Why the Aye-Aye Taps Trees Eight Times a Second

In the wet canopies of eastern Madagascar, a slender primate finger strikes tree bark up to eight times per second, and the animal doing it is hunting by sound.

A primate that hunts like a woodpecker

The tapping is not decorative behavior. It is percussive foraging, and the aye-aye, Daubentonia madagascariensis, is the only primate known to use echolocation to find its prey. In the entire mammalian class, this method appears only in one other lineage, the striped possum. The aye-aye works in complete darkness, listening for the acoustic signature of hollow galleries where wood-boring insect larvae hide inside a living branch. According to the published behavioral literature summarized on the animal's Wikipedia entry, percussive foraging accounts for between 5 and 41 percent of the aye-aye's total foraging time. That is not a curiosity at the margins of its diet. It is a working strategy, refined enough to substitute for the ecological niche that woodpeckers fill on other continents where no woodpecker ever evolved.

The animal doing this is the world's largest nocturnal primate. Head and body span 36 to 43 centimeters, and the tail, at 56 to 61 centimeters, is significantly longer than the body itself. Adult weight sits at roughly 2 kilograms, about 4 pounds, with captive males averaging around 2,615 grams and females around 2,570 grams. Aye-ayes spend up to 80 percent of the night moving through the forest canopy, and a single animal may travel as much as 4 kilometers, or 2.5 miles, between dusk and dawn. Males defend home ranges of up to 32 hectares. Females hold smaller ranges that never overlap another female's. Everyone works alone.

The anatomy of the tap

The hand is the story. The aye-aye's middle finger is dramatically thinner and longer than the digits on either side of it, and at the base sits a ball-and-socket joint that no other primate possesses. That joint lets the finger swivel freely in almost any direction, a range of motion that turns the digit into a probe rather than a grasping tool. Because the aye-aye's other fingers are so specialized for reaching into narrow spaces, they are poor at gripping wood, so the animal has evolved a pseudothumb, a sixth digit, to keep hold of the branch while it works.

Once the tap picks up the hollow signature of a tunnel, the animal has to breach the bark. It does this with continuously growing rodent-style incisors, an adaptation shared with rodents but with no other primate in the world. After the incisors have gnawed a hole through the wood, the same slender middle finger reaches in and hooks the grub out, most often the larva of a long-horned, or cerambycid, beetle. Echolocating ear, chisel tooth, ball-and-socket probe: three specializations, working in one sequence, so precise that biologists reach for the woodpecker comparison every time.

The lineage carrying this equipment is entirely alone. Daubentonia madagascariensis is the only living member of its genus and of its whole family, Daubentoniidae. A second species, the giant aye-aye, Daubentonia robusta, survived into the last 1,000 years before going extinct. What remains is a single surviving branch on a single island.

From Sonnerat's notebooks to Jersey Zoo

Western science met the aye-aye late, and understood it slowly. The name was first written down in 1782 by the French naturalist Pierre Sonnerat, who took it from the Malagasy word for the animal and described it, in his own phrasing, as a "cri d'exclamation et d'étonnement," a cry of exclamation and astonishment. Thirteen years later, in 1795, Étienne Geoffroy Saint-Hilaire formally named the genus Daubentonia in honor of the French naturalist Louis-Jean-Marie Daubenton. The family Daubentoniidae was established by John Edward Gray in 1863.

Naming an animal did not save it. Scientists officially declared the aye-aye extinct in 1933. It took 24 years for a living population to be rediscovered in 1957. In 1966, nine individuals were translocated to Nosy Mangabe, a small island off Madagascar's eastern coast, to seed a protected reserve population. Captive work followed slowly. Jersey Zoo imported six adult aye-ayes in 1990 and recorded the species' first captive birth two years later, in 1992. Today, roughly 50 aye-ayes live in zoological facilities worldwide, a small demographic buffer against a wild population that no one can precisely count.

The animal's biology does not help matters. Females typically begin breeding at three or four years of age, and they produce a single offspring only once every two to three years. Gestation runs 152 to 172 days. Newborns weigh 90 to 140 grams. In captivity, aye-ayes have lived to 23.3 years and beyond, with some records reaching 24.3 years; the estimated wild lifespan is roughly 20 years. It is a life history that recovers from setbacks the way old-growth forests do, which is to say, not quickly.

Marked for death by its own finger

The most severe threat to the aye-aye's survival originates at the same anatomical feature that makes it unique. In many parts of Madagascar, the aye-aye is considered a harbinger of death. The local taboo, called fady, dictates that if the animal points its narrowest finger at a person, that person is marked to die. In Sakalava tradition, the belief goes further: "Aye-ayes sneak into houses through the thatched roofs and murder the sleeping occupants by using their middle fingers to puncture their victims' aorta." Because of this fear, villagers frequently kill the animal on sight to prevent the perceived curse.

The population data reflects the pressure. In 1992, the IUCN estimated the total wild population at between 1,000 and 10,000 individuals, and the trend today is decreasing. The IUCN's current listing, dating to 2014, classifies the species as Endangered, and the aye-aye is also listed on CITES Appendix I, the strictest tier of international trade protection. A primate that survived a formal declaration of extinction is now being pushed back toward a real one, and it is being pushed for the exact anatomical masterpiece that keeps it alive.

That is the shape of the aye-aye's story. Eight taps per second against wet bark, in complete darkness, from a hand no other primate has ever grown. And in the villages below the canopy, the same finger, held still and pointing, is read as a death sentence.

Sources

• Wikipedia, Aye-aye
• Wikipedia, Daubentonia
• Animal Diversity Web, Daubentonia madagascariensis
• National Geographic, Aye-aye
• National Geographic Kids, Aye-aye