Research identifies key driver for infanticide among chimpanzees

Led by PhD student Adriana Lowe and Dr Nicholas Newton-Fisher from Kent's Living Primates Group, the research team analysed the records of infanticides and failed attempts at infanticide over a 24-year period in the Sonso community of chimpanzees in the Budongo Forest.

While relatively rare, infanticide is well documented in chimpanzees but those in the Sonso community are more prone to practice it than others that have been studied, with infanticide the most common cause of infant death there. However, although various explanations have been proposed, this study is the first to thoroughly investigate the phenomenon.

For the study, the team analysed 33 attacks on 30 victims from a single community of chimpanzees between 1993 and 2017. From this they were able to determine that 11 of the attacks were 'definite' infanticides, four 'almost certain', nine 'suspected', and nine 'attempted'. Most attacks were by adult males, and the victims were often very young: two thirds were under one week old. The team also noted incidents of partial cannibalism.

From their analysis and observations they concluded that the sexual selection hypothesis -- the idea that male chimpanzees will kill infants other than their own so they have a better chance of fathering the mother's next infant -- was the main reason for the high rates of infanticide. For example, females who lost an infant to infanticide conceived on average around seven times more quickly than if their infant had survived, with the majority of mothers who lost infants going on to reproduce again within the community.

The team found no evidence to suggest that infanticide was part of a male strategy to eliminate future competitors.

Dr Newton-Fisher, who has studied infanticide in the Sonso community since the 1990s, said: 'Sexually-selected infanticide involves the reduction in time until the mother is free to mate again that a male could bring about by killing her present infant. However, we did identify examples of infanticide by females. Although rare, in those cases we suggest that they may use infanticide to drive away rival mothers, who would otherwise compete with them for food.

'We also discovered rare occurrences of partial cannibalism within the community, suggesting meat acquisition was a by-product of the infanticide.'

The report, 'Intra-community infanticide in wild, eastern chimpanzees: a 24-year review' (Adriana Lowe and Nicholas Newton-Fisher, University of Kent; Catherine Hobalter, St Andrews University; Caroline Asilmwe, Budongo Conservation Field Station, Uganda; and Klaus Zuberbühler, Institute of Biology, Neuchatel, Switzerland) is published in the journal Primates.