A lemur from Madagascar has been hiding a sixth finger on its hand
Naturalists have studied the aye-aye for two centuries — and missed its extra finger
On the island of Madagascar off the east coast of Africa, an exotic lemur called the aye-aye has been found to have a sixth finger on each of its hands.
"[It's] more than just a little nub. There's bone; there's cartilage. When we looked carefully at it, we noticed there's even a fingerprint," Adam Hartstone-Rose told Quirks & Quarks.
This is an exotic bit of evolution comparable to the panda's thumb — a unique adaptation to its lifestyle.
A fascinating animal on a fascinating island
The aye-aye is unusual in many ways. It's a nocturnal primate, about the size of a cat, with big eyes, giant bat-like ears and rodent-like incisors. But its most distinctive feature is its extraordinarily long and thin fingers. Its middle finger is particularly strange — even thinner than the others and strangely jointed.
"If the aye-aye's arm is scaled up to the size of a human arm, then the fingers would be about a foot long," explained Hartstone-Rose, a comparative anatomist at North Carolina State University, who led the research.
The aye-aye's spindly fingers are adapted for a very peculiar foraging behaviour called tap foraging, he explained.
"They'll find a piece of rotting wood or dead tree and take that creepy long third finger and tap along it to echolocate like a bat, to create a mental map of the trails under the wood that grubs have carved," he said.
Once they've located the grubs, the aye-aye will use its sharp teeth to open a hole into the wood. It will then thread its long, jointed, middle finger through the grub tunnels and snag nutritious grubs with its claw.
A surprising discovery
The discovery came as part of a larger research enterprise. Hartstone-Rose and his colleagues had been doing systematic dissections of a range of primate specimens, including monkeys and lemurs, paying particular attention to the forearms and hands. These can reveal a great deal about the kinds of adaptations that suit tree or ground-dwelling lifestyles.
They were doing detailed anatomy on an aye-aye specimen when they discovered something strange.
A muscle from the forearm that allows humans to stick out our thumb, was sub-divided — and part of the muscle didn't go to the thumb at all.
About 20 per cent of it went to a previously unnoticed structure in the wrist that had a bone, a little piece of cartilage and connections to two other muscles. They realized it was a stubby little appendage that, when they looked carefully at it, even revealed a fingerprint.
"It's only maybe a centimetre long, hidden within the palm of the hand, which made it hard to notice. But with all of that fine muscle control, it can produce more than a kilogram of force, about half of the body weight of an aye-aye," said Hartstone-Rose. "So this little itty-bitty anatomy might have an important functional role."
Nifty fingers customized for the aye-aye
It's far more common in evolution for fingers to disappear, as is seen in animals like the horse and the cow. But there are a few animals that have added digits, like the giant panda.
The giant panda's famous sixth digit was evolved to help it clutch bamboo when it abandoned its carnivorous diet. Hartstone-Rose thinks something similar is behind the aye-aye's pseudo-thumb. It likely evolved to help the lemur grab onto things and perhaps navigate better in trees while foraging.
The aye-aye's long, thin fingers are an example of hyper-specialization, he explained. They've evolved in an extreme way to help the animal get grubs out of trees. But this has drawbacks. These extended fingers have little grasping strength, so the animal has difficulty using them effectively to do things like holding onto tree branches while climbing.
The implication is that this short pseudo-thumb evolved to help it with this kind of forceful grasping.
Hartstone-Rose suggests this kind of radical adaptation was probably helped by the fact that the aye-aye evolved on Madagascar, which was an isolated and empty island when the lemurs arrived.
"There was so much space and so many ecological niches that would normally be filled by other animals, and those other animals simply didn't exist in this weird and wonderful space," he said.