Biologists find sixth finger on aye-aye's paws
Oct. 21 (UPI) -- The aye-aye, the world's weirdest primate, boasts 12 fingers, six on each paw, according to a study. A team of researchers, led by scientists at North Carolina State University, found the pseudothumb while studying the tendons that support the primate's strange hands.
Aye-ayes, Daubentonia madagascariensis, are the world's largest nocturnal primates. Native to Madagascar, the rodents boast a variety of unusual features, including large ears, rodent-like teeth that never stop growing, and long, skinny fingers.
"The aye-aye has the craziest hand of any primate," lead study author Adam Hartstone-Rose, an associate professor of biological sciences at NC State, said in a news release. "Their fingers have evolved to be extremely specialized -- so specialized, in fact, that they aren't much help when it comes to moving through trees. When you watch them move, it looks like a strange lemur walking on spiders."
Digital imaging of the wrist and hands of six aye-ayes revealed tendons supporting a hidden thumb, or pseudothumb. Further analysis revealed the thumb's bone, cartilage and musculature structures. The secret digit can move in three directions, and scientists suspect it helps grip objects and branches when they climb trees.
"Using these digital techniques allows us to visualize these structures in three dimensions, and to understand the organization of the muscles which provide movement to the digit," said NC State postdoctoral researcher Edwin Dickinson.
In addition to wiggling in space, the pseudothumb can also produce a force equivalent to nearly half an aye-aye's body weight.
"The pseudothumb is definitely more than just a nub," Hartstone-Rose said.
Scientists fond the new digit, described this week in the American Journal of Physical Anthropology, in the left and right hands of both males and females.
The study's authors suspect the lemurs adopted their sixth finger to compensate for their other five highly specialized digits.
"Other species, like the panda bear, have developed the same extra digit to aid in gripping because the standard bear paw is too generalized to allow the dexterity necessary for grasping," Hartstone-Rose said. "And moles and some extinct swimming reptiles have added extra digits to widen the hand for more efficient digging or swimming. In this case, the aye-aye's hand is so specialized for foraging an extra digit for mobility became necessary."
While a few primates have lost a finger to improve their range of motion, the aye-aye is the first primate documented to have added a digit.
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