A 20-year demographic study of a relatively undisturbed and exceptionally large community of eastern chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, southwestern Uganda, has revealed that our close primate relatives can lead surprisingly long lives in the wild.
A member of the Ngogo community of chimpanzees in Uganda’s Kibale National Park. Image credit: Brian Wood, Yale University.
The study, led by Yale University researcher Brian Wood, establishes an average life expectancy of about 33 years in its sample of 306 chimpanzees, nearly twice as high as that of other chimpanzee communities and within the 27- to 37-year range of life expectancy at birth of human hunter-gatherers.
“Our findings show how ecological factors, including variation in food supplies and predation levels, drive variation in life expectancy among wild chimpanzee populations,” Dr. Wood said.
“They also inform the study of the evolution of human life history, helping us to imagine the conditions that could have changed mortality rates among our early hominin populations.”
Dr. Wood and co-authors have monitored births, deaths, immigrations, and emigrations in the unusually large Ngogo chimpanzee community since 1995, producing the largest demographic dataset available for any community of wild chimpanzees.
This study reveals that Ngogo chimpanzees have the highest life expectancy on record for any group of wild chimpanzees.
Favorable ecological conditions largely account for the Ngogo community’s high life expectancy, according to the study.
The forest in Ngogo provides a relatively consistent and abundant supply of high-energy and nutritious foods, including easily digestible figs.
“This rich food supply helps buffer the Ngogo chimpanzees against periods of hunger, and helps fuel their ability to stave off diseases that would otherwise lead to higher mortality,” the scientists explained.
The Ngogo chimpanzees also benefit from a low risk of predation, because leopards are not found within Kibale National Park, and from the fact that during the study, the chimpanzees did not experience major disease epidemics, either introduced by humans or due to other causes, like those that have affected wild chimpanzees at several other long-term research sites.
In the same national park, not far from Ngogo, other researchers have studied the life expectancy of chimpanzees in the Kanyawara community.
Like Ngogo, this community lacks natural predators, but its life expectancy at birth is nearly 13 years shorter than that of Ngogo.
“The Ngogo chimpanzees’ higher survivorship appears to be an adaptive response to a more abundant and less varied food supply than that of Kanyawara,” the authors said.
“It has long been proposed that there are extreme differences in the life expectancies of human hunter-gatherers and chimpanzees,” said Yale Professor David Watts, co-author of the study.
“Our study finds that while maximum lifespan differs a great deal, the differences in average lifespan are not as dramatic as typically thought, especially when chimpanzees are not subjected to major negative impacts caused by humans.”
“In fact, the Ngogo community’s pattern of survivorship more closely resembles that of human hunter-gatherers than those documented for other chimpanzee communities.”
The findings will be published in the April 2017 issue of the Journal of Human Evolution.
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Brian M. Wood et al. 2017. Favorable ecological circumstances promote life expectancy in chimpanzees similar to that of human hunter-gatherers. Journal of Human Evolution 105: 41-56; doi: 10.1016/j.jhevol.2017.01.003
