Exclusively located in the northeastern parts of Madagascar, aye-aye (Daubentonia madagascariensis) is a dark brown or black primate, distinguished by the bushy tail of a squirrel larger than its body, the ears resembling a bat and the oversized, slender middle finger. Although the species has been known to science since the mid-1800s, scientists have only now discovered that aye-aye has a sixth digit – a tiny pseudothumb that probably helps the creature squeeze branches.
Aye-aye uses its excessively long middle finger for food, usually insects that lie inside the hollow branches. The primate first uses sharp ears to listen to the activity of the sleepless hump, after which it uses rodent-like incisors to bite through the bark of the branch to make a hole for the middle finger, slender enough to run straight through the hollow branch of the tree. The finger has a ball-and-socket joint, similar to the human shoulder, which provides a fantastic degree of movement. At the tip of the finger, aye-aye has a hooked nail that can capture and pull prey.
Unfortunately, for our sympathetic breath, the people of the island do not think it overly lovely. His bizarre appearance terrifies the locals, and his cheerful invitation does not help too much in this regard. According to ancient Malagasy legends, aye-aye is considered a symbol of death. The natives believe that if the middle finger ever points to you, death will surely occur soon. For this reason, poor aye-ayezi are often killed out of sight by superstitious locals. Due to this persecution, as well as habitat loss, aye-aye has been listed as critically endangered, with experts estimating fewer than 1,000 individuals in the wild. Maybe that middle finger was called.
With new anatomical insights things are even more complex. According to a new study conducted by Adam Hartstone-Rose, a biologist at North Carolina State University, aye-aye actually has six digits, meaning no middle finger. Take it!
"Aye-aye has the craziest hand of any primate," says Adam Hartstone-Rose, associate professor of biological sciences at NC State. "Their fingers have evolved into extremely specialized ones – in fact, so specialized that they are not very helpful when it comes to moving through trees. When you watch them move, it looks like a strange spider-walking lemur."
The reason why he has not been found so far has to do with the anatomy of the pseudothum. The bone itself is very small, while the rest of the finger is mostly cartilage and muscle, which do not appear on the x-ray.
The researchers accidentally found a small thumb while using six ayeas to use digital dissection techniques. They noticed that one of the hand tendons detaches from the base of the thumb and moves toward a wrist joint called a radial sesamoid (people do not have it).
"Using these digital techniques allows us to visualize these structures in three dimensions and to understand the organization of the muscles that allow digit movement," says Dickinson, who built the digital model of anatomy and co-authored the paper's first author.
"Pseudothumb is definitely more than just a fixture," Hartstone-Rose says. "It also has a bony and cartilaginous extension and three different muscles that move it. The pseudothumb can move in space and stretch an amount of force equal to almost half the body weight. So that would be very helpful for the grip."
According to the researchers, gymnastics of the elongated middle finger does not allow the branches to hold properly. Like the panda, which also has a pseudothumb, Aye-aye probably uses this extra digit to capture branches. In addition to aye-aye and pandas, moles also have six digits that they use to move more dirt.
"Other species, such as panda bear, have developed the same extra digit to aid in capture, because the standard bear's paw is too generalized to allow the dexterity needed to catch it," says Hartstone-Rose. "Both moles and some extinct reptiles for swimming have added extra digits to extend the arm for more efficient digging or swimming. In this case, the aye-aye arm is so specialized in finding the extra mobility number.
"Some types of primates have reduced numbers to help them move. Aye-aye is the first primate to dial numbers instead of dialing them. And it's amazing that he's been there all along, in this weirdest of all primates, but so far no one noticed. "
The findings are described in a journal American Journal of Physical Anthropology.