Photograph by Nory El Ksabi
Read Caption

A juvenile mandrill eats bark in Gabon. The highly social creatures rely on grooming to soothe conflict and strengthen bonds.

Photograph by Nory El Ksabi

How Sniffing Poop Helps Monkeys Stay Healthy

Like humans, mandrills of central Africa have strategies to avoid getting sick.

We don’t eat food off dirty floors. We dutifully wash our hands. We steer away from the clearly infected. It's all part of avoiding things likely to make us sick, and new research shows our primate cousins do it too.

Except the mandrill's method is a bit more unsavory: smelling each other's poop.

By detecting the odor of intestinal parasites in their group members' feces, these central African monkeys identify who is ill—and then avoid grooming them.

Grooming is important to mandrills: It soothes conflict and builds relationships, as well as keeps fur and skin free of pests. But this social behavior can also spread parasites, such as E. coli bacteria and other microbes that cause dysentery.

“We found that gastrointestinal parasites were present on the fur. So it’s risky to groom a parasitized individual,” says study leader Clemence Poirotte, an ecologist wo works with the Mandrillus Project, a multinational collaboration to study the world's only population of wild mandrills used to people. (See "Animals Have Evolved Into Parasites At Least 200 Times.")

Poirotte and colleagues studied a group of about 150 mandrills over two years as they roamed the savannahs and rain forests of Gabon.

“The population is so well habituated, they just don’t care at all about us. We have the privilege to just observe what happens," Poirotte says.

"Every day I spent with this population was maybe the coolest thing I had done in my life.”

Monkey See, Monkey Do

During their fieldwork, the team observed that “when an individual has parasites it will be less groomed, but particularly less groomed on the bottom,” says Poirotte—a smart idea, since parasites are transmitted through feces. It’s a little like using the toilet paper in the public bathroom but steering clear of the bowl.

Why Do These Monkeys Have Colorful Butts?

To test their theory that mandrills were not grooming infected individuals to avoid getting parasites themselves, the researchers treated several with anti-parasite medication, orally and with IV medication after trapping. (Read about mindsucking parasites in National Geographic magazine.)

After treatment, 12 of those individuals enjoyed more frequent grooming, including three that received ten times more. Further bolstering the theory, being infected did not influence how much a mandrill groomed others, only how much grooming the animal received.

“The results were totally in accordance with our predictions, so that was super nice,” says Poirotte, whose study appeared recently in the journal Science Advances.

The next step was figuring out how the mandrills knew their kin had parasites.

The researchers presented captive mandrills housed in forested enclosures in Gabon with feces-smeared bamboo sticks. One stick held mandrill feces full of parasites, another mostly parasite-free poop. A researcher who did not know which sample was which recorded the mandrills’ behavior. (Read: "Oldest Non-Human Stone Tools Outside Africa Created by Monkeys.")

As expected, the monkeys sniffed and investigated the bamboo sticks, but avoided the highly infected samples. (Poirotte could not distinguish between parasitized and non-parasitized samples herself, though she says there were samples that smelled worse than others.)

Her next task is to examine how the parasites may be affecting the mandrills' health.

Sniffing Out More Parasites

Benjamin Hart, a veterinarian and professor emeritus at the University of California, Davis, says that the mandrills likely keep their parasites at a manageable level.

If an animal acquires too many parasites, it can kill them, but moderate infections appear not too much of a threat, he adds.

Hart expects that this research will inspire similar examinations in other species.

“This study provides a thorough analysis and can serve as a model for others wanting to pursue the role of olfaction in parasite avoidance,” he says. “I thought the paper was excellent.”