Some say the eyes never lie – for Madagascar’s ring-tailed lemurs, it’s their glandular secretions that always tell the truth.

A new study from a team of Duke University researchers shows that lemurs can detect if a fellow primate is weakened by injury just by sniffing the wafty odours they leave behind.

Does this smell weak to you? 

"Our study shows that physical injury from peers dampens an animal's scent signature, and in a way that its counterparts can detect," Duke professor of evolutionary anthropology Christine Drea explained in a press release.

For ring-tailed lemurs, scent is an important means of communication. The island primates are equipped with genital scent glands that produce a musky substance – perfect for smearing on branches and twigs if you’re a lemur wanting to send olfactory messages to your fellow primates. The pungent paste is made up of 200 to 300 chemicals that carry important messages like “I was here” or “let’s start a family”. Each lemur’s distinctive musk also reveals clues about its health.

Working with captive ring-tailed lemurs at the Duke Lemur Center in North Carolina – a primate facility that houses the world's greatest number and diversity of lemurs outside of Madagascar – researchers went about swabbing the scent glands of their test subjects.

Over a nine-year period, 23 individuals were swabbed while they received treatment for recently sustained injuries. Similar to most primates, ring-tailed lemur dominance battles involve a fair share of biting, pulling and swatting, and these tussles can result in cuts and wounds.

Ring-tailed lemurs live in social groups known as troops that are led by a dominant female.

By analysing their swabbed samples and comparing them to the scent signatures of perfectly healthy lemurs, the researchers determined that injuries can noticeably alter a lemur’s chemical cocktail. Wounded individuals dial back their signature body odour, and the number of compounds in their scent decreases by as much as 10 percent.

When wounded, “their entire olfactory signature kind of flat-lined or disappeared,” Drea told National Geographic.

This is particularly problematic for male lemurs wounded during breeding season – a time when fights are more common as males jostle for mating rights. It’s also a period of heightened stress for the males who struggle to “sustain their olfactory signals” Drea points out. Mating season is an energetically costly time for male lemurs and while those nursing an injury may be able to put on a brave face, their weakened musk will give them away.

In lemur land, social status is ranked by the quantity and quality of these smelly secretions. In addition to slathering their scent on trees and branches, males will also "re-mark" over the secretions of their rivals in an attempt to out-stink the competition. Males that leave behind a weak odour are more likely to have their scent smeared out by rivals, indicating that the over-marking males are able to detect a weaker competitor.

“These animals constantly monitor the physical condition of their competitors and respond quickly to any opportunity to climb the social ladder,” says Rachel Harris, who conducted the research as a postdoctoral associate in evolutionary anthropology at Duke.

For ring-tailed lemurs, social rank is all about the scent. And the scent never lies.