Why chimpanzees carry dead infants: an empirical assessment of existing hypotheses

Abstract

The study of non-human primate thanatology has expanded dramatically in recent years as scientists seek to understand the evolutionary roots of human death concepts and practices. However, observations of how conspecifics respond to dead individuals are rare and highly variable. Mothers of several species of primate have been reported to carry and continue to interact with dead infants. Such interactions have been proposed to be related to maternal condition, attachment, environmental conditions or reflect a lack of awareness that the infant has died. Here, we tested these hypotheses using a dataset of cases of infant corpse carrying by chimpanzees in Gombe National Park, Tanzania ( n = 33), the largest dataset of such cases in chimpanzees. We found that mothers carried infant corpses at high rates, despite behavioural evidence that they recognize that death has occurred. Median duration of carriage was 1.83 days (interquartile range = 1.03–3.59). Using an information theoretic approach, we found no support for any of the leading hypotheses for duration of continued carriage. We interpret these data in the context of recent discussions regarding what non-human primates understand about death.

Beyond orphaned infants: novel effects of maternal death in wild primates

“…The impact of grief on wild animal behavior and survival are much more challenging to identify in natural populations as compared to humans, but the last decade has seen efforts to identify these effects (48)(49)(50). Within the populations studied here, an example of the death of a white-faced capuchin mother from the Lomas Barbudal population is consistent with our expectations of what grief might look like in a wild primate.…”

Reactions to infant death by wild vervet monkeys (Chlorocebus pygerythrus) in KwaZulu-Natal, South Africa: prolonged carrying, non-mother carrying, and partial maternal cannibalism

“…Birth-related hormones cannot explain this case, although the mechanisms through which siblings bond may be a factor. Dead infant carrying by non-mothers has also been reported for geladas (Fashing et al 2011 ), gorillas (Warren and Williamson 2004 ), chimpanzees (Lonsdorf et al 2020 ) and baboons (Carter et al 2020 ), although Carter et al ( 2020 ) note that carrying by non-mothers is usually of short duration, as in our sibling vervet monkey. The learning to mother hypothesis has been proposed to explain why some non-mothers carry dead infants: behaving maternally towards even a dead infant can contribute toward gaining skills required to be a competent mother (Warren and Williamson 2004 ).…”

“…It has been suggested that extended carrying occurs especially in places with more extreme climates (e.g. the habitats of mountain gorillas and geladas), or during dry seasons (Biro et al 2010 ), the conditions of which may slow the decomposition of a body (Fashing et al 2011 ), although a recent analysis found no evidence that chimpanzees carried infant corpses for longer in the dry season (Lonsdorf et al 2020 ). Beminde’s infant was born relatively early in the birth season (October), when climatic conditions were fairly dry compared to later in the season (November–December), which may have helped mummify the body.…”

“…Some researchers have also suggested that mothers may be unaware or unsure that their infants are dead, and so continue to provide maternal care (Watson and Matsuzawa 2018 ). However, behaviours directed at deceased infants that are not normally directed at live infants, such as carrying them slung across their backs (observed in chimpanzees and baboons), appear to contradict this (Biro et al 2010 ; Gonçalves and Carvalho 2019 ; Carter et al 2020 ; Lonsdorf et al 2020 ). Our observations neither support nor refute this hypothesis, although the severity of decay of Beminde’s deceased infant suggests that unawareness of death may not fully explain continued care-taking by vervet mothers.…”

“…Again, hormones may play a role in this, and stronger attachment may lead to stronger responses after infant death (Kaplan 1973 ; Anderson 2016 ). One prediction is that the death of older infants should cause stronger post-death responses in mothers, although a recent analysis of chimpanzee infant corpse carrying found no relationship between infant age at death and duration of carrying (Lonsdorf et al 2020 ). In our sample, most cases involved newborns; the few older infants that died were not carried for longer than newborns, although our sample is too small to draw firm conclusions from this.…”