Back in 2016, researchers studying red-fronted lemurs in the forests of central Madagascar observed a bizarre incident: lemurs rubbing toxic millipedes all over their genitals and anuses, as well as chewing the noxious critters in their mouths. And it wasn't a one-off affair. The lemurs seemed to repeat this ritual frequently, typically after heavy rains.

At first the never-seen-before behavior was a mystery, but researchers now believe they have cracked the case. It turns out that these primates are using the millipede toxin for its medical benefits, according to a press release.

"Self-anointment combined with eating millipede secretions may be a way of self-medication by red-fronted lemurs," said Louise Peckre, an expert in primatology at the Leibniz Institute for Primate Research in Germany, and one of the discoverers of the behavior.

That the lemurs seem to engage in the behavior more frequently after heavy rains is simply because this is when millipedes are driven out into the open, so they're easier to catch. And the lemurs' choice of venue for the rubbing probably has to do with those regions being particularly irritated by parasites, which the toxins help to eliminate.

Parasites are a pain

Most specifically, the parasites that are most bothersome to red-fronted lemurs are Oxyuridae nematode, a common worm living in and near an animal's anus. The rashes and itching caused by this worm's residence in the nether regions can be quite overwhelming, thus the extreme need to go so far as to rub toxic millipedes over these sensitive areas as a remedy.

"Strikingly, during the fur-rubbing observations, we noticed the presence of bald areas on the lower back of many animals. These are known as sit spots, and are likely caused by repeated rubbing," explained Peckre. "These bald areas may then indicate the presence of infections by Oxyuridae in the population at the time."

Although this is the first incident of medication being used by lemurs, the behavior is not unheard of elsewhere in the animal kingdom. Chimps, bonobos and gorillas have all been witnessed swallowing rough leaves as a way to rid their digestive systems of parasites. Some ants even incorporate antimicrobial resin into their nests.

Researchers next plan to further investigate the behavior by seeing whether the most frequent millipede-rubbers also tend to be the most inflicted with parasites. It will then be possible to better determine if the behavior is primarily a reaction to being infected, or whether it is more premeditative — a preventative medicine.

The study was published in the journal Primates.

Wild lemurs found using millipedes as medicine
Scientists thought there must be a good reason red-fronted lemurs were caught regularly chewing millipedes and rubbing the toxins over their genitals.