How to become a parasite

Daniel and Kelly talk about whether or not dinosaurs had parasites, how often parasitism has popped up in the tree of life, and more. 

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Episode Transcript

Speaker 1 (00:09):
Imagine sitting out on a porch on a warm spring evening.
You hear the spring peepers calling out to each other
in a nearby pond. Birds are chirping in the trees,
preparing to turn into their nest for the night. Off
in the distance, you see a deer walking with her fawn.
It's a beautiful scene, reminding you how wonderful it is

(00:30):
to live in Virginia when I hear and see these
denizens of the forest, though I see something else as well,
walking ecosystems for parasites. You see on average a bird
species harbors more than three species of tapeworm nearly three
nematode species. Probably call that frog home. Those deer are

(00:51):
harboring treematodes, tapeworms, nematodes, and more. Though hidden from sight,
parasites are thought to make up over half of the
biodiversity of multicellular organisms on our planet. Today, we're going
to answer listener questions about the evolution of parasitism. Hope
you're not listening to the show while eating dinner. Welcome

(01:13):
to Daniel and Kelly's Extraordinarily Infected Universe.

Speaker 2 (01:30):
Hi, I'm Daniel, I'm a particle physicist, and I'm looking
forward to meeting alien parasites.

Speaker 1 (01:35):
Hello. I'm Kelly Wiener Smith. I study parasites and space,
but not parasites in space. But maybe one day I'll
study Yeah, maybe one day I'll be studying parasites in space.

Speaker 2 (01:45):
So my question for you today, Kelly, is what is
your favorite depiction of parasites in science fiction?

Speaker 1 (01:51):
Probably aliens don't need to think about it. Aliens, it's
a great parasitoid in there. I went through a phase
where I was just love all of the zombie movies
and I was actually taking notes on like what mechanism
was producing the zombies. So for a while, you know,
when there were big fears of nuclear bombs getting dropped
on the US for example, you know, nuclear material was

(02:13):
the cause of zombies. But then it became like infectious diseases,
and I've gotten off course. I was thinking about writing
a whole book on this actually.

Speaker 2 (02:22):
And why didn't you?

Speaker 1 (02:23):
It sounds fascinating, you know enough people told me that, like,
the zombie craze is behind us, and that this is
a book nobody would want to read because it would
be two of the time. Yeah, two of the moment, right,
like when people weren't watching the zombie movies like The
Last of Us Sword, which I guess that's running still now.

Speaker 2 (02:39):
Yeah, but zombies aren't a moment, They're a movement.

Speaker 1 (02:42):
You know, that's right. Well, and I gave a talk
at the Zombie Apocalypse Medicine Meeting, or I think I
for Sam. Yeah, and it's happening in October of this year.
I highly recommend folks go to it.

Speaker 2 (02:55):
These are legit scientists.

Speaker 1 (02:56):
These are legit scientists. Yes, wow. And I gave a
talk on that topic and it was like the most
fun research I've ever done in my life.

Speaker 2 (03:07):
Well, I think I speak for the audience and saying
that we all want to read your book on the
science of zombies. That sounds fantastic.

Speaker 1 (03:12):
You know. I've got a link to the talk that
I could share if anyone wants to see the talk,
But I don't think it's going to become a full
length book, not with what I've got on my agenda
right now.

Speaker 2 (03:21):
All right, Well, today we're not talking about space or
science fiction or zombies. We're answering a really fascinating question
from a listener about the context, the evolution, and maybe
even the future of parasites in our world.

Speaker 3 (03:33):
Take it away, Zach, Hey, they're extraordinaries. The recent episode
on the history of Trickinella, I should say, the recent
history of trick and Ella got me wondering about the
more ancient history of parasites. I assume that being a
parasite is a strategy that existed back in the time
of the dinosaurs, maybe even further back. I'm curious if

(03:55):
we have evidence of that, and if parasites tend to
become extinct when their host species does as well, or
if we're still living with the same kind of parasites
the dinosaurs dealt with. I'm also curious if we know
about how parasites evolve, how does an organism go from
being not a parasite to a parasite? Do we have

(04:16):
any idea of that evolutionary history. Looking forward to hearing
more squishy wormy biology from you guys.

Speaker 2 (04:23):
Thank you Zach for sending in this question. If you
have questions about the nature of the universe, or the
history of parasites, or anything in this extraordinary universe, we
want to hear from you. We want to answer your questions.
We want to scratch your itch because everybody's curious about
the universe, so don't be shy. Write to us two
questions at Danielandkelly dot org. Everyone gets an answer.

Speaker 1 (04:44):
And I am currently researching why we scratch itches, which
is a question sent by a listener. So you even
get those kinds of questions answered.

Speaker 2 (04:52):
Amazing, yep, So dig into this for us, Kelly, tell
us about the history of parasites and why we call
something a parasite and what fraction of species we could
classify as parasites.

Speaker 1 (05:02):
Well, Daniel, let's pretend that you didn't read the outline,
which maybe maybe you didn't, although you're usually very well prepared.
If you had to guess what percent of multicellular organisms
were parasites, what percent would you guess?

Speaker 2 (05:15):
It doesn't seem to me like most things are parasites.
Bunnies are not parasites, flies are not parasites. Whales or
not parasites, So you know, it seems to me unusual.
So I guess one percent is my absolutely no information.

Speaker 1 (05:28):
Guess are you playing along? Are you really guessing one percent?

Speaker 2 (05:31):
I'm really guessing one percent?

Speaker 1 (05:32):
Oh wow, that's fantastic. Okay. I love those moments where
you can be like, you're totally wrong. So Dan, you're
totally wrong. Yes, yay. So if you look at that rabbit, So,
rabbits are mammals, and on average, mammals harbor one to
two species of trematodes, one to two species of tapeworms,
and four nematode species.

Speaker 2 (05:51):
Well, I didn't want to eat rabbits before, but I
certainly don't want to eat rabbits now.

Speaker 1 (05:55):
Well, then let's not talk about more typical food animals
like cows and pigs. We'll stay away from those. But
it's estimated that this was from a two thousand and
eight paper that was estimating about forty four thousand host
species with vertebrates and about seventy seven thousand species of trematodes, tapeworms, nematodes,
and acanthoscephalins. So this is four kinds of parasites, almost

(06:17):
twice as many parasites as there are hosts. And that
doesn't even include things like lice and ticks and stuff
like that.

Speaker 2 (06:25):
What hold on a second, you're telling me that like
every species of critter out there has like two unique
species of parasites, So there's like a two to one
parasite to creater ratio.

Speaker 1 (06:36):
This is a great question. This is where it gets
a little bit complicated. So, for example, if you have
two different species of rabbit, they might share one of
their trematode species for example. So there are some parasites
that are generalists and you can find them in more
than one host species, but there are also a lot
of parasites that are found in one species. And so
in general, when we look at how many parasites are

(06:58):
generalists and how many are specially listener just found in
one host, we get this estimate where we feel comfortable
saying that probably more than fifty percent of the multicellular
biodiversity on Earth is using the parasitic strategy in some way.

Speaker 2 (07:10):
Oh my gosh, we're in the minority. Yeah, for not
sucking on some other critter, we are like unusual.

Speaker 1 (07:17):
We're total chumps, man.

Speaker 2 (07:19):
So we got to rethink this.

Speaker 1 (07:22):
Wow, I know there's a lot of interesting questions that
arise from there. So we know that a bunch of
species have this strategy of being a parasite, which essentially
means that you rely on another organism where you're extracting
their energy. It's not benefiting them, and you need them
to get the energy.

Speaker 2 (07:39):
Well, then can I ask a technical question like, yeah,
is a cow a parasite because it relies on grass
and you know it's extracting the energy from the grass.
Why isn't a cow listed as a grass parasite?

Speaker 1 (07:50):
So it is eating lots of different blades of grass,
and so this is usually an intimate, endurable interaction between
two INDs.

Speaker 2 (08:00):
Intimate, wow, intimate.

Speaker 1 (08:02):
Well, you know, if you're living inside, for example, of
something and sharing gets dinner with it every night, that's
pretty intimate. And so, yeah, cows are eating lots of
different grass. We call mosquitoes and lamb preys and ticks
and stuff micro predators. So they take little meals from
lots of animals. They don't kill them, which predators usually do,

(08:25):
but they just take little meals from lots of animals.
And so the idea is that to be a parasite
you have to be on one organism for a long time.

Speaker 2 (08:33):
Okay, so mosquito's technically not parasites then, right.

Speaker 1 (08:36):
You would be amazing how much fur flies at parasitology
conferences when the mosquito and the leech people are like,
these are parasites. And then some of the community is
like they're micro predators, and the leech people are like,
we belong here, and I'm like, they belong here, don't
kick them out.

Speaker 2 (08:51):
Well, they're parasites in spirit, even if they don't qualify technically, right,
because otherwise, like a giraffe which nibbles on this tree
and then the other tree in the other trees basically
has the same relation with a tree as a mosquito
does with people, right, So.

Speaker 1 (09:03):
That's because it's not killing all of them. Yeah, yeah,
And so sometimes you can get into debates with some
folks about to what extent herbivores can be thought of
as parasites solely. You know, if like an insect lives
on the same tree for a long time, is it
a parasite or is it a different category altogether because
it's a plant And anyway, I guess at the end
of the day, nature doesn't care that humans like to

(09:25):
have very nice clear boxes to put things in. She's
always giving us edge cases to make things complicated.

Speaker 2 (09:31):
And life is an ecosystem, right, It's a web, and
there's nothing isolated. We're all munching on each other and
eating each other and being eaten by each other. It's
a big party, right.

Speaker 1 (09:42):
I don't want to go to the parties you go to, Daniel.
Based on what you've just described, we have fun.

Speaker 2 (09:47):
We have fun. But how useful are these categorizations? I mean,
does it matter what we call a parasite or not?

Speaker 1 (09:55):
So I think it does. And the main reason I
think it does is because often folks will try to
model the impact of parasites on populations, or use some
modeling to try to understand how to control a parasite population,
and those models depend on the assumptions you make about
what that organism is doing, how many organisms it's feeding on,

(10:15):
how many babies it produces. Like, having these details helps
you create models that can describe different groups of strategies,
and those models can be important for a lot of things,
including epidemiology.

Speaker 2 (10:27):
I see, all right. And also it's nice to get
like minded people together, right, And so yeah, you gotta
people want to know which conference do I go to?
Do I go to the zombie conference or the parasite
conference or the zombie parasite conference or whatever? And so
we got to put names on things so we can
get ourselves organized, even if we admit that the names
are fuzzy. You know, is Pluto a planet or not

(10:47):
doesn't really matter. We're interested in studying this stuff fluting
in outer space.

Speaker 1 (10:51):
I wants organized a symposium called how to Make a
Zombie with my friend zen Folks, who has like the
coolest name ever, and then it was me, who has
the least cool name ever. And it was a very
interesting combo. But anyway, it was all about parasites that
manipulate host behavior and how they do that.

Speaker 2 (11:06):
Right, right, Yeah, parasites and zombies are connected in the end.

Speaker 1 (11:09):
Sure.

Speaker 2 (11:10):
So the bottom line is that there are lots and
lots and lots of parasites out there. I was totally wrong,
and there's lots of different kinds of parasites out there.
But what about by body mass? Are parasites like a
tiny fraction of the mass or if you made a
big pile of all the parasites on the planet, would
be bigger than the pile of non parasites.

Speaker 1 (11:28):
This is a great question. So I didn't necessarily prepare
for you to ask this question, so I don't have
all the numbers that I want at my fingertips. But
there was a system where this question was studied explicitly
and it was the estuaries in southern California. And essentially
they took the biomass of all of the free living organisms,
and they extracted the parasites from them, and they took
the biomass of them. And in these systems, the main

(11:51):
predators are predatory birds. They eat the crustaceans, they eat
the fish. They're like the main predator out there. And
the biomass of trematode parasites was equal to and maybe
I think in some cases greater than the biomass of
the top predators in the system, which is to say,
greater than the biomass of the birds.

Speaker 2 (12:09):
Wow.

Speaker 1 (12:10):
And that's because this system has a lot of trematode
parasites that infect the snails, and when the snails get infected,
they get castrated and all of the energy that would
have gone to making baby snails now becomes parasite biomass.
And so the ground it's like a carpet of snails
in these estuaries. There's just loads and loads of them.

Speaker 2 (12:27):
I'm imagining stepping on them to the crunch, crunch, crunch.

Speaker 1 (12:30):
It's not crunch crunch, crunch, crunch, because it's very muddy
out there, so you're like squishing them underground, like they're
getting covered in mud. So you do feel bad about that.
I did feel bad. It was great exercise working in
this system though. This is what I did for my PhD,
because every step you take you need to like steal
your foot back from the mud. And anyway, good exercise.

(12:51):
Point is some systems have really really high parasite biomass.
But for example, if you were to collect a deer,
the biomass of parasites in a deer is probably a
very small percent.

Speaker 2 (13:04):
Like one percent.

Speaker 1 (13:05):
Yeah, I guess you know, probably like one percent. That's
where you came from.

Speaker 2 (13:08):
Day I tell what I meant. Yeah, yeah, let's see.

Speaker 1 (13:13):
Totally right. You hit the nail on the head. It
was perfect.

Speaker 2 (13:18):
If we keep asking questions, we'll eventually find one that
lines up in my answer.

Speaker 1 (13:21):
That's right, And we got there pretty quick. We got
there pretty quick. And and this biomass question that you
asked depends a lot on what the strategy of the
parasite is. And in our next segment we're going to
talk about different strategies. But basically, when the strategy involves
killing your host, you can take up more of that
host biomass because the host is going to die anyway.
But now I want to talk about you know, so
we've determined that maybe as much as fifty percent or

(13:43):
more of the number of species on the planet that
are multicellular are parasites. But how many times did you
get a transition from free living to a parasitic strategy?

Speaker 2 (13:54):
Right? Like? Did this evolve one time and now we
have a huge explosion to parasites or is it a
common thing that's been developed independently multiple times, like flight
for example.

Speaker 1 (14:04):
I mean, do you want to give a like, guess
at how many times parasitism has evolved.

Speaker 2 (14:09):
This time a good sport. I would guess that it's
evolved multiple times because it seems like kind of an
obvious strategy and it doesn't require like some huge technological
innovation like photosynthesis or the engineering a flight. Even so
I would guess it's pretty common. I don't know a
dozen times.

Speaker 1 (14:28):
When we had Joe Wolf on the show, she pointed
out that the crab body plan has popped up I
think eight times. I think you undersold the difficulty of
parasitism a little bit. So if you are trying to
infect a host, you need to find that host you
need to survive its immune system. You're now in a
completely foreign environment that is actively trying to kill you

(14:50):
all the time. But despite those challenges, we think that
parasitism has shown up at least two hundred and twenty
three times in Kingdom Animalia.

Speaker 2 (15:00):
Sorry, two hundred and twenty.

Speaker 1 (15:02):
Three times, two hundred and twenty three times.

Speaker 2 (15:05):
Like you can identify individual moments when something went from
non parasite to parasite in the evolutionary record hundreds of.

Speaker 1 (15:13):
Times, hundreds of times.

Speaker 2 (15:16):
Yes, wow, why aren't we all parasites?

Speaker 1 (15:19):
Well, you know, there's got to be some.

Speaker 2 (15:20):
Hosts, I see. For the grifters to keep drifting, we
need some suckers, that's what you're saying.

Speaker 1 (15:26):
And you and I are the suckers, I guess. But
so sometimes you get this transition to parasitism that results
in a lot of additional species. So I think when
you ask people about parasites, they usually think about things
like tapeworms and trematodes and nematodes and stuff like that.

Speaker 2 (15:43):
Unfortunately, that is my mental image, and that's why it
gives me the willies.

Speaker 1 (15:47):
Yeah, I'm sorry.

Speaker 2 (15:48):
It's intimate, though, it's.

Speaker 1 (15:50):
Very intimate So in phylum plati helminthis, so that's where
you get the trematodes and the tapeworms and stuff. They
were about transitions to parasitism last time we counted. But
in arthropodas, so these are invertebrates with exoskeletons. So we're
thinking about like wasps and crustaceans and stuff like that.

(16:12):
It evolved one hundred and forty three times. Wow, So
lots of transitions and lots of times it happened in insects,
and it happened sixty times in flies. So flies you
often get a transition from free living two parasitic strategies.

Speaker 2 (16:28):
How do we trace these transitions? Like I understand we
can look at the fossil record and see how whales
return to the ocean this kind of stuff. But insects,
we don't have fossils. How do we know this history.

Speaker 1 (16:39):
We do have some insect fossils. Often they get trapped
in things like amber, and amber actually does a really
great job of preserving things and making it so you
can still see their features. Yeah, but we probably are
undercounting because there probably are a lot of like transitions
that happened and then we're lost those species went extinct,
or a lot of just instances where there was like
a free living host that we'd just never found. And

(17:01):
so these you should imagine that all of these estimates
are very likely to change over time. But we have
been lucky to get a fair number of fossils in
the record, and the fossils are usually not beautiful exactly
what you want fossils. But for example, there's a lot
of tapeworms and monogeneans. So these are monogeneans are parasites

(17:21):
that you often find on the outside of fish. They
have these hooks that they use to hold onto the
fish and then they suck like the mucus. Sometimes they
suck blood, but they live on the outside.

Speaker 2 (17:32):
The less intimate side, we'll say.

Speaker 1 (17:34):
Yeah, slightly less intimate, but no less durable, and so
they use these hooks. And then tapeworms also tend to
have these hooks and they use them to hold on inside.
So now we've got intimate and durable. But these hooks
have very distinctive shapes, and sometimes we can find in
fossilized animal poops called coprolites, which we've talked about before,

(17:57):
these hooks in like the pairings that you would expect
and in the shapes that you would expect and inside
of or of, like very adjacent to organisms that you
would expect to find them in. And we found evidence wow,
of these sorts of relationships from the Mesozoic area, which
was sixty six to about two hundred and fifty million
years ago. Wow. This includes like the Jurassic period where

(18:18):
you know Jurassic Park and the Paleozoic period, which is
even later. I think Paleozoic means something like old life.
And we found some evidence of these parasites from back
then too, so it's totally reasonable to assume that like
t Rex was probably carrying around some nematodes. These are
ancient interactions.

Speaker 2 (18:36):
I love thinking about when things arose because it's a
way to figure out, like how hard is it to happen?
And also is it like that happen on alien planets?
Like we think about how quickly did we get life
after there were the conditions for life on Earth? Pretty quickly?
How quickly did we get intelligence after they were their
conditions for intelligence? Not that quickly? And it's only end

(18:56):
equals one, but it's like a hint as to like
is this likely to happen elsewhere, And so how far
back is the evidence of parasitism? How quickly did people
start to slurp and munch intimately on each other?

Speaker 1 (19:08):
Well, so we are mostly focusing on multicellular organisms going
after other multicellular organisms. I think it would be really
hard to know if there was like a bacterial parasite
going after other organisms in the like primordial soup era
of life.

Speaker 2 (19:26):
I guess bacteria phages could be considered parasites and viruses
that infect.

Speaker 1 (19:29):
Bacteria, right, Yeah, I meant conferences. We like to joke
that the first organism was free living, in the second
one found some way to exploit it. But you know,
we don't know that for sure.

Speaker 2 (19:39):
This economy that's just happening.

Speaker 1 (19:40):
Yeh, that's right, that's right. And so you know, I
think pretty early on, when you start getting multicellular organisms,
you get parasites also.

Speaker 2 (19:48):
And remind us how early did we get multicellular organisms
For those of us who are not evolutionary.

Speaker 1 (19:53):
Biologists, So multicellular life is probably something like seven hundred
million years ago, and I think we have decent evidence
of parasites in multi cellular organisms from at least two
hundred and fifty million years ago, and I'm guessing that
before that it gets even harder to find evidence of parasites,

(20:13):
and you have to be super careful that you're not
tricking yourself. So, for example, there are these isopods that
attached to fish, and sometimes they'll like live on top
of their gill arches, so like underneath their gill flap,
and then other times they'll live on their tongue. And
so if you find an isopod next to a fish,
you might be like, oh, there's a parasitic isopod, But
how do you know that it wasn't just like fossilized

(20:35):
next to the fish. How do you know it was
actually a parasitic relationship. So this stuff can be really difficult.

Speaker 2 (20:40):
These glimpses into deep time are super fascinating. One of
my favorite things in science. You know, to think that
like Earth had life on it for billions of years
before it had multicellular life. You know, like so many years,
so many seasons, so many days past with just like
frothing bacteria and stuff floating in the oceans. Now really interesting,

(21:00):
which is also kind of depressing because it means like
if you find a planet with life on it. There's
a good chance it's just a bunch of single cell
organisms nobody really to talk to. It's incredible how rapidly
things have changed. Like seven hundred million years ago, we
get multi cellular life and then you know, things explode.
And only recently have we had intelligence and technology and podcasts.

Speaker 1 (21:22):
Why I promise to not rat you out because you
essentially said there's just bacteria and that's not interesting. I
promise I won't tell Katrina that you said that, but
I agree, Well, this is.

Speaker 2 (21:33):
Our way to find out if she listens to the
podcast or not. If I get in trouble over.

Speaker 1 (21:36):
That, that's all right, Well, keep me posted, all right.
So when we come back, let's talk about categories that
parasites fall into. So we were talking about how do
you define and categorize these things. We'll talk about the
categories that we think make the most sense.

Speaker 2 (22:11):
All right, we are back, and we are talking about
a very intimate topic today. We're talking about the incredible
community of folks that live off of you, that you
are supporting your parasites and the parasites of everybody else
on earth. We're digging deep into the history to understand
how often this happens, how it happens, why it happens,
and what is the future of parasitism. So let's keep

(22:34):
digging into the past. Kelly tell us about like how
these things become parasites, How does the species decide from
being a maker to becoming a taker?

Speaker 1 (22:43):
I love that phrasing. And the answer is really like,
we don't have a great handle on this. So you know,
we talked about how this transition has happened probably more
than two hundred and twenty three times, and there's this
long running belief in the community that the what usually
what happens is that an organism starts specializing on dead stuff.
So you go from a decomposer to a parasite. And

(23:06):
the idea is you get good at living off of
flesh when it's dead, and that sort of lowers the
threshold to becoming a parasite and living on that thing
and taking its energy while it's still alive.

Speaker 2 (23:18):
So first you identify like, hey, these corpses are rich
in nutrients, I might as well eat them, and then
you start eating the living versions of them.

Speaker 1 (23:26):
Yeah, And so my friend Dan Metz and I wrote
a paper. He was leading this paper I was just
on the support team, but we wanted to find like
the papers that showed this, because this is a thing
that you can see found in the literature for like
one hundred years or more, but when you look into
the literature, it actually hasn't been tested that often. So
I think a lot of us have this gut feeling

(23:48):
that you know, being a decomposer is the step before
becoming a parasite, But folks really haven't done the nice
kind of analyzes you need. Where you'd need a tree
of life. You'd need to know who the free living
organisms and the parasites are are, and you'd need to
know what that free living organism was doing before the
transition to parasitism, and then look to see if certain
kinds of free living organisms were more likely to transition

(24:10):
to parasites. And you need some control groups too, and
that requires a lot of natural history data. And so
you and I have talked in the past about, you know,
the value of taxonomy and the value of going out
and collecting information, and I think it's critical because a
lot of times we'll want to do these sorts of
analyzes and then you'll get your tree of life together
and you'll be like, oh, we have no idea what
any of these pieces are doing in the wild, and

(24:32):
so you can't ask these questions that really help us understand,
you know, the paths that life take. You need that
natural history data.

Speaker 2 (24:40):
And you always need data, right, especially to check your intuitions. Yeah,
because you know, science is a story and we're putting
together a history of what happened, and when we don't know,
we're inclined to like fill in with our intuition, say well,
here's an idea. Maybe this is reasonable, and if it
sounds natural to us, we accept it. We're like, okay, yeah,
I believe that. Yeah, but sometimes the you verse is

(25:00):
not intuitive. So you always got to go out there
and check, especially when you're telling a reasonable sounding story
that doesn't have any actual data to support it. So
more science, people.

Speaker 1 (25:10):
Right, So much more science, Yes, so much more science. Okay.
So we were talking a little bit earlier about why
you would want to try to create boxes to describe
what parasites are doing, what different animals are doing, and
I was arguing that you need those boxes so that
you can create models to help understand what's happening. And
there's a couple of different folks who have come up
with categories that they think keep popping up as strategies

(25:33):
that parasites take. And I saw a quote in one
of these papers that made me think, oh, man, this
is for Daniel uh oh, so here it is. This
is from Poola twenty eleven. It seems safe to speculate
that if life exists on other planets, as long as
alien ecosystems show some basic similarities to ours, then parasitism

(25:54):
will not only have evolved, but alien parasites would be
using exploitation strategies very close to the ones to find.

Speaker 2 (26:01):
Here well parasites in the universe.

Speaker 1 (26:07):
So he was really going out on a limb here,
very strongly worded. He believes these categories explained like truths
about paths taken by life.

Speaker 2 (26:16):
I think it's fascinating when scientists do that. When they
find something here, they understand it, and then they say
this must be universal. We found it here, and therefore
it's everywhere. It seems like it could be the only way,
And that's so often just reveals like a lack of
the broader context, like you're within a certain set of
box of ways of thinking about the universe and it's

(26:36):
hard to imagine. Outside that box doesn't mean that outside
the box doesn't exist. And like, as I was just saying,
like this is why we got to go and find
alien parasites and blow earthling parasitologists minds about ways that
you can suck on other people. Right, there may be
even more intimate ways to be a parasite we never
even imagined, totally true, as you know. And that's why
I wrote this whole book about how aliens might do science,

(26:59):
trying to think outside the box.

Speaker 1 (27:01):
Daniel, what is that book called.

Speaker 2 (27:03):
It's called Do Aliens Speak Physics? And it's for sale
in November. Check it out at.

Speaker 1 (27:07):
All fine bookstores. Yeah, no, it is a fantastic book.
I got to read it.

Speaker 2 (27:11):
So I love that this parasitologist is thinking about aliens.
But I'm a little skeptical that our earthly categories of
parasites are the only ones in the universe. But anyway,
walk us through the kinds of parasites we have found
here on Earth, and everybody out there, if you're eating dinner,
maybe finish up before we dig into all of these
intimate details.

Speaker 1 (27:31):
Okay, So, one of the exciting things about these strategies
is that organisms from very different locations on the tree
of life seem to have ended up with very similar strategies,
which suggests that there's something critical about these boxes that
we're creating.

Speaker 2 (27:47):
Yeah.

Speaker 1 (27:47):
So one of the strategies, which we've already talked about,
is parasitoids. So the idea here is that you infect
one host species, you use all of its energy. You
have to kill it as part of your life cycle,
and so we get parasit wasps that do this. We
get nomatomorphs. So these are sort of nematode looking things
that live in cricket So have you ever seen that

(28:08):
video of a cricket that jumps into a pool and
a worm that's like ten times the length of the cricket,
maybe even more, glides out of its back end.

Speaker 2 (28:17):
Mm hmmm.

Speaker 1 (28:18):
Nightmare fuel, Yes, absolute nightmare fuel.

Speaker 2 (28:20):
There's one with the praying mantis also, I think yeah, yeah.

Speaker 1 (28:23):
Yeah, yeah. So sometimes those animals survive actually after they
jump into the water, which blows my mind because you
would think something that big comes out of you, you're
a goner.

Speaker 2 (28:31):
Yeah, I think that some mornings. Yes, oh god, I
think there's an intimate detail for everybody.

Speaker 1 (28:41):
That's right intimate, I hope not durable. I hope we
can all forget that. Okay, So you get those like
wormy things that eat almost everything inside of the organism,
and it's kind of amazing because they've managed to take
everything from the organism without killing it, so they're able
to like avoid the critical organs, but all the like
fat has been slurped out. Anything that's not absolutely critical

(29:04):
has been slurped out and turned into parasite. And those
zombie ants that you see with the Cordyceps fungus, they
also are exhibiting a parasitoid strategy. They kill the ant
and then they use all of that energy to make
their own babies.

Speaker 2 (29:16):
So explain to me the word here parasitoid, because as
a non expert, I hear that, and I think that's
sort of like a quasi parasite or a semi parasite
or something oid. Makes me think, like, not quite, what
does the word mean parasitoid?

Speaker 1 (29:31):
This is a great question. I think the original phrase
came from a German scientist, and that doesn't necessarily mean that,
like their prefixes and suffixes meant something different. You're right,
I'm looking up Merriam Webster and the first definition of
parasitoid is resembling a parasite, which must be the oid
thing the.

Speaker 2 (29:50):
Way that like our jokes resemble humor but aren't always
right on the money. You know, it's humoroid.

Speaker 1 (29:55):
That's right, that's right, we're humoroid kind of podcasts, which
sounds maybe too close to hemorrhoid.

Speaker 2 (30:01):
But uh so a lot of intimate jokes today, yes.

Speaker 1 (30:05):
Right, so many, my guess would be and I this
is total spitball. Okay, So there are no parasitoids that
infect humans, and so I think that when we think
of parasites, we think of things that are like living
in your guts or you know, like we think of
the kind of parasites that we have. And so it's
possible that when parasitoid was first named, they were like, oh, like,

(30:28):
it's living in another thing. We're familiar with that. But
then it like kills them and bursts out of them,
and it's this is something different. So let's call it parasitoid.
And I'm totally spitball. Like you have asked me yet
another question. I don't know the answer too, but I'm
gonna double down on my answer.

Speaker 2 (30:43):
All right, So these guys don't live in humans. They
infect their host, slurp up all the in crucial bits,
and then eventually kill them.

Speaker 1 (30:51):
Yep, that's a strategy, all right.

Speaker 2 (30:53):
And so if that wasn't gross enough for you, let's
hear about the next category, which I shiver to even say.
Parasitic castrators.

Speaker 1 (31:02):
Yes, lovely. So they don't kill the hosts, but they
do reduce their evolutionary fitness to zero. So essentially they
make it so that that organism will never be able
to pass down its genes again. It won't be able
to have any more of its babies.

Speaker 2 (31:18):
Wow.

Speaker 1 (31:19):
But now the parasites get to.

Speaker 2 (31:20):
Live cute little parasite babies. You know.

Speaker 1 (31:25):
The parasite studied for my PhD has these two little
eye spots, and it has like a little suckery kind
of mouth that opens and closes. And by the end
of my very long PhD it took a really long
time to finish, I kind of thought that they were cute.
So I am capable of thinking some parasites are cute.

Speaker 2 (31:42):
They've infected your brain. See, they're working their magic on you.

Speaker 1 (31:45):
That's right, that's right.

Speaker 2 (31:46):
So tell us about what these guys do.

Speaker 1 (31:48):
All right, So at the top of the show, we
talked about those snails where a bunch of the biomass
of those snails is actually going towards the production of
parasite babies. That's an example of castration. All of the
energy that would have gone TODs making snail babies is
now making parasite babies. And those snails can live for
like I think it's like a decade after getting infected.
And sometimes when the tide comes in on a warm day,

(32:09):
they can release up to two thousand of those parasites
into the water column. And so we think of them
as essentially like extended versions of parasites. At that point,
they're not working towards their own evolutionary goals anymore. Also,
side note, the parasites that live in the snails have
social behaviors, Like do you know how ant communities have

(32:30):
different jobs for the different ants, and the ants have
different shapes. Yeah, you've got like a million mile stare
right now? Is this too much?

Speaker 2 (32:38):
No, I'm thinking about ants?

Speaker 1 (32:39):
Oh all right, okay, I was thinking.

Speaker 2 (32:41):
About that time they poured aluminum into an ant farm.
I remember, like the shape of it.

Speaker 1 (32:45):
That's a mean but informative way of getting that information. Okay,
So ants have individuals who have different jobs, and they
have different shapes, and they have some individuals who never
have their own offspring. Their job is just to support
the colony. The same thing is happening inside of these snails.
So there are attacker morphs that are essentially just like

(33:08):
giant mouths that go around and when a parasite tries
to invade the snail, because sometimes a new parasite species
will come in invade a snail that already has parasites
living in there, and it could kick out the parasites
that are living in there and then take over the gonad.

Speaker 2 (33:22):
So it's like parasite versus parasite battle inside the snail.

Speaker 1 (33:25):
Yes, wow, yes, And some kinds of trematode parasites have
essentially these mouths that never reproduce on their own. Their
whole job is to just patrol the snail, and if
a parasite comes in, they go and they suck a
hole into the side of the tree with toad and
like suck out the insides and kill it that way.

Speaker 2 (33:43):
Wow, this is like gang violenceers. This is our territory.

Speaker 1 (33:46):
Yes, yeah, they have like more interesting social lives than
I do. They're fascinating.

Speaker 2 (33:50):
But how high bar is that?

Speaker 1 (33:52):
Kelly tell me, Oh my god, it's like it's like
on the ground. You don't have to lift your foot
up much at all. I don't go out much. But anyway, okay,
parasitic castrators, trematodes. Let's do one other example. There are
barnacles and so like if you've been to the ocean,
you've probably seen barnacles. There are barnacles that stick to
the sides of like whales and turtles. There are also

(34:12):
barnacles and isopods. These are these little crustacean things that
are both able to castrate crustaceans like crabs. So the
barnacle in particular does this very interesting thing where it
has a sack where female crabs would usually have their babies,
and it sets up shop there, and females usually will

(34:33):
use their claws, so this is on the underside of
what you'd think of as like the crab's belly. They
use their claws to like pass water over it to
make sure that their eggs stay oxygenated. They will do
that when the barnacles down there to keep the barnacle oxygenated,
and males will start doing that too, So it looks
like not only do they castrate male and female crabs,
but they get the males to start acting like females

(34:55):
to take care of the barnacles. We call this feminization,
and it's kind of poorly understood, but anyway, So barnacles
are castrating crabs, trematodes are castrating snails, and there's insects
castrating other insects. This is another very common strategy.

Speaker 2 (35:12):
You just said the word castrating way too many times
from that country. Let's move on.

Speaker 1 (35:18):
There's a total of six categories, but rather than making
you all sit through all six, let's pick one more
to give you a flavor of the amazing strategies that
parasites have taken on.

Speaker 2 (35:30):
Let's do vector transmitted parasites. Because that sounds physics, y vectors.

Speaker 1 (35:35):
Ooh, all right, Well you can try to connect this
with physics at the end, we'll see. But so, what
we mean by vector transmitted is that you have one
animal that is bringing a parasite to another animal and
essentially injecting it into that other animal. Probably the most
famous vector transmitted parasite is malaria. So mosquitoes will take

(35:56):
a blood meal from a person who has malaria, they
suck the parasite up. A parasite does some developing inside
of the mosquito and eventually goes to live in the
mosquito's salivary glands. And then when a mosquito goes to
take a blood meal from another human who isn't infected yet,
the parasite ends up in that person's blood, and then
that person gets infected and the cycle starts again.

Speaker 2 (36:16):
I think it's fascinating. You call this a blood meal.
Makes it sound so wholesome, you know, like it's just
going home to have dinner after a long day of work.
You know it deserves it, But like, this thing is
sucking my blood, you know.

Speaker 1 (36:28):
I've read stories from the early nineteen hundreds in the
Journal of Parasitology where they sort of mentioned in an
offhanded way that like the mosquitoes and the lice in
the colonies were fed blood meals by volunteers, which means
somebody was like sticking their arms in the mosquito cage.
Anna Phillips was telling us in the Leech episode that
now you can like fill medical gloves up with blood

(36:49):
from you know, a hospital or something and feeds them
that way. But I love the faces you make when
we talk about parasites. It's so entertaining for me. But Mike,
you know I if I I tell you my big
goal here, I feel like when you start learning about parasites,
you can't get over the disgust hump Ye. It's just
like so upsetting. But the more you learn about it.

(37:10):
This doesn't happen for everybody, but maybe you'll get there.
I feel like the more you learn about it, the
more kind of amazing it is that evolution has resulted
in this super complicated strategy. Like I mean, imagine being
able to live in environments as different as a mosquito
and a human, like to conquer both of those hosts
in a way that like human ingenuity has just not

(37:32):
been able to outsmart. There's good evidence that malaria is
able to essentially like change the proteins on the outside
so that our immune system can't recognize it. Like we
start to get a handle on it, and then it
shows up again in like a whole different outfit, and
our body is like, where did the malaria go? But
it's still there and it's like racing us and you
know we have We've created all of these vaccines that

(37:53):
keep you know, not quite keeping up. These are kind
of incredible creatures. You can hate them, and I wish
there was no malaria because I don't want to see
any child with malaria ever again. But you kind of
have to respect sort of the amazing strategies that they've
been able to come up with with the help of
natural selection.

Speaker 2 (38:10):
Yeah, I totally appreciate you leaning into the wonders of
the universe, and you know, that's our whole brand, and
the universe is extraordinary and these things are incredible. And
I guess I'll try to think about that next time
I see a mosquito biting Meia be like, wow, think
about all the amazing science going into this itch that
I'm going to be suffering for the next three days.

Speaker 1 (38:29):
Well, that's a micro predator, Daniel, I'm not interested, just kidding.

Speaker 2 (38:36):
All right, So let's take another break and we come back.
Let's hear some good news about how parasites can go extinct. Okay,

(39:03):
we're back, and Kelly is grossing me out with intimate
details of how parasites infect their hosts, castrate them, and
pass on diseases. Yay, we're all supposed to think. Yay.
So let's turn to some good news. We heard about
how parasites have evolved many many times in the record.
Do parasites also go extinct? We have evidence for parasites disappearing.

(39:25):
How does that happen?

Speaker 1 (39:26):
They do go extinct, and so, you know, we talked
about how parasitism as a strategy popped up, you know,
over two hundred times in the tree of life. It
appears it's also been lost a few times.

Speaker 2 (39:36):
Ah, I know.

Speaker 1 (39:38):
So sometimes it's hard to get a handle on those
losses because if an animal loses it and then doesn't
end up in the fossil record, it can just be
kind of complicated. But yeah, parasites go extinct, and in fact,
some parasites are more susceptible to extinction than their hosts.
And I know that's not sad for you, but we're
losing diversity. So one parasite strategy that we didn't talk

(39:58):
about in the last section are trophically transmitted parasites.

Speaker 2 (40:02):
What does trophic mean?

Speaker 1 (40:03):
So, like, think about trophic levels in a food web,
and so trophic transmission is essentially a predator consumes an
animal that was infected, and the parasite gets into the predator.
So the trophically transmitted system I studied for my PhD
was California killifish. There are these small fish in southern
California estuaries and they get eaten by predatory birds like egrets,

(40:26):
and the parasite goes from living on top of the
fish's brain to living in the egrets gut.

Speaker 2 (40:30):
I wouldn't want to live in either of those places.
I mean, you're in southern California. There's lots of beautiful
places to live. Why would you choose a fish brain?
But hey, you do you?

Speaker 1 (40:39):
I mean, I'd rather live in Virginia. But that's all right.
So everyone knew that was coming. But this parasite actually
has an even more complicated life cycle. So parasites can
have different strategies depending on where they are in the
life cycle. So there you've got trophic transmission from fish
to bird. The parasites then find love in the bird's
gut and they may eggs that pass with the bird

(41:02):
species into the salt marsh, where they're accidentally consumed by
those snails that we talked about that get castrated and
then this is.

Speaker 2 (41:10):
Such a horror movie. This whole thing is a horror movie.

Speaker 1 (41:13):
I love it, I eat it up. My mom loved
horror movies. Maybe that's what's set me up for all
of this.

Speaker 2 (41:17):
It's her fault.

Speaker 1 (41:18):
Yes, yeah, yeah. So anyway, in order for this parasite,
this trematode parasite, to complete its life cycle, you need
to have the fish, you need to have the birds,
and you need to have the snail. If you lose
any of those hosts in the cycle, or any of
them falls to numbers that are small enough where it's
hard for the parasite to find them, then that animal
can go extinct. And so parasites have all of these

(41:40):
extra requirements that depend on what's happening with their hosts,
and so in some cases we think that parasites might
be more susceptible to extinction than a lot of free
living animals because there's so many different components that need
to work well for the parasite to complete the life cycle.

Speaker 2 (41:54):
It's kind of a fragile system, a bit of.

Speaker 1 (41:56):
A fragile system, and so parasites can be used as
what we call ecological indicator. So if you go to
an estuary and you find a lot of parasites, that
probably means a lot of host species are there, and
they're there in pretty high numbers, and so a lot
of parasites can be indicative of a healthy ecosystem, which
I think is counterintuitive to some people but makes perfect
sense to my parasitologists heart.

Speaker 2 (42:18):
Like if you go to a new neighborhood and you
discover there's lots of gangs there, you're like, oh, wow,
this must have a thriving economy for the gangs to
be siphoning money off of.

Speaker 1 (42:30):
Wow, where do I go with that? I mean, I
guess it means that the police aren't really doing their job,
which maybe means there's immune systems and hosts that are compromised,
perhaps by pollutants. But maybe we're pulling this analogy a
little bit too tight, so let's move on.

Speaker 2 (42:45):
All right. So they do go extinct, and maybe even
more often than their hosts because they rely on like
a complex system and getting passed from one to the other.
But then they're everywhere and they're very common and they've
evolved many many times. How do we understand that altogether?

Speaker 1 (42:58):
I mean, there's a lot of hosts all and you
can be losing a lot of hosts, but you still
look around and there's a lot of birds outside and stuff.
It's just, you know, hard to keep track of all
of this stuff. But if I can, just for one
second try to convince you that you should care about
parasites going extinct, and I won't.

Speaker 2 (43:12):
You can try.

Speaker 1 (43:13):
Yeah, So, parasites are important parts of ecosystem. So we
mentioned that those snails that are castrated can release up
to two thousand parasites into the water. Those parasites are
free swimming bags of energy that get eaten by a
lot of organisms, so they provide food to the system. Also,
you know, we talked about those crickets that have those

(43:34):
long worms in them, and the crickets are forced into
jumping into the river and then the worms, the nomatomorphs,
come out the back of them. Those crickets are terrestrial
organisms that jump into creeks, And in Japan there's this
endangered trout where a lot of the food that it
gets comes from crickets that are jumping into the creek
that they wouldn't have access to otherwise. And so these

(43:55):
parasites often make lots of food available to other animals
or make that food easy to catch. They play important
roles in ecosystems that we don't always understand very well,
and if you lose them that could be bad in
some cases.

Speaker 2 (44:08):
Yeah, I totally appreciate that. And it's sort of curious
to me, like, why do parasites have this ick factor
that like predators don't, you know, like a jaguar, it's
still like finding another critter and taking it and eating
its resources. Why is this somehow grosser to be like
living inside as you consume somebody for some reason, it
is definitely grosser though like morally it's not any better

(44:30):
or worse. So I don't know, there's just some sort
of like human ick factor there. And maybe when we
meet the aliens they'll be like, yay parasites, and Kelly
will be like, I found my people.

Speaker 1 (44:40):
That's right, that's right. I understand you guys, you no,
I agree. There's definitely a judgment called people think of
parasites as like these super degenerate organisms. They have fewer
sort of abilities to like sense the exterior environment, but
they do manage to thrive in really complicated systems where
they're being attacked. And yeah, anyway, people do tend to
find parasites to be much gross to the lions. I

(45:01):
get that some people could look at a lion and
be like, that looks much nicer than a tapeworm. But
if you know what to look for in a tapeworm,
some of them are quite beautiful. I've got my tapeworm
picture right here.

Speaker 2 (45:12):
Oh wow. Do you know anybody who like has a
coat made out of tapeworms or something like that, You know, tapeworms.
Maybe we need a fashion industry, you know, to replace
fur with parasites somehow.

Speaker 1 (45:23):
I think that any parasite themed fashion that is available
is found at the American Society of Parasitologists Student Travel Auction.
We bring it in and we all pay way too
much for it to try to pay for student travel
so that they can come to our conferences. But anyway,
we're getting off track. We've got one more question from
the listener, and the question is do parasites go extinct

(45:43):
with hosts? So there's a number of different things that
can happen when hosts go extinct or when hosts sort
of split into multiple species, and so yes, often when
a host goes extinct, the parasite can go extinct. Also,
if the parasite was infecting lots of other host species,
then maybe it will be fine. But we already talked
about complex life cycles where if you lose one host

(46:03):
in the system, you can lose the parasite too. But
there's also examples where you get these splits in the
tree of life, and as you get those splits in
the hosts, the parasites also split into new species over
time because you now get like separations of parasite populations,
and over enough time they become their own species. And
so there's a sort of famous example where pelicans, as

(46:25):
they sort of speciated over time, they're lice speciated with them.
So pelicans, if you like create the tree of life
for pelicans and then you create the tree of life
for their lice, you could almost lay them on top
of each other. They follow very similar paths. But you
can also get cases where the parasites don't seem to
be tracking what's happening with their hosts. So, for example, guppies,

(46:47):
they are these these tiny little fish they get infected
by monogenians. These are these external parasites we were talking
about that have the hooks and they hold onto the
outside of the fish. They don't seem to be following
the evolutionary trajectory of their hosts. And right now we
don't have a really good framework for when you should
expect the tree of life for parasites and species to
essentially mirror one another, and when you shouldn't. But we

(47:10):
get all of these different options because nature is amazing
and it depends, but we don't.

Speaker 2 (47:15):
Always know on what, and the incredible variety of strategies
we see helps us explore like the space of possibilities
and maybe even helps us think outside the box to
like what might be happening elsewhere in the universe.

Speaker 1 (47:26):
Absolutely, yes, I would love to find a planet that
was rich with free living hosts and dissect a bunch
of them to see if they have parasites, but only
if it was humane and ethical.

Speaker 2 (47:40):
All right, aliensf you're listening, come on by. Kelly wants
to look inside you.

Speaker 1 (47:44):
Kelly wants to say huh.

Speaker 2 (47:46):
In an intimate sort of way.

Speaker 1 (47:48):
All right, Well, thank you so much to Zach for
sending in his questions. Let's see if Zach feels like
we've answered the many questions he posed about the evolution
of parasitism.

Speaker 3 (47:57):
Hey, they're extraordinaries. Thank you so much for this credible
primer on parasites and their history. I feel like I
got way more than I bargained for, which is awesome.
You definitely answered my essential question that, yes, t Rex
probably had tapeworms or something like it, that we can't
be totally sure. I also really enjoyed hearing about all
the different categories of parasite strategies, and really appreciated hearing

(48:21):
that there are no parasitoids or castrating parasites for humans,
so I can sleep a little better at night. I
was also definitely convinced that parasites have an important role
to play in our ecosystems, so I'll try to be
a little bit more appreciative and a little less grossed out.
I am still a little curious. Seems like it's still
an open question how something goes from being a free

(48:42):
living organism to being a parasite, even if we can
count when it happens. I'm also curious if anything has
ever gone from being a parasite back to being a
free living organism. In any case, you guys rock. Thank
you again so much.

Speaker 1 (48:57):
All right, thank you so much to Zach for all
of his many questions. If you would like to send
us questions, we would love to hear from you. Send
them to questions at danieland Kelly dot org. And I
hope that you enjoy your meal or you ate it
before we began this conversation.

Speaker 2 (49:11):
And before you heard Kelly say the word castraid uncountable
number of times.

Speaker 1 (49:15):
I don't think there's a limit on how many times
you can say castrat. It's a great word.

Speaker 2 (49:20):
All right, everyone, thanks for joining us on this tour
of one more aspect of our extraordinary universe.

Speaker 1 (49:32):
Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We
would love to hear from you.

Speaker 2 (49:38):
We really would. We want to know what questions you
have about this extraordinary universe.

Speaker 1 (49:43):
We want to know your thoughts on recent shows, suggestions
for future shows. If you contact us, we will get
back to you.

Speaker 2 (49:50):
We really mean it. We answer every message. Email us
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Speaker 1 (49:56):
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Speaker 2 (50:06):
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