MANY animals are herbalists. Pregnant elephants eat particular leaves to induce the births of their calves. Birds keep bloodsucking bugs at bay by weaving insect-repelling plants into their nests. Fruit flies lay their eggs on fermenting matter that is rich in ethanol, which drives away parasitic wasps. There is, in other words, a whole pharmacopoeia of botanical products out there. Examples of animals employing the products of other animals for medical purposes are, by contrast, rare. But one case has just come to light. Louise Peckre of the German Primate Centre, in Göttingen, has found that red-fronted lemurs treat threadworm infestations in the gut and around the anus with millipede juice.

Unlike their fellow myriapods the centipedes, which are venomous, millipedes have no chemical weapons. But they have chemical defences, particularly benzoquinones. These can blind, burn and poison would-be predators, and also act as insect repellents. It was in this context, as she describes in a paper in Primates, that Ms Peckre watched with fascination the habit of some red-fronted lemurs she had under observation in the Kirindy Forest, in Madagascar, of gnawing on benzoquinone-rich millipedes and rubbing the remains around their anuses, then swallowing them. She saw six lemurs doing this and was left wondering, why?

Some monkeys rub millipede juice onto their skin to ward off biting insects, so what she had seen was not completely unexpected. But Ms Peckre’s lemurs were not behaving in a way that suggested repelling insects was their purpose. Lemurs’ anal regions are furry and are rarely attacked by bloodsucking arthropods. Nor would swallowing dismembered pieces of millipede seem likely to deter something that was attacking the skin. Pieces of the puzzle started to come together, though, when she and her colleagues noticed, by analysing the lemurs’ faeces, that times of peak millipede use coincided with threadworm infestations in the lemurs’ guts.

Threadworms have the repulsive habit of slithering out of their host’s anus at night and laying their eggs in the soft flesh nearby. The site where the eggs are laid itches. The infested individual either scratches or licks the site, gets the eggs on its fingers or tongue, and ultimately either swallows them or passes them on to others during grooming sessions.

Human beings, who are frequent hosts of threadworms, can deal with them using drugs such as benzimidazole, which are similar in structure to benzoquinone. Ms Peckre therefore suspects that her lemurs are employing millipedes in lieu of a trip to the pharmacy.