The Mysterious Middle Finger of the Snot-Nosed Aye-Aye

Observing the natural world every day is a task that is always educational, often wondrous, and sometimes a little gross. Case in point, an article published this week in the Journal of Zoology: “A review of nose picking in primates with new evidence of its occurrence in Daubentonia madagascariensis.”

The primate in question, D. madagascariensis, is the aye-aye of Madagascar, an endangered lemur roughly the size of a house cat. Researchers led by Anne-Claire Fabre, an assistant professor at the University of Berne and an associate of London’s Natural History Museum, observed the nose-picking (or “rhinotillexis”) in captive aye-ayes at the Duke Lemur Center in North Carolina.

The aye-aye is equipped with a long, skinny middle finger – a digit unique in the animal kingdom in that it pivots around a ball-and-socket joint. When seeking food, an aye-aye will tap on the bark of a tree until it perceives a hollow within (called “percussive foraging”), then it gnaws a tiny hole in the wood, sticks its skinny, mobile digit deep inside, and retrieves a meal of insect grubs. The little primate has another use for its special finger, we now know.

The researchers note that “this animal inserts the entire length of its extra-long, skinny and highly mobile middle finger into the nasal passages and then licks the nasal mucus collected.” In layman’s terms, he picks his nose and eats it.

This discovery brings the number of animals known to pick its nose to 12 – primates all, including humans. Why do they (and we) do this? The authors admit that "there is no scientific consensus about the potential costs or benefits of this behaviour." Clearly more probing is necessary.

The video depicting rhinotillexis in aye-ayes is here: youtube.com/watch?v=zzXNZ_Z_M6M&t=22s.

Photo credit: nomis-simon / Wikimedia Commons