Infanticide across apes: a meta-analysis of life history traits to estimate risk in orangutans (Pongo spp.)

Abstract

Infanticide is most commonly observed in species with slow life history traits, where females have long intervals between births, and as a way of increasing reproductive success males may force females to return to estrus sooner by killing their dependent offspring. Orangutans exhibit some of the longest inter-birth intervals (IBI) among mammals, making them a key system for evaluating the Sexual Selection Hypothesis. Orangutans, in particular, are known for their extensive parental care, and they are characterized by having the longest gestation length among primates after humans. In species with extended gestation and lactation lengths as well as delayed weaning, such as orangutans, the incentive for infanticide is expected to be especially strong because the dependent offspring suppress female fertility for prolonged periods. In this study, comparative data from both great apes (Hominidae) and lesser apes (Hylobatidae) are used to test whether variation in life history traits such as gestation length, weaning age, and inter-birth interval (IBI) can predict the likelihood of infanticide in orangutans, despite no occurrence having been documented in wild populations. Statistical models were applied to assess which life history traits in apes show higher probabilities of infanticide occurrence. There are many authors that claim that infanticide is an important selection pressure in orangutans, yet, there is no documentation of these events in the wild. Could it be that infanticide is not an important selection pressure for orangutans despite being a proven pressure for some of it's closest relatives? Or perhaps infanticide is occurring and the rural habitats that orangutans live in make it too difficult for humans to observe? The objective of this meta-analysis is less about proving or disproving the presence or absence of infanticide in orangutans, and more about analyzing if infanticide is occurring, why would orangutans be susceptible to it, under the assumption that it is occurring in wild populations for the purpose of this study.