Weighing in at only 40 grams, brown mouse lemurs are one of the smallest species of primate in the world. Their diminutive size, as well as their nocturnal, tree-dwelling lifestyle, makes them difficult to track and observe. Sarah Zohdy, then a graduate student at the University of Helsinki in Finland, and her colleagues came up with an ingenious way to study the interactions of these small lemurs: they followed their lice.

Scientists have estimated that lice originated at least 130 million years ago, when they fed off feathered dinosaurs, although they now live on just about all species of birds and mammals. They tend to be very host-specific, meaning they only live and feed on one species or a set of closely related species. And for lice to reproduce and spread, their hosts have to be in fairly close contact (like, as many parents know, kids in a kindergarten classroom).

In wild species, lice rarely switch hosts unless the animals interact physically, whether through wrestling, nesting together or mating.

Zohdy and her colleagues had been studying lemurs in Madagascar, using traps to monitor their movement. The team tagged Lemurpediculus verruculosus, a species of lice that is specific to the brown mouse lemur, with a unique color code using nail polish. Over time the researchers continued to trap lemurs and look at their lice to see if any of the tagged ones had switched hosts.

They documented 76 transfers among 14 animals—all males—over the course of a month, which happened to be during the breeding season. The researchers hypothesized that the male-only transfers most likely occurred during fights over females. But perhaps more interestingly, the lice data found 13 new social interactions that the traps had failed to predict. Among these was the finding that lemurs travel more than had been thought: some lice transfers occurred between lemurs that had last been trapped more than 600 meters apart.

This is not the first study that used lice to look at a bigger scientific picture, but it is one of the first to use lice to study behavior in a living wild species. The team hopes its work shows the usefulness of this technique.

This article was published in print as "Lice Don't Lie."