I thought
it would be interesting because of a specific study I
found to look at the cannibalistic burrowing wolf spider, the
Lycosa Hispanica. And this is from a paper called does
female personality determine mate choice through sexual Cannibalism? It was
published in the journal Ethology in and Uh. It was
(23:28):
carried out by some research group at the Experimental Station
of Arid Zones e Z A C S I C
in Spain. And and here's what's going on. Sometimes you
want to eat your mate, but you cannot wait for
the mating. This is a problem because obviously, so we've
talked about the benefits for the female of sexual cannibalism.
(23:51):
She gets to mate, and she gets to pass on
her genes, and she also gets a meal. And we've
been debating the benefits for the male. But what if
the mail eats She wants to eat the mate so
bad that she eats him before they mate. I mean,
that seems like a maladaptive. It seems like it's gotten
(24:12):
out of control here. Yeah, And what they found is
that this happens a lot. Actually, so sometimes this specific tarantula,
This wolf spider doesn't even wait for sex before eating
the potential mate. So why is that? What determines if
the female wolf spider attacks a potential mate before the mating.
(24:34):
Is it how hungry she is? If so, this would
support the adaptive foraging hypothesis perhaps or is it her
personality and if that's the case, it might support the
aggressive spillover. Is it the fitness of the male? This
might support some form of sexual selection or mate choice.
So the researchers decided to try to isolate these and
(24:56):
figure out which one is the most likely. So they
studied a group of female wolves by to see how
aggressive they were in feeding. And some were insanely aggressive
that they were just going to town immediately on any
prey such as beatles, while others were more cautious or
they used the term docile, which I thought was funny.
(25:17):
Essentially more reticent hunters, they just didn't jump on prey
quite as quickly. And then they took these same females
they had studied and offered the females a random selection
of males for mating to see what would happen. Uh,
And you know what. The female personality actually seemed to
play a big role. So I want to read a
quote from a press release by one of the study authors,
(25:39):
Reuben robin Ada Bueno uh And. The author said, more
cannibalistic females were also more voracious towards their prey and
thus better nourished. This is surprising since they have more
nutritive resources to invest in their offspring before finding the
first males, and then said their priority should be in
(26:00):
suring the firm s fertilization of their eggs instead of
eliminating their potential donors. So this is kind of counterintuitive.
You might expect that the hungry or the female, the
more likely she would eat the male, but that's not
what they found. And the more aggressive females they pounced
on the pre war they also ate more, They were
less hungry, and they were more likely to attack, kill,
(26:24):
and eat the males. So it sounds a little bit
more like aggressive spill over to me. Yeah. So unless
it's an aggressive spill over, it would seem to be
evolutionarily counterintuitive. Is that she she's well nourished, she should
be prioritizing the fertilization of her eggs. Instead, she just
kills and eats her potential mates. But this makes it
(26:45):
look like baseline predation tendencies play a larger role in
whether or not the male gets eaten. Uh. Though you
could possibly also look at this and interpret it as
a form of mate testing, like you know, the weaker
males are the ones who are more were likely to
get eaten. Yeah, alright. The next one we have here
(27:05):
is the raft spider, and this is UH Dolomedes fimbriatis.
And according to a two thousand fifteen study from the
University of Melbourne, these spiders do seem to be testing
the males. This would be mate fitness rather than engaging
in an aggressive spill over. Okay, how does this work? Well,
this is how they they They studied it. They watched
(27:26):
a bunch of spiders mate, of course, yes, they watched. Yeah.
The first trial consisted of eleven solving that eleven of
sixteen females copulated then attacked the males deering or immediately
after population. Uh. Four of these attacks were fatal. Then
they did a second trial where six of eight females
that copulated attacked the males with two fatalities. So this
(27:47):
is interesting. Even in these UH I think one thing
we should point out is that even in these species
that practice sexual cannibalism, it doesn't always happen, right, Yeah,
So in these cases, aggression love did not seem to
play a factor. So again they're they're looking at how
aggressive a hunter does. This female appear to be outside
the context of mating. Rather male size, female age, and
(28:12):
her virginity seem to be more important factors. So females
were marginally more likely to attack smaller males. I don't
know if there's another word for this, but I kept
seeing this pop up over and over again, the virginity
of a spider. It just seems like it would that
seems like such a human word. Uh, Like it's weird
(28:32):
to think of a spider. I don't know, it would
just seem to make more sense to say a spider
who had not yet made it. Yeah, spider virginity, or
certainly virginity of a spider sounds like a wonderful name
for a short horror collection, but actually will explore in
a in another example I'm gonna get to in a bit,
you'll see where spider virginity, for lack of a better word,
(28:54):
does become more of an important consideration in sexual cannibalism. Yeah. Now,
one other inter seeing consideration to take into account is
the male decision making process in sexual cannibalism cases. So,
so a male is approaching a potential mate. He I
want to say he knows. I mean again, it's an insect,
(29:16):
so he probably doesn't really consciously know, but something about
his behavioral programming quote knows that he might get eaten
in some scenarios. So he has to do a kind
of behavioral cost benefit analysis. What what makes a delicious
bachelor insects decide to roll the dice with a hungry
(29:36):
female in the case of something like a praying mantis,
where the female in many cases eats the male during
or after sex. So there's a paper published in Plos
one that explores these decision factors and mate choice in
sexual cannibalism. UH in praying mantises is called low mate
encounter rate increases male risk taking in sexual cannibalistic praying mantis. Um.
(30:03):
So you've got a male of a praying mantis species
like tin Odera sinensis and this is this is a
common praying mantis. So I think it's referred to as
Chinese mantis. But it uh it has to make a
calculation of risk versus reward when approaching a female. Now,
I've read in one source that about sixteen percent of
the time a male of this species copulates with a
(30:25):
female in the wild, he gets slaughtered and eaten. So
those aren't good numbers. I mean, you wouldn't want to
copulate if you've got slaughtered sixteen percent of the time.
Uh So there's a chance of getting eaten. But of
course there's also the positive, a chance of reproducing. So
how does the male decide whether to roll the dice?
And the authors of this study point out two major
(30:46):
factors to consider. Number one is the level of predatory
risk imposed by the females, so some females are more
likely to eat you than other ones. And number two
is the frequency of mating opportunities for males. How many
chances has this male had to copulate with other females.
So to study these things, the researchers artificially controlled the
(31:09):
dating pool. Essentially, specifically, they allowed some males of this
praying man of species to encounter and court more females
than others, and then they controlled the risk of predation
by the females on the males. One thing to notice
that a male, a male can tell when a female
is hungry, and the hungrier she is, the more cautious
(31:29):
they are. However, what they found is that if the
male is sex starved, he will take risks with hungrier females.
And in the wild, it's worth pointing out, as we
said earlier, most of the time you're going to be
on the verge of starvation. Females in the wild are
usually hungry. So what what did they find? They found
sexually frustrated males are and those are again sorry anthropomorphic language.
(31:53):
The males who had had fewer chances to encounter females
in court them the ones that are sexually frustrated. We're
willing to take more risks with more dangerous females in
return for chances to mate. And the males who had
had more time around females, had been exposed to more females,
they were more cautious, approached the females more slowly and
stayed farther away. Um, And the ones who had not
(32:17):
been exposed to females, they basically just said to hell
with it and they ran in there. Okay, So yeah,
it comes down to what kind of risk are they
willing to take to carry out their genetic mission. Yeah,
and then there was also a second experiment where the
researchers tried this with hungry females and well fed females,
and they found that the hungry females plus sexually frustrated
(32:38):
males that that combination did in fact lead to cannibalism. Uh.
They said, quote greater risk taking behavior by males with
low mate encounter rates resulted in high rates of sexual
cannibalism when these males were paired with hungry females. So
it essentially paints a picture of how to make a
male praying mantis suicidal. You take away his access to
(33:01):
two lady friends. Hey, yeah, there you go. Okay, but
we we should look, we should get back to some spiders,
because the classic example of sexual cannibalism in the wild
is going to be the black widow. But but let's
let's look at a relative of the black widow. That's right,
We're gonna look at the red back spiders who are relatives,
and they have males, they're seemingly quite willing to take
(33:22):
the risk of being cannibalized. It's another one of these
species where we see some some rather intense uh sexual
dimorphism here because the uh, the the females are are
far larger while the males are tiny about the size
of a grain of rice. Females live for up to
two years, while males generally only make it four to
(33:43):
eight weeks. Oh man, Yeah, so so that's one example.
Like we were talking about at the beginning, like we
we can't help but overlaying our sort of social values
onto the lives of these insects. But it really does
look there like the female is what's being valued somehow
by nature and the male is just kind of there
to provide some sperm and disappear. Yeah, and then you
(34:04):
know it also comes from our attachment to life, I
think too, where you say, well, one lives longer, one's
got a bigger body, one seems to have more of
a sensory experience of the world, whereas I mean, it
really comes down to they each have their genetic mission,
they both carry it out. Ultimately, Hey, you could say
the male wins because he did it faster, we got
(34:25):
to the finish line mission accomplished, uh, no longer wasting resources.
But during copulation among the red back spiders, the smaller
male spider positions itself above the female's jaws. And this
apparent to male here I am, yeah, here, i am,
I'm right here, And it's apparent the male complicity, complicity
and sexual cannibalism is favored by sexual selection because cannibalized
(34:49):
spiders receive a number of paternity advantages. Oh really, so
they're they're documented cases here that show that it's better
to be cannibalized. So it would seem so genetically better,
not so much for the individual. Yeah, but again then
we're ben we're projecting conscious experience onto these these guys.
So cannibalized males copulate longer and they fertilize more eggs
(35:13):
than those who survive. Okay, so that's one advantage. Another advantage,
females are more likely to reject subsequent suitors after consuming
their first mate. Uh. And these results represented to empirical
evidence for uh what is sometimes referred to as male
copulatory suicide as an adapted behavior. And of course, but
(35:35):
of course that's a highly anthropomorphizing term. So two, back
to our analogy that this is again saying, this strangely
counterintuitive seeming behavior is itself being selected for. It's not
like a byproduct of something, it is itself the trait
that's beneficial. Kicking people in the face is good, yes,
(35:55):
And then there's this Uh. They deposit a sperm plug
and so this is a small part of their copula
tory organ that stays inside the female uh. And then
it prevents any future males from successfully fertilizing her eggs,
meaning that males have to mate with a virgin spider
to maximize their reproductive success. So competition is tough then
(36:19):
for the male redback spider for a couple of different
reasons here. And they only get that one mating opportunity
to carry it out. That's it. And given the whole
sperm plug thing, they have to act fast and they
also might get eaten, so so be it. But they
also have to make sure that they have enough energy
to carry out the deed. Interesting yeah, and this is
so they might not they might fail, they might be
(36:42):
too weary to survive the copulation long and or not survive,
but to perform the copulation correctly, right, And this is
where we get into I think it's helpful to think
of the mail and a weaponized sense as a he is.
He is a warhead, that is that has sent on
a mission to reach a destination. He to have the
appropriate navigational tools and fuel to to reach that destination.
(37:05):
But as it is with you know in rocketry, you
you don't you don't want to supply too much fuel.
You want just enough fuel to get where you're going, right,
And that actually plays into the development of the male
redback spider. According to a two thousand six University of
Toronto study, the speed of their development actually depends on
the density of females in the surroundings. When males can
(37:28):
smell females in abundance, they develop rapidly, settling for a
smaller body size and less fat reserves. All also that
he can get to those virgin females sooner and if
there are if there are plenty around, he doesn't need
the fat preserves in order to survive a long search
for a mate. But likewise, if he can smell um,
(37:51):
you know, he doesn't smell any females in the vicinity,
or they're very few, then they're gonna go ahead and
develop those those fat reserves so that he can actually
survive a longer trip. Oh wow, So it's sexually strategic
storage of energy. Yeah, exactly. And if there are a
lot of males in the area already, they tend to
go smaller on size, but with greater fat stores to
(38:12):
sustain them through. And I love this. The extended courtship
rituals in which the male has to essentially play music
on the female's web, just like strumming like the chords,
for up to eight hours at a time. What Yeah,
And if apparently she doesn't like the song he's playing,
she doesn't necessarily eat him. She may just swat him
(38:33):
off the web. We we are just we are jumping
the anthropomorphism shark here, I am. I am so sorry
to the people who don't want to hear the anthropomorphism
because we have gone off the demon. How is there
any way we could do this topic otherwise? For some reason,
it just demands this biased, inappropriate language. I think the
thing is, if you strip away the anti anthropomorphism, it
(38:56):
ultimately becomes less interesting to talk about. That's interesting to
listen to and harder to convey because instead of saying,
he plays their love song on a spiderweb, I have
to get into a little into far stuffier um biological
language about what's actually going on, and ultimately I'm conveying
the same thing, Like it's either you're easier just to say, hey,
(39:17):
this is gonna be a little anthropomorphic, so just be aware.
Well I didn't mean that as a criticism. I mean
I'm doing it too. Uh yeah, I mean, what what
does it take though, for a spider to become good
at playing the strings? Does he have to practice? I
don't know. It's certainly not extensive practice because he really
has to jump right in there, right. Yeah. So here's
(39:40):
one thing I was wondering. It seems like in every
case we look at of sexual cannibalism, it's the female
eating the male. And this obviously makes sense because the
female is the egg carrying member of the species. So
I mean, it just wouldn't make sense for a male
to eat a female after copulation, because that would destroy
all chances of reproduction happening from the you know, you
(40:02):
would eat the female and then the fertilized eggs would
not survive probably, and then what was the point of copulation?
That just seems like wasted energy resources. But is there
any case where some version of this could happen? There is,
interestingly enough we do find an example of reverse sexual
cannibalism in the mi Carria sociabilist spiders. It essentially boils
(40:25):
down to a form of mate male mate selection, much
like that employed by various female cannibals. So I see
that it depends kind of when you when you discuss
like how unrealistic would male sexual cannibalism be, It kind
of depends on which theory you're throwing at it. So
if you're going with the mate selection, yeah, you could
see where a male could say, Nope, don't want to
(40:48):
breathe with that that female. That breeding with that female
is not going to give me optimal results on my
genetic mission here. Better off that I eat her, okay, okay.
And so in two thousands thirteen, a team of researchers
from Messeric University in the Czech Republic found that young
males from the summer generation of these particular spiders were
(41:10):
the were the most likely cannibals here, and they tended
to cannibalize older females of the previous spring generation. So
male size and aggressions seem to play a role in
all of this. Uh, and female virginity or lack thereof,
did not. So this is another case where we're also
getting a little bit of the aggression spillover model in
(41:33):
there as well. Okay, but for the most part, it
seems to come down to mate selection. Yeah, So it
seems like we've seen a couple of scenarios where those
two hypotheses are not necessarily in conflict. They can sort
of work together, right. You know. One of the things
that this is kind of a side note, but one
of the things that's been interesting to me in looking
at this research is the different examples of uh I
(41:55):
guess males that can continue the mating process after they
have been partially consumed or dismembered. Particularly you see this
with with some of the mantis species out there, right,
some mantids, the males can continue to mate after their
their heads bitten off, and I think this is the
case with some of the spiders too. Write they can
like they can inject a sexual organ into the female
(42:19):
and then they can basically be eaten while those individual
organs are still doing their work. Yeah. I mean, we
already discussed in one case where the female made it
longer with the males who were cannibalized. So it's kind
of like if I'm cool again to anthropomorphize, if I'm
cool with having my head eaten off, I actually am
going to get in there longer doing the thing that
(42:41):
I am designed to do. So it makes sense, right,
it makes so much sense. Yeah, it's like she's gonna
eat me, but I'm gonna be there twice as long. Right,
it all evens out. Now. There was another interesting finding
that I wanted to talk about, bree Fulee, which was
about sexual deception in a cannibalistic mating system. Yeah. So,
(43:07):
so there was a paper published inteen called sexual Deception
in a Cannibalistic Mating System testing the film Fatale hypothesis.
And this was by the scientist Catherine L. Berry in
Biological Sciences, Proceedings of the Royal Society b And this
was a study of the female false garden mantis. Now,
(43:27):
what the false garden mantis does is she has pheromones
that are taken as good faith signals of her fecundity,
of of how likely she is to produce numbers of
healthy offspring. So mantis is that are in better health,
they're in better shape, they're more well fed. They give
(43:48):
off pheromones that advertise to males like, hey, I'm good
to go. We we are going to have lots of
beautiful children together if you come mate with me. But
also within this mantis ecs, there is some rate of
sexual cannibalism where the female will eat the male uh
after or during copulation. So this study was supposed to
(44:10):
examine UH whether or not the females could lie using
pheromones about how likely they were, when what and how
good health they were, you know, uh to to the
males who were coming up to them. So the females
were given different feeding regimens. Essentially some of them got
(44:31):
very good nutrition, others got medium nutrition, some got poor nutrition,
and some got very poor nutrition. And then males were
allowed to uh they were they were given the opportunity
to sort of approach a female that they couldn't see.
They were visually obscured in the terms of the study
and UH and so they were just operating off of
(44:52):
these pheromones that were put out by the females. And
what the study found is that for most of these
female mantis is the the rate at which the males
would approach them seemed to be indicating there was good
faith communication with the pheromones so males were most likely
to approach the good the you know, the ones who
had been on a good feeding schedule, and then a
(45:12):
little less likely to approach the ones on the medium
uh feeding schedule. So so there was a pretty much
correct advertisement system there except for one case, and that
one case that did not fit were the females who
had had the very poor feeding schedule, the ones who
got fed the worst, and those actually attracted more males
(45:35):
than any of the other female feeding regimens, even though
that meant the females were in the least good health
and they were the least likely to produce good numbers
of healthy offspring, but they were the most in need
of a good cannibal meal, right exactly, And so a
previous study had found that these females were more likely
to cannibalize males the ones who had had the least
(45:57):
eat So the hungry of the female is, the morely
she is to eat her mate. Now, studies necessarily or
have not necessarily found the same thing about other insects
and aragnets, but in this one particular species, it seems
that despite the fact that there's basically a correct or
good faith pheromone signaling system. UH, there are some cases
(46:18):
in which individuals will violate it in order to eat
somebody interact, so we get we get deception on top
of everything else that we're dealing with. You. Now, there's
one more paper I'd like to talk about that I
thought was pretty interesting. It's not directly addressing sexual cannibalism
as its main focus, but it had some cool observations
(46:39):
about it. And it's a paper called conditional monogamy Female
Quality predicts Male Faithfulness and it was published in Frontiers
in Zoology in So the premise of this paper is
that males from lots of animal species display polygony, you know,
having lots of wives, mating with more than one female,
(46:59):
you really as many females as he can. But some
animal species display I guess the inverse of that monogamy,
wherein the male only mates with one female in his
entire lifetime. Now, this might be for multiple reasons. The
male might be involved in uh, for for example, parent
parental duties maybe, but there there are some species where
(47:21):
that doesn't really seem to be the case yet there
is still just mating with one female in the lifetime.
And this would include especially to interest to our interests
several species of spiders. Now that this is a quote
from the paper monogamy is associated with curious adaptations like
lifelong associations between males and females. I like, how that's
(47:42):
a curious adaptation, but also extreme sexual size dimorphism, genital damage,
and sexual cannibalism. Okay, so that just lays out the
obvious that sometimes you meet that special person and that's
the only person you want to be with for the
rest of your life. Other times they kill you and
eat you or damage your genitals. Yeah. So, uh so, Now,
(48:02):
if you look at a male spiders sex organs on
one thing you might see is that the male spider
has these two copulatory organs called the pet a palps
that are used for copulation. These are sort of like
miniature arms coming out of the spider's head. Also, it
is likely that these organs will be damaged in mating.
So essentially, the male spider has two detachable penises available
(48:27):
for his mating budget and the females, on the other hand,
can mate multiple times. So the female spider has a
pair of incimination ducts. These are her her mating openings
and these can be in a sense plugged by a
broken off piece of male spider copulatory organs sense, so
(48:47):
he can break off part of his pet a palp
in her incimination duct to prevent other male rivals from
successfully coming along and mating with that same female, like
a stabbing in a Cormack McArthur novel where they snapped
the blade off in the wound. Yeah. Yeah, it sounds
pretty violent in our terms, but but I guess this
(49:07):
is how the spiders work. We we shall not judge now.
The study in particular is focused on the orb web
spider known as the wasps spider or our guy a
pee bruiniqui, and a few mating stats from previous research show.
One is that in the laboratory, females are quote highly
cannibalistic and eighty percent of males are killed during their
(49:28):
first copulation. That's a lot pretty high. Yeah yeah. They
also say genital damage is very common in a bruin bruiniqui,
and it occurs in eighty five percent of copulations into
unused genital openings. So there's a really good chance that
you get your sexual organ broken off inside the female.
And then on top of that, they say the broken
(49:49):
off pieces remain in the female insemination duc to nine
percent of these cases, so that these things, when they
get broken off, are highly effective in plugging the hole
so that other males can't come along and mate with
the same female. So that's one positive reason for detaching
one's penis. Y yeah, among others, I assume. Uh no,
(50:12):
I bet that's the only one that may be. But anyway,
so since the male sex organs tend to get broken off,
he has a choice. He has two of these things,
and he can spend them both on the same female.
This would be the case of monogamy, mating with only
one female in his lifetime, or he can try to
mate with two different females in his lifetime. Okay, so
(50:34):
we see us a form of mate selection here that
he can decide if this is definitely the one so
much so that he wants to double down on this
mate selection, or if he wants to save one keep
one in a chamber for labor later used. Right, It's
like if you're you know, you know, you're you're playing
roulette and you have two chips. Do you put them
on two different numbers or do you put them both
(50:55):
on the same number? All in on Black Widow right.
So the researchers tested interactions between these fighters, and they
found lots of interesting correlations. For example, the size of
the male did not seem to have any impact on
the mating strategy, but the size of the female did so.
When when a male encountered a female for the first time,
the bigger the female, the more likely the male was
(51:18):
to use his entire mating budget on her to give
her both of his sex organs. Also, when a male
mated with two different females, when he decided to spread
it over two different two different numbers on roulette, the
second female he broke off a sex organ with tended
to be heavier than the first, so he would tend
to trade up for a larger female. The larger the
(51:41):
female usually the more fecand well like a lot of
what we've discussed here in this episode, and that that
makes perfect economic sense, of course. Now a few other
interesting things they observed. One of them was that the
rate of sexual cannibalism they saw in their study was
much lower than what had been previously reported that number
we cited earlier. They found that the frequency of cannibalism
(52:02):
on the first copulation. So the first time male and
female meet up was thirty one point four percent, so
that's lower than than previous studies. But there was also
one really funny complication of how this worked out. In
the lab, they write, quote, the monogamous males either copulated
once in one and this is what they called the
(52:23):
group m one um, which are males cannibalized after a
single copulation or twice with the same female. In the
M one group, it cannot be decided whether males would
have followed a bigginess or monogamous tactic if only they
had survived their first copulation. So sometimes the male he
gets one broken off sex organ into the female, but
(52:46):
then she eats him and you can't tell whether he
was planning on using both on her or moving on.
That's right, because she has enacted her a mating choice
exactly he can exercise h Yeah. And and related to
that is that one thing they noticed was I think
this goes along with some of the other stuff we've read.
How long a male mate it during his first copulation
(53:07):
was directly positively correlated to the rate of sexual cannibalism.
So the longer he sticks around, the more time he
spends locking down his copulation with this one female, the
higher the chance that he gets slain and devoured. All right, now,
I want to read one more long ish quote from
their discussion section because I thought it was really interesting
related to what we've been talking about today. They say, quote,
(53:30):
some of the monogamous males were cannibalized after their first copulation,
while others used both of their mating options with the
same female. While the latter can be interpreted as a
clear cut male decision. So if the male uses both
on the female and then gets eaten, sort of makes
sense for him to get eaten. Um Uh. They picked
(53:52):
back up. Interpreting the decisions of single mated males is
more difficult. At least, some of these males may have
been forced into anogamy by aggressive females, while others may
have chosen to sacrifice themselves to their mating partner already
during the first copulation. Whether a male falls victim to
sexual cannibalism during the first copulation is a direct function
(54:14):
of its duration. Any copulation longer than ten seconds, remember
that number will most likely end with the death of
the males. Well, males that jump off before ten seconds
have a relatively high chance of surviving. Indeed, copulation durations
of single mated males show a high variance, which suggests
(54:34):
that some of them chose to copulate for longer in
spite of sexual cannibalism being the likely consequence, while others
attempted to escape but failed. So, I don't know, I
feel like that sort of drives home that when you're
speaking about evolution, it's worth remembering that your body is
sort of of little use to your genes once you've
(54:55):
lost all of your reproductive potential. Yes, the male individual
here is is the delivery system for the genetic material.
But anyway, if you want to read that study, it's
again conditional monogamy, female quality predicts male faithfulness. It's pretty
interesting and had some other observations too. All right, so
there you have it. Uh, we've we've discussed sexual cannibalism,
(55:17):
some of the theories regarding uh, it's the evolutionary development
a handful of species why it seems like it might
not make sense, but but it probably does. And of
course we've engaged in a fair amount of anthromomorphism. But
still I felt like I was holding back in some
of these, So was I Now, if you're a professional
researcher in this field, don't take a page from us,
(55:38):
try to do do the right thing, be boring, but
be accurate, and think like an insect than ever possible.
All Right. In the meantime, if you want to check
out more episodes of Stuff to Blow your Mind, head
on over to our website that's the mothership stuff to
Blow your Mind dot com, where you'll find all the
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(56:00):
Mind on both of those. We are Stuff to Blow
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