The Tarsier Is One Weird Primate, and Yes, We're Related

To Myron Shekelle, Ph.D., an instructor and research associate in the biology department at Western Washington University, tarsiers are way more than their weird mashup of features; they are a fascinating example of primate evolution. Shekelle has conducted decades of research contributing to what we know about tarsiers and primate evolution.

"When I handed in my first grant proposal, which would have been in 1993, or 1994, one of the reviewers commented, 'Shekelle 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," he says.

Though every species has different characteristics, tarsiers tend to range in size from 3.5 to 6 inches long (8.9 to 15 centimeters), with tails that can be twice that length, making them among the smallest known primates. Some species of tarsiers have hairless rat-like tails, while others have tufts of fur along the length of the tail or at the end.

Tarsiers, like owls, can rotate their head 180 degrees and have round, furry bodies with very long legs. Their ankle bones (tarsals) are particularly long — that's where they get their name, tarsier. Their long legs and ankles allow them to jump and leap like lemurs. Their 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.

They are the planet's only primate species that is entirely carnivorous. They eat the best that the Southeast Asian forests offer, including insects, lizards and even snakes.

It's impossible to talk about tarsiers without also discussing what large eyes they have. Even for nocturnal mammals, their eyes are enormous compared to their body size — they're the biggest eyes of any mammal relative to their body weight. Tarsiers lack a tapetum lucidum, a reflective eye layer that most other nocturnal animals have.

"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 straight back at you. Cats and dogs, lemurs, bushbabies and lorises," Shekelle explains. "Tarsiers don't have that. [Humans], monkeys and apes don't have that."

The tapetum lucidum is important to nocturnal vision because it creates more chances for the retina to gather light. "So the tarsier eye had to compensate and became a lot bigger than you'd expect," Shekelle explains.

He says that the tarsier's lack of this reflective layer offers researchers an exciting insight into primate evolution. When primates became diurnal, meaning active during the daytime, they eventually lost the need for a tapetum lucidum.

"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 tapetum lucidum," Shekelle says.

In 1994, Shekelle and his colleagues identified several new species of tarsiers on the Indonesian island of Sulawesi, where there was thought to be just a single species. Since the first scientific description of tarsiers was published around 1777, scientists have debated where tarsiers belong on the primate evolutionary tree.

They jump, climb and live like lemurs. In contrast, they have similar characteristics to apes and humans, like their lack of a tapetum lucidum.

"At the level of genomic data, where we can look at really major changes, the evidence is becoming clearer that tarsiers are our cousins that are related to the monkeys, apes and humans, and they belong in a group we call Haplorhini," Shekelle explains.

Wait — tarsiers are our cousins? Based on the available data, Shekelle says, yes, that's what the data show.

Tarsiers are notoriously elusive and difficult to study, especially without the dead giveaway eye shine that other nocturnal species have. Don't bet on seeing them at the zoo, either, because they do poorly in captivity.

"Every single group that has been brought into captivity has seen its population go down either very quickly or less quickly, but they all die eventually," explains Shekelle. "I don't think that it has to be that way."

Tarsier conservation is becoming an increasingly important topic as some species of tarsiers have made the endangered or vulnerable species list due to their small size, limited population, lack of protected areas, and deforestation and mining. The island of Sangihe in Indonesia has an endemic species of tarsiers — the Sangihe tarsier (Tarsius sangirensis) — meaning they are only found there. The Sangihe tarsier was recently added to the list of 25 most endangered primates in the world after the island opened to gold mining.

Because there are more tarsier species than scientists studying them, Shekelle says it's difficult to know precisely how endangered some species are across these many islands. In fact, he says there's an even smaller island between Sangihe and Sulawesi called Siau Island. It's only about 38 square miles (100 square kilometers) and is home to a critically endangered species he wrote about in 2008 called Tarsius tumpara.

Shekelle hopes that more tarsier species won't become critically endangered with some intentional conservation efforts.

"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," says Shekelle, "they stay alive, and they breed."

It's a systematic process to get tarsiers into a properly controlled environment that requires time, expertise and financial investment.