Madagascar's aye-aye lemur is a delightful aberration of an animal.
He has huge ears, a bushy tail, mammary glands between his legs and white hairs that resist when aroused. Over a vampiric nose and a mouth of incisors that never stop growing, candy colored eyes glow like full moon eyes. Greedy fingers sticking in curled claws stab their hands. The middle finger, a spindle-shaped rod, turns on a ball joint. Sometimes an even longer fourth finger is flipped over.
If this was not enough to reach the odds for eccentric adjustments, scientists would simply find another ̵
"We've studied the function of these strange, spindle-shaped middle fingers for so long that no one has ever looked at such a low structure on the wrist," said Adam Hartstone-Rose, an anatomist at North Carolina State University, leading the team that led the has completed the research. "But this structure probably has more evolutionary significance than this strange, small, spindle-shaped finger."
Other animals also have additional attachments. But the replacement thumb of yes-yes described on Monday in the American Journal of Physical Anthropology may have evolved in circumstances that have not been observed in any other animal. The study shows that there is much more to learn about how the various vertebrate lines evolved – even though we looked at the anatomy that we thought we understood.
A century of study and numerous X-rays overlooked the pseudo-palate of the eye. Dr. Hartstone-Rose and his team found it accidental when they examined the forearm muscles of the eye. They followed a muscle that nestled for movement on the hands of all primates, but surprisingly found tiny tendons that also connected it with a cartilaginous stretch with the palm and a bulbous bone between the thumb and wrist.
"We found that there were those different muscles that controlled this whole thing," Dr. Hartstone Rose, allowing the structure to move like a thumb from the other fingers and into the palm of the hand.
They confirmed that this structure exists in seven in total aye-ayes, suggesting that it was part of the anatomy of the misfit mammal. And the reason seems to lie in the peculiar foraging.
At night, Aye-Aye hunters use echolocation. They tap on branches with slender fingers and look for voids in the wood. When a branch bounces back into a certain cavity – insect tunnels that cut under the bark – the maki bites and rips open the wood. He sticks his spindle-shaped middle finger in the hole and turns it around, felling maggots with his claws.
These long, delicate fingers are perfect for foraging, but in many other ways they are lousy. When the animal tried to travel with the middle finger, it snapped under the weight of the aye-aye.
The hands of the Aye-Aye so specialized in this foraging, "that they lost the ability to grip," Dr. Hartstone- said Rose. They compensated, he thinks, with a pseudo-palate.
If the researchers confirm their hypothesis by examining aye-aye lemurs living at the Duke Lemur Center in Durham, NC, the aye-aye will be the first animal to be described for reasons the dexterity develops an extra digit.
In other cases, animals developed extra digits to compensate for when their fingers were less specialized or their hands widened. For example, the original thumb became just another finger for the bear line, but the giant panda developed a pseudo-thumb because it helped bamboo eat. And additional digits turned the moles into better graves.
But Aye-ayes may not be alone for long: "We're starting to study some of the bats' wrist structures," Dr. Hartstone-Rose. "We think they have some similar little grips they put on their wrists."