Aye-aye lemur 'heats up' its special foraging finger

The aye aye's cold finger shows up black on thermal images (c) G Moritz / N Dominy Cold finger: the special digit shows up black on thermal images

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Madagascar's mysterious aye-aye warms up its extra-long finger when searching for dinner, scientists have found.

The lemur, the world's largest nocturnal primate, taps its specialised middle finger on tree trunks to find nutritious beetle larvae.

Studying thermal images, researchers found that the digit was colder than the others but warmed by up to 6C during foraging.

Scientists suggest that the aye-aye saves energy by keeping the digit cool.

The findings are published in the International Journal of Primatology.

Finger facts

The aye-aye has a delicate middle digit (c) David Haring
  • The aye-aye's middle finger is long and very thin - less than half the width of its other digits
  • It has a ball and socket joint so it is far more flexible when it comes to extracting grubs from trees
  • The finger is also used for grooming, scraping out coconuts and drinking. The animal uses it to move water or nectar rapidly into its mouth

The team from Dartmouth College in New Hampshire, US, wanted to investigate the surface temperature of sensitive structures.

The aye-aye's unusual middle finger has already been found to be super-sensitive to vibrations, so provided the perfect subject for their study.

"It was striking to see how much cooler the third digit was while not in use and how quickly it warmed to [match] the other digits when engaged in an active foraging task," said graduate student Gillian Moritz, who carried out the study under the guidance of her supervisor, Dr Nathaniel Dominy.

Black and white

When not in use, the finger appeared black on thermal images. This indicated a large difference in temperature between it and the white (hot) ears and eyes.

But when the animal was looking for food, the finger rose in temperature by up to 6C.

The finger is hot as it probes a boiled egg (c) G Moritz / N Dominy The finger heats up as it probes a food source

"We think the relatively cooler temperatures of the digit when not in use could be related to its [long, thin] form," said Ms Moritz.

"This form results in a relatively high surface-to-volume ratio [but] such a ratio is bad for retaining heat."

In order to sense the vibrations of beetle larvae through the bark of a tree, the finger is "packed with sensitive nerve endings", the scientist explained.

Because of its specialist sense receptors, using this tapping tool is very costly in terms of energy.

"Like any delicate instrument, it is probably best deactivated when not in use," Ms Moritz told BBC Nature.

Kink in the flow

The question of how the lemur controls the heat of a single digit remains unclear.

Ms Moritz suggested two explanations. The first was simply that the blood vessels that supplied the digit could be constricted or dilated.

The second more unusual possibility, she said, was that the creature might employ temperature control method that was linked to the flexibility of its finger.

Ms Moritz explained: "Because the finger is fragile and vulnerable to injury, it is often extended back and out of the way during locomotion and periods of inactivity," she said.

This extension could cause a "kink" in the artery that supplies warm blood to the digit.

Start Quote

Like any delicate instrument, it is probably best deactivated when not in use”

End Quote Gillian Moritz Dartmouth University

In the same way a bent garden hose supplies less water, the artery could supply less blood, keeping the finger much colder than its fully supplied neighbouring digits.

Aye-ayes are the only primates known to have this strange adaptation.

The species is listed as Near Threatened by the International Union for Conservation of Nature (IUCN), mainly because of threats to its habitat.

But the odd-looking primate also suffers direct persecution. Superstition in Madagascar describes the species as a bad omen. Those that are pointed at by the creature's mysterious finger are said to meet their death.

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