Photo: Simon Bardet from Pixabay
Lethal aggression pays off for chimpanzees. Territorial expansion ultimately leads to far more offspring.
Dhe winners of a chimpanzee war in Uganda have gained increased fitness as a result of their territorial conquest. Females in the winning group produced more than twice as many offspring following their group’s territorial victory. Infant mortality among those young was also lower. These are the conclusions of an American-German research team, based on years of field observations, published in PNAS on 17 November. Between 1998 and 2008, chimpanzees from the so-called Ngogo group killed no fewer than 21 members of neighbouring groups. The neighbours were weakened and eventually forced to cede ground: from mid-2009 onwards, the Ngogo group took over approximately 6.4 square kilometres of additional territory, expanding their range by 22 per cent.
Birth rate
In the three years before the Ngogo chimpanzees had settled into their new territory, females in the group produced a total of fifteen infants. That birth rate rose sharply in the three years following the territorial expansion: no fewer than 37 infant chimpanzees were born – well over double the previous figure. A baby boom in chimpanzees is sometimes the result of high infant mortality: when young die quickly, mothers conceive again sooner. That was not the case here. Whereas young chimpanzees faced a mortality risk of 41 per cent in their first three years of life before the territorial expansion, that figure fell to just 8 per cent after the victory. The improved survival prospects of offspring amplify the fitness advantage conferred by territorial expansion, as the researchers calculate. Before the expansion, a female living to the age of 50 would on average raise 2.2 offspring to at least 3 years of age. After the expansion, that figure would rise to 7.4 offspring.
Food resources
The trees in the Ngogo area performed equally well before and after the territorial victory; relatively speaking, they bore just as much fruit. The territorial expansion did, however, give the chimpanzees access to more trees and therefore more food. This is presumably why females were able to produce more offspring. The greater availability of food resources also meant reduced competition for food within the group. In addition, infanticide by rival chimpanzees is fairly common. The Ngogo chimpanzees were less troubled by this now that they had effectively weakened their neighbours. According to the researchers, these findings indicate that chimpanzees wage war out of self-interest. Lethal violence between chimpanzee groups has arisen through natural selection, and not as a result of artificial external influences – the habitat of the Ngogo chimpanzees is virtually undisturbed and no humans live along the edges of their territory.
Lethal violence between chimpanzee groups has arisen through natural selection