10 Wild Facts About the Aye-Aye, a Most Improbable Animal

Like all lemurs, aye-ayes are primates, in the same order as monkeys, apes and humans. But aye-ayes resemble something between a raccoon and a rat. They are small — about 12 to 16 inches (30-40 centimeters) in length and 5-6 pounds (2.3-2.7 kilograms) on average and have long, bushy tails (18-22 inches or 45-55 centimeters) that are larger than their bodies. Their big yellow-orange or sandy brown eyes give them a perpetual look of surprise. While somewhat small, they are also the world's largest nocturnal primate.

One of the aye-aye's most distinguishing features is its huge, triangular ears — the largest compared to body size of any other primate. These ears are made up of a network of ridges that fine-tune the aye-aye's hearing so it can hear the subtle movement of grubs and larvae lurking in the chambers of old, decaying trees.

Unlike other primates, aye-ayes have continuously growing incisors like those of a rodent (as opposed to the characteristic lemur toothcomb), according to McGrath. Because of this, they were originally classified as rodents. These ever-growing incisors help the aye-ayes chew through wood, bark and nuts — even concrete if they're bored. There's no concern if the teeth wear down or even break because they continue to grow throughout the aye-aye's life.

Aye-ayes have long fingers on each hand ending with long curled claws (unlike the characteristic primate fingernails). The fingers are so exceptionally long that, when uncurled, they take up about 41 percent of the total length of the aye-aye's forearm. When walking on the ground, aye-ayes raise their delicate, clawed fingers to protect them, which makes their gait appear weird and clumsy. They also have a recently discovered sixth finger, a so-called pseudothumb, on each wrist made of bone and cartilage that helps these lemurs climb, grasp and dangle from branches.

Perhaps the strangest of the aye-aye's unusual traits is the long, thin middle "tapping" finger on each of the front hands. This scrawny finger can rotate 360 degrees around the joint, much like a human's arm at the shoulder joint.

That middle "tapping" finger isn't just for show. The aye-ayes use it to tap and forage for food, a practice called percussive foraging. In fact, aye-ayes are the only primates known to do this, McGrath says. And, since there are no woodpeckers in Madagascar, they fill a specific ecological niche on the island. Aye-ayes practice this echolocation by using their tapping finger to rapidly tap (up to eight times per second) along a branch or trunk of a tree while positioning those large, sensitive ears to listen for the subtle auditory feedback that indicates grubs and larvae are inside. Then they dig into the hard layers of the wood with those ever-growing incisors and use that flexible tapping finger to reach in and hook unsuspecting bugs on their claw.

Even though aye-ayes are social at times, these lemurs are rather introverted. They prefer to spread out so they can cover more ground for foraging, and then reconnect later with their group. Females tend to get a bit feisty, though, if another female outside her family unit comes creeping into her area in search of food.

There was once a giant aye-aye, called the Daubentonia robusta, which lived on Madagascar within the last 1,000 years. This now-extinct lemur had massive, robust limb bones suggesting it weighed as much as 2.5 to 5 times that of the aye-aye, which translates to upward of 25 pounds (11.3 kilograms).

If their looks don't startle you, the sounds they make will. Aye-ayes scream when they become aggressive, whimper when competing with others over food, make a "tiss" noise when confronted with other lemurs, and yell "hai-hai" when attempting to flee from captors. This "hai-hai" noise is likely where they got their unusual name.

Natives of Madagascar believe aye-ayes are harbingers of bad luck and that if an aye-aye points its long, skinny finger at someone, they are marked for death. Others believe that aye-ayes can creep into homes and use their tapping finger to pluck out the hearts of humans.

But that reputation seems unfortunate. "They do not show aggression toward the animal care staff, and their high level of intelligence makes them excellent participants in positive reinforcement training for behaviors like voluntary ultrasounds or blood draws, which make veterinary care much easier on all parties involved," says McGrath.