This bizarre primate has a newly discovered digit

The aye-aye of Madagascar has a pseudo-thumb that may help it grip trees, showing there’s still much to be learned about anatomy.Tuesday, 22 October 2019

Aye-ayes have a mishmash of peculiar features, such as huge ears, bushy tails, rodent-like teeth, and a very long fourth finger used to pry grubs out of trees.
Aye-ayes have a mishmash of peculiar features, such as huge ears, bushy tails, rodent-like teeth, and a very long fourth finger used to pry grubs out of trees.
photo by Pete Oxford, Nat Geo Image Collection

Adam Hartstone-Rose studies the muscles of forearms, which are surprisingly intricate and easily overlooked. The delicate movements of our hands, for example—like the ability to play a Mozart piano concerto—are only possible thanks to these sinews.

But Hartstone-Rose doesn’t study only human forearms: he specialises in those of many primates, and comparing anatomical differences between species. When his lab at North Carolina State University happened across a dead aye-aye specimen, he was thrilled. “They have these famously strange hands and bizarre fingers,” he says—all the better for research.

The aye-aye (Daubentonia madagascariensis), if you’re unfamiliar, is one of nature’s more absurd creations. These house cat-size lemurs, native to Madagascar, have super-long, spindly third and fourth fingers that they use to tap on trees to find grubs. Their brains, the largest of any lemur relative to body mass, allow them to find the larvae’s tunnels. They then gnaw into the bark with rodent-like incisors, and remove the goods with their chopstick-like digits.

Examining the aye-aye specimen, Hartstone-Rose and colleagues began to trace the route of a muscle called the abductor pollicis longus down into the forearm. This is one that, in humans, extends the thumb away from the body, a motion called abduction. “It’s the muscle that allows you to hitchhike,” he says.

In most primates, it starts in the forearm and attaches to the base of the thumb. But in the aye-aye, part of it splits off, and connects with a bone called the radial sesamoid, which is usually quite small in other primates, but elongated in this endangered species.

This MRI image shows the various structures of the aye-aye’s hand. The pseudo-thumb is made up of the radial sesamoid bone, in red, and its cartilaginous extension called the prepollex, in pink. It’s controlled by the adductor pollicis longus, in purple, and two other muscles shown in light and dark green. Other tendons and muscles are shown in light blue.
This MRI image shows the various structures of the aye-aye’s hand. The pseudo-thumb is made up of the radial sesamoid bone, in red, and its cartilaginous extension called the prepollex, in pink. It’s controlled by the adductor pollicis longus, in purple, and two other muscles shown in light and dark green. Other tendons and muscles are shown in light blue.
photo by Courtesy Edwin Dickinson, NC State University

The bone is also topped, the team reported in a study published October 21 in the American Journal of Physical Anthropology, with an extension of cartilage. Further investigation revealed two other muscles are connected to the radial sesamoid, which allow the bone to move in a gripping motion. Hartstone-Rose and colleagues have named this a “pseudo-thumb,” and suggest that it functions as sixth digit to help the arboreal animals hold onto tree limbs.

“It’s really cool to find this kind of anatomy in a primate for the first time, especially a primate as weird as an aye-aye,” Hartstone-Rose says. Such studies of arm and hand anatomy, and the differences between lineages, could help better understand how these structures have evolved in different species, including humans.

Getting a grip

The team hypothesises that throughout evolution, the aye-aye lost some ability to grip due to the extreme specialisation of its other fingers. The aye-aye’s fourth finger accounts for more than two-thirds the length of its hand; if humans had such a digit, it would be nearly a foot long. Its third digit, which is primarily used for tapping, is very thin and has a wide range of motion, equipped with a unique ball-and-socket joint.

A close-up image shows the fleshy pad above the aye-aye's pseudo-thumb.
A close-up image shows the fleshy pad above the aye-aye's pseudo-thumb.
photo by Image by Adam Hartstone-Rose

The aye-aye’s first finger, or thumb, is not fully opposable like in some other primates; it rather sits in line with the other digits. Thus, the species may have evolved this pseudo-thumb to help it stay aloft, and perhaps even to pick up different items or foods.

A similar scenario likely happened with the giant panda, which is well known for having a sixth digit, also called a pseudo-thumb. The ancestor of these bears, like other ursines, have digits in a single line, allowing them to walk on the ground; opposable digits get in the way of such footwork.

But panda bears evolved to feed upon bamboo, even though they need to eat more than 12 hours each day to digest it. “Any self-respecting carnivore shouldn’t be digesting all that fibre,” Hartstone-Rose says, jokingly. More to the point, though, climbing bamboo trees is difficult without an opposable digit.

Enter the panda’s pseudo-thumb, which is also composed of an enlarged radial sesamoid and cartilaginous extension, and is controlled by the same three muscles as in the aye-aye.

This seems to be an example of convergent evolution, a process by which very distantly related species come to have similar bodily structures.

“The panda bear and the aye aye essentially have the same anatomy in the pseudo-thumb,” Hartstone-Rose says, “which is pretty neat.”

The team next plans to study how living aye-ayes and pandas use their special digits.

Dorothy Fragaszy, a primatologist and National Geographic Explorer who wasn’t involved in the paper, says she’ll be particularly interested to see how the aye aye uses this little nub; she imagines it could help them grip trees branches as well as potential food items.

What most fascinates her, she says, is that the pseudo-thumb has its own fleshy pad, visible in photographs. “Those clearly allows them to press or rub the [pseudo-thumb] bony projection against things they’re holding in their palm,” says Fragaszy, who is also an emeritus professor at the University of Georgia.

Anne-Claire Fabre, an evolutionary biologist at the Natural History Museum in London who also wasn’t involved in the paper, says she seen aye-ayes “maintaining food items in the palm of their hand while eating,” and this pseudo-thumb may help explain how they do that.

Distracting fingers

As to why the aye-aye’s extra thumb escaped attention, Hartstone-Rose can only speculate.

“The only thing I can figure is, the aye aye’s hand is so interesting, specifically the fingers, so weird, that people have never really noticed that there’s a bizarre thing happening with the other part of the hand,” he says.

The animals are also rare and in decline, perhaps another reason for lack of study. They are listed as endangered by the International Union for Conservation of Nature, and in decline due to hunting, logging, and habitat fragmentation in Madagascar. 

The finding is a good example of how much remains to be discovered in his field, as is true throughout biology.

Hartstone-Rose often gets the question, “Don’t we already know almost everything about anatomy?”

The answer, of course, is no. Just ask the aye-aye.

Read More