Parents of young children in daycare quickly learn how good toddlers are at spreading disease. Now, researchers have discovered that something similar may happen in chimp colonies plagued by human respiratory diseases. The tendency of 2-year-old chimps to play with each other seems to make such outbreaks in chimp communities in Côte d'Ivoire much deadlier, according to a study published 18 June in PLoS One. The study suggests that chimpanzees may pay an evolutionary price for learning to be social.
Since Christophe Boesch began studying wild chimpanzees in the Taï forest in Côte d'Ivoire in 1979, the animals' populations have declined by more than two-thirds. Boesch, now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues have tied some of the losses to outbreaks of two viruses--respiratory syncytial virus and human metapneumovirus--that the researchers themselves unwittingly and repeatedly brought into the forest. A closer look at the outbreak patterns revealed that the most severe epidemics seemed to happen in a 3-year cycle. Peter Walsh, a researcher at the same institute who was helping a student with a study on the social development of young chimps, wondered if something akin to daycare outbreaks was at work in the forest.
Walsh, Hjalmar Kuehl, and their colleagues examined records of births and deaths since 1984 and found that the outbreaks seemed to occur primarily when there were many 2-year-olds in the troop. An initial outbreak apparently killed many of the group's youngest members; their mothers then all became fertile at the same time and gave birth roughly a year after the previous outbreak. Two to 2.5 years later, the troop had an unusually high number of 2-year-olds, known for their eagerness to play. That budding sociability had deadly consequences, however: It was the perfect way to spread viruses. Again, many of the youngest chimps died, and the cycle started over. The scientists found a matching pattern in another study group living elsewhere in the Taï forest. They could not find any similar correlation with weather patterns, food availability, or predator abundance.
"Play behavior apparently has a huge potential cost for these animals," says Tony Goldberg of the University of Illinois, Urbana-Champaign, who was not involved in the research. That means sociality must also have big benefits, or it would have been selected against, he says. Those benefits presumably include learning the social skills that characterize chimpanzees, Goldberg says. Because introduced human diseases lead to more deaths, play behavior may eventually decrease in these groups, he speculates.
The results are bad news for ecotourism, Walsh says. "Ecotourism has been sold as a way to save the apes. But tourists are going to be bringing in their airplane colds ... right to the apes," he says. "An outbreak that kills half the population is worse than years of poaching." But knowing that 2-year-olds are especially vulnerable could help researchers and tour operators be more careful, he adds. Also, researchers have now introduced strict measures, including face masks, to avoid transmitting any new viruses to the study animals.
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