Killing neighbors and seizing their land led to a baby boom in Uganda’s chimpanzee community. This may indicate why chimpanzees find it advantageous to start wars.
Chimpanzees (Pan troglodytes) have long been known for violent conflicts or “wars.” This was first recorded by British primate researcher Jane Goodall, who observed in 1974 that a chimpanzee population in Tanzania’s Gombe National Park split into two warring groups, leading to a four-year battle that killed all the males in one group. However, it was not clear why the animals continued to be violent for so long.
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From 1998 to 2008, Kibale’s Ngogo chimpanzees were involved in violent conflicts with their neighbors. At least 21 chimpanzees from neighboring groups have been killed during this decade of conflict, and in 2009 the Ngogo chimpanzees expanded into areas previously inhabited by their rivals, expanding their territory by 2.5 square miles (6.4 square kilometers), or 22 percent.
Records show that a female Ngogo chimpanzee gave birth to 15 offspring in the three years before expanding her territory. But over the next three years, they gave birth to 37 children, more than doubling the birth rate.
In addition, infants born after dilation were more likely to survive, with a 41% chance of dying before age 3, but now their chance was only 8%. The study was published in the journal PNAS on November 17th.
“At the time, it was very clear to field workers that chimpanzees were experiencing a baby boom. We expected it to show up in the data, but we didn’t expect survival rates to increase,” Wood told Live Science.
This study provides the best evidence to date that for chimpanzees, expanding their territory after killing rivals can directly increase reproductive success, he said. Wood added that chimpanzee territory expansion meant more food was available, and subsequent improvements in nutrition and health likely led to increased female fertility and improved survival of young.
The improved survival rate can be attributed to two factors. Wood said the first is to improve the mother’s health and energy, and the second is to eliminate rival males.
Michael Wilson, who studies chimpanzee behavior and biology at the University of Minnesota and was not involved in the study, told Live Science: “The main cause of mortality in baby chimpanzees is being killed by neighbors, so it makes sense that they have a higher survival rate.” “This study supports the idea that under certain conditions, chimpanzees will adaptively protect group resources and kill members of neighboring groups. Chimpanzees are essentially seeking out their own group.”
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But if there are benefits for the winners, there will also be costs for the losers, Wood said. He believes this is likely to be a zero-sum game, with winners benefiting and others losing out, so overall chimpanzee numbers will probably not increase.
Scientists behind the study say the discovery could help shed light on the evolution of violence in humans. Because of the lethal violence present in our closest living relatives, chimpanzees and bonobos (Pan paniscus), some scientists have previously suggested that this trait may have been present in our common ancestor, who lived perhaps 6 or 7 million years ago, Wood said.
Competition over access to land and resources remains a constant part of the human condition, he said, but is generally altered by humans’ ability to mediate and avoid conflict.
“There are similarities to what chimpanzees are doing in the conflicts that are going on around the world over resources, but I don’t think that’s a good comparison if you’re going to get involved,” Wood said.
In general, Wilson said there are notable differences between humans and chimpanzees when it comes to intergroup relationships. “If a chimpanzee finds a male from a neighboring group, the only way to benefit that male is to impose some kind of price on him, take his territory, or take his life.”
When people see strangers from another group, they may benefit from interacting with them, he said.
This allowed humans to build multi-layered societies with trade, kinship, and ritual ties to form larger units of social organization.
“In the modern world, the benefits to be gained from interactions between groups have become so enormous, and the costs of war have increased so enormously that it’s generally a pretty foolish idea to go to war,” Wilson said.
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