For young chimpanzees, play can be fatal. Germs spread by play explain why one African group shows waves of infant mortality that peak every three years, say experts who have studied them.
Primatologist Christophe Boesch and his colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, have monitored the mortality of a chimpanzee group living in the Taï National Park of the Ivory Coast for more than 20 years. They noticed that outbreaks of respiratory disease strike roughly every three years. In the worst events, some 20% of the troop died.
These cycles do not match fluctuations in any external environmental factor, such as food abundance. Instead, the researchers report in PLoS One 1, the chimps have become locked into a cycle of reproduction and disease, driven by their own social behaviour.
If they lose their offspring, female chimpanzees soon breed again. Boesch suspects that an initial disease outbreak at some time in the past killed a large proportion of the group’s youngsters, causing many adult females to breed at once, and creating a cohort of youngsters of a similar age.
For their first year or so, baby chimps stay close to their mother. But when they reach 18 months they become boisterous and sociable. “They spend almost all their time playing and in very close contact, rolling around, wrestling, pulling each others’ hair and biting,” says Boesch.
The young chimps have the most physical contact of any group members — play results in twice as much contact as adult grooming, for example. It's not just the juveniles that wrestle, mothers also get drawn into the play. This makes a large group of youngsters ideal for spreading disease — just like in a human kindergarten.
It’s obvious when an outbreak is under way, says Boesch: “Every [chimp] gets ill within three to five days.” The respiratory viruses do not kill the young chimps, but they make them susceptible to other infections, which do. This creates a group of childless females, and the cycle begins again.
The initial respiratory infections are probably caught from humans — the genetic sequences of viruses isolated from the chimps are almost identical to those of viruses that can cause respiratory disease in people2.
The researchers now wear surgical masks whenever they are in sight of the chimps. Ecotourists should do the same, says Boesch.
The cycle of infant mortality plays out on a more general pattern of decline, caused by poaching, leopard predation, and other diseases such as the Ebola virus and anthrax. Two decades ago, there were more than 80 adults in the chimpanzee group. Now there are about 25.
“We have to be extremely careful,” says Victoria Horner, who studies chimpanzee behaviour at Emory University in Atlanta, Georgia. “By habituating chimpanzee groups to people we are potentially putting them at risk.”
Chimp sanctuaries in Africa already typically inspect visitors’ vaccination certificates and bar anyone with a cold, she notes, but these precautions are rarely applied to contact between wild chimps and people.
Different factors compound one another, says Horner: poaching and logging increasing contact between humans and chimps, and smaller groups are more vulnerable to disease.