How Did the Smallest Primates Get Such Big Eyes?

Tarsiers are our palm-sized primate cousins that evolved in the forests of Southeast Asia to have long limbs and huge, round eyes. Learn how we've learned about them in today's episode of BrainStuff, based on this article: https://animals.howstuffworks.com/mammals/tarsier.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff, a production of iHeartRadio, Hey brain
Stuff Lauren bogelban here. The unique characteristics of tarsiers have
caused people to call these tiny primates many names, from
goblins to gremlins to grogu. To be fair, they do

(00:22):
have prominent and sometimes pointed ears and an uncanny eyeballed
ahead ratio for a creature that's so closely related to us.
The tarsier is a small type of primate found living
in the forests of Southeast Asia and the Pacific Islands.
The taxonomical tarsiar family contains three genera, some sixteen species

(00:42):
and seven subspecies depending on who you ask. It's an
ongoing field of research, and I'm pretty sure biologists enjoy
arguing about taxonomy. But whatever their exact classification, tarsiers all
occupy the dense forests of Malaysia, Indonesia, Brunei, and the Philippines,
though every species has different characteristics. Tarsiers tend to range

(01:05):
in side from around three and a half to six
inches long that's about nine to fifteen centimeters, making them
among the smallest known primates. Like owls, they can rotate
their head nearly one hundred and eighty degrees in both directions.
Tarsiers have round heads with a small snout and large
ears that look a bit like muscleshells. They also have round,

(01:28):
furry bodies. Their thick, short fur can range in color
from gray to black or tan to yellow, orange to
red to brown, or some combination. They have long arms
and legs. Their ankle bones, or tarsals, are particularly long.
That's where they get their name. Those long legs and
ankles allow them to jump and leap like lemurs. Their

(01:52):
fingers are also exceptionally long, with small sticky pads at
the ends to help them cling to trees where they
live and hunt for prey. Their tails can be twice
their body length. Some species of tarsiers have hairless, a
rat like tails, while others have tufts of fur along
the length of the tail or at the end. Altogether,

(02:12):
they look sort of like someone stuck frog legs on
a highly caffeinated teddy bear. Tarsiers like hanging out in shrubs, trees,
and other vegetation above ground level. This vantage point allows
them to rest more at ease from predators and also
to hunt for their small prey. They are the planet's
only primate species that is entirely carnivorous, but within that category,

(02:37):
they eat whatever their forests have to offer, including insects, frogs, crabs, lizards, birds, bats,
and snakes, and different species are variably social, but they
tend to form small groups that gather to share food,
protect each other from predators, play, groom, snuggle, and mate.
They communicate with calls that are outside the range of

(02:58):
human hearing. But okay, we have to talk about what
large eyes they have. Even four nocturnal mammals, their eyes
are enormous compared to their body size. They're the biggest
eyes of any mammal relative to their body weight. Their
eyes are very round in shape and often a striking
gold color. A tarsiar's lack a tpedum lucidum, which we've

(03:21):
talked about before on the show. It's a reflective layer
inside the eye behind the retina that most other animals
have if they are active at night. The tpedum lucidum
is important to nocturnal vision because it bounces light back
at the retina, giving the eye a second chance to
gather that light. For the article of this episode is
based on how Stuff Works. Spoke with Myron Shekel PhD,

(03:45):
a biological anthropologist imprimatologist with Western Washington University. He said,
one of the things we know is when you take
your flashlight and you shine it into a forest at night,
lots of animals have a very strong eye shine that
comes to right back at you. Cats and dogs, lemurs,
bush babies, and lorises. Tarsiers don't have that. Humans, monkeys

(04:08):
and apes don't have that. So the tarsier I had
to compensate and became a lot bigger than you'd expect.
Shikehl has conducted decades of research contributing to what we
know about tarsiers and about primate evolution in general, he said.
When I handed in my first grant proposal, which would
have been in nineteen ninety three nineteen ninety four, one

(04:30):
of the reviewers commented, Shakelle has outlined a career's worth
of research, and now I'm just about to the point
where I can publish the answer to the questions I
posed in that research grant proposal all those years ago.
It's exactly the tarsiers lack of this reflective layer that
offers an exciting insight into primate evolution. When primates became diurnal,

(04:55):
a meaning active during the daytime, they eventually lost the
need for a pedem lucidam, Schakel said, So it looks
like tarsiers spent some amount of time in their history
as a diurnal primate. If they then reversed and became
nocturnal again, they had perhaps already lost that topedum lucidum.

(05:17):
Since the first scientific description of tarsiers was published around
seventeen seventy seven, scientists have debated where tarsiers belong on
the primate evolutionary tree. They jump, climb, and live like lemurs,
but have physical characteristics similar to apes and humans, like
that lack of a topedum lucidum and the fact that

(05:38):
they have a dry nose instead of a wet one,
which is one of the main ways that researchers categorize primates.
Chakel said, At the level of genomic data, where we
can look at really major changes, the evidence is becoming
clearer that tarsiers are our cousins. However, tarsiers are notoriously
elusive and difficult to study, especially without the dead giveaway

(06:01):
eyeshine that other nocturnal species have. Chakel and his colleagues
identified several new species of tarsiers on the Indonesian island
of Sulawesi, where there was long thought to be just
a single species. But unfortunately, tarsiers do poorly in captivity.
No group that's been brought into captivity for study or

(06:22):
tourism has survived. It's hard to replicate their prey rich
natural environment, and like other primates, they're easily stressed out.
Tarsier conservation is becoming an increasingly important topic, as some
species of tarsiers have made the endangered or Vulnerable species
list due to their limited population, a lack of protected areas,

(06:45):
and deforestation and mining of what territory they do inhabit.
Just for one example, take the main island of Sangi
Hey in Indonesia. Sangi He has an endemic species of tarsiers, endemic,
meaning that the species is only found there. The saying
gie Hey tarsier was recently added to the list of
twenty five most endangered primates in the world after the

(07:08):
island opened to gold mining. Because there are more tarsier
species than there are scientists studying them, it's difficult to
know precisely how endangered some species are across these many islands,
but hopefully some intentional conservation efforts will help bring species
back from the brink and prevent more species from becoming

(07:30):
critically endangered. It'll take a concerted effort and coordination with
local researchers and experts. Shaquel said, If we keep tarsiers
in their habitat like in Indonesia, and we simply throw
a cage around them and use somebody who actually knows
how to keep tarsiers alive, they stay alive and they breed.

(07:56):
Today's episode is based on the article the tarsier is
one weird primate and Yes We're related on how stuffworks
dot Com, written by Alison Trautner. Brain Stuff is production
of iHeartRadio in partnership with HowStuffWorks dot Com and is
produced by Tyler Klain. Four more podcasts from my heart Radio,
visit the iHeartRadio app, Apple Podcasts, or wherever you listen
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