Humans -- like most non-human primates -- are social beings and profit in many respects from the benefits of a community. However, their closeness to conspecifics is an opportunity for pathogens and parasites to infect new hosts. It is therefore advantageous to avoid sick individuals. Scientists including Clémence Poirotte from the German Primate Center investigated how mandrills, an Old World monkey species inhabiting equatorial rainforests of Gabon, recognize conspecifics infected with intestinal parasites and avoid an infection. The monkeys are able to smell an infected group member and consequently groom them less than healthy individuals. This component of the "behavioral immune system" of mandrills plays a crucial role in the co-evolution of host and parasite.

Clémence Poirotte studied 25 wild mandrills for a period of two and a half years. The Old World monkeys live in the dense rainforests of Southern Gabon. Mandrills are facing intensive parasite pressures. They excrete gastro-intestinal parasites in their feces and thus the skin and fur of parasitized individuals, particularly in their peri-anal area, is highly infectious. Social grooming is of enormous importance for the group members since it minimizes conflicts and increases the well-being of both grooming partners. However, the resulting physical proximity also increases the risk of transmission of pathogens.

The scientists studied both the extent of the parasite infestation as well as the grooming activities of the mandrills. They found that animals infected with parasites were less frequently groomed than healthy individuals, particularly at risky body parts (i.e., the peri-anal area). As soon as the infected animals received medical treatment, the animals concerned received significantly more social grooming than before. Olfactory tests with mandrills in open enclosures showed that they avoided the immediate vicinity to heavily parasitized feces samples.

"As a species that lives in close contact with conspecifics, the ability of mandrills to recognize safe social partners and avoid risky contacts shows a strategy that minimizes the transmission of parasites" explains Clémence Poirotte, a scientist at the German Primate Center. The pathogens cause a selection in favor of anti-parasitic behavior. "The coevolution of parasite and host has not only influenced the evolution of defense mechanisms but also the evolution of social systems," says Poirotte.