Many who have wished for an especially long middle finger have directed their envy at the aye-aye, a nocturnal lemur.
Lemurs, including the aye-aye, are native only to the island of Madagascar. As on many islands, animals there have evolved to fill various ecological roles that are usually occupied by entirely different groups of animals. The absence of those other animals on a particular island means there are unusual opportunities to take advantage of resources.
In the case of the aye-aye, the resource they exploit is insect grubs inside trees. These primates tap on trees to locate this plentiful food source. They tap at around 8 times per second and listen to the echo to find hollow chambers, chew holes in the wood and reach in with the aforementioned long, skinny, and bony finger. This digit is unusual in having a ball-and-socket joint, much like a human shoulder, so it can swivel around to make extracting grubs easier.
It’s their long skinny fingers that allow these animals to fulfill a niche usually occupied by woodpeckers. The evolution of this percussive foraging behavior enabled them to feed successfully, but at the cost of their ability to grip branches and other objects with their hands. That trade-off led to strong evolutionary pressure for a new way to grip things, which is presumably why they evolved a sixth digit.
Scientists recently discovered that the aye-aye has another digit, which is called a pseudo-thumb. Biologists who study lemur anatomy were dissecting the forearm of an aye-aye to explore the musculature of that limb. They followed a tendon from the arm into the hand and discovered that it split in a Y shape. One part went to a finger, and the other branch led to a little projection, now understood to be an extra digit, next to that finger. It was the first time anyone had realized that the aye-aye had a sixth finger. The pseudo-thumb has both bony and cartilaginous sections. Further investigation revealed that there are three distinct muscles attached to this digit which give the aye-aye the ability to move it in multiple directions.
The evolution of extra digits (from the primitive condition of five digits that humans and many other animals have) is rare. The best-known example of an additional digit is the pseudo-thumb of the giant panda, which it uses to grip bamboo shoots. The additional digits of the aye-aye and the giant panda developed from the same bones of the wrist and the same muscles, too, even though they evolved independently. That makes these digits an example of convergent evolution — the evolution in distantly related organisms of traits that are similar in form and function that were not present in their most recent common ancestor.
The 2019 discovery of the aye-aye’s pseudo-thumb means they have six fingers, so we need not covet the long middle finger of this lemur. Your finger may not be as long relative to your body as an aye-aye’s — but at least you have a middle finger.
Karen B. London, Ph.D. is a Certified Applied Animal Behaviorist, Certified Professional Dog Trainer, and an author of six books on canine training and behavior, including her most recent, Treat Everyone Like a Dog: How a Dog Trainer’s World View Can Improve Your Life.