According to him, such cycles do not match fluctuations in any external environmental factor like food abundance. The researcher says that the chimps, instead, have become locked into a cycle of reproduction and disease, driven by their own social behaviour. Writing about their observations in the journal PLoS One, they have revealed that female chimpanzees start breeding again when they lose their offspring. Boesch believes that initial disease outbreak in the past might have claimed a large number of young chimps, causing many adult females to breed at once, and creating a cohort of youngsters of a similar age. He points out that baby chimps stay close to their mother for about a year, but become boisterous and sociable at the age of 18 months.
"They spend almost all their time playing and in very close contact, rolling around, wrestling, pulling each others' hair and biting," Nature magazine quoted him as saying. Highlighting the fact that the young chimps have the most physical contact of any members, Boesch says that play results in twice as much contact as adult grooming.
He also points out that it is not that only young juveniles that wrestle, but mothers also get drawn into the play. "Every (chimp) gets ill within three to five days," when an outbreak takes place, says Boesch. He says that the respiratory viruses, though do not kill the young chimps, make them vulnerable to other infections.
All that results in a group of childless females, and the cycle begins again. The researchers are of the opinion that the initial respiratory infections in chimpanzees might have come from humans because the genetic sequences of viruses isolated from the animals are quite similar to those that may cause respiratory disease in humans. Boesch says that researchers these days wear surgical masks whenever chimpanzees are around, and suggests that eco-tourists to do the same too.