Why do some primate mothers carry their infant’s corpse? A cross-species comparative study

Abstract

Non-human primates respond to the death of a conspecific in diverse ways, some which may present phylogenetic continuity with human thanatological behaviours. Of these responses, infant corpse carrying by mothers (ICC) is the most-frequently reported. Despite its prevalence, quantitative analyses of this behaviour are scarce and inconclusive. We compiled a database of 409 published cases across 50 different primate species of mothers’ responses to their infants’ deaths to test hypotheses proposed to explain between- and within-species variation in corpse carrying. Using Bayesian phylogenetic regressions, we preliminarily identified three factors as possible predictors of ICC occurrence. However, using an information-theoretic approach, no combination of these predictors performed better than the null model, offering no support for any of the hypotheses we tested. In contrast, for those cases in which infant corpses were carried, infant age affected ICC duration, with longer ICC observed for younger infants. This result may provide support for hypotheses that suggest that ICC is a by-product of a strong mother-infant bond. The results are discussed in the context of the evolution of emotion and their implications for evolutionary thanatology are considered.

Why do some primate mothers carry their infant's corpse? A cross-species comparative study

“…Data accessibility. The compiled comparative data and the R code used in this study are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.np5hqbzsk [71].…”

Dead infant carrying by chimpanzee mothers in the Budongo Forest

“…Her apparent use of a ‘substitute’ in her carrying of a twig together with the even more prolonged second carry suggest she had a particularly strong motivation to carry. These observations, combined with the fact that all recorded instances of carrying in our dataset concern infants who died before weaning age, indicate that maternal behaviours, which are not limited to maternal care, likely play an important role (Fernández-Fueyo et al 2021).…”

“…Thus, rather than mummification being the result of favourable climactic conditions, it is possible mummification was observed because the extended carrying durations allowed for it. Recent explorations of several large datasets also found no support for the ‘climate hypothesis’ (Das et al 2019; Lonsdorf et al 2020; Fernández-Fueyo et al 2021). The evidence for the ‘post-parturient condition hypothesis’ is similarly mixed; of the three extended carries two infants were new-born, while the other was 2-years old.…”

“…Here, we focus on a particularly remarkable behaviour seen in many primates, dead infant carrying by mothers, which in chimpanzees typically occurs for a period of up to three days (Gonçalves and Carvalho 2019). Dead infant carrying (also referred as infant corpse carrying) is the most frequently reported thanatological behaviour and shows substantial variation in accompanying behaviours across primate species (Fernández-Fueyo et al 2021), as a result, it offers rich data against which to explore possible explanatory factors. This behaviour has not only been reported in chimpanzees (Matsuzawa 1997; Hosaka et al 2000; Kooriyama 2009; Biro et al 2010) but also in bonobos (Fowler and Hohmann 2010; Tokuyama et al 2017), gorillas (Warren and Williamson 2004; Masi 2020), chacma baboons (Carter et al 2020), red colobus (Georgiev et al 2019), geladas (Fashing et al 2011), bonnet and lion-tailed macaques (Das et al 2019), Japanese macaques (Sugiyama et al 2009; Takeshita et al 2020), and vervet monkeys (Botting and van de Waal 2020), while failed apparent attempts at carrying have been observed in ring-tailed lemurs (Nakamichi et al 1996) and marmosets (Thompson et al 2020).…”

“…Infants that died of sickness were carried for longer than those who were stillborn or victims of infanticide, while the age of the infant did not influence duration of carrying (Das et al 2019). Although dead infant carrying is a shared but rare behaviour in primates, carrying duration seems to be the longest in great apes (Fernández-Fueyo et al 2021), particularly in chimpanzees (Das et al 2019). Wild chimpanzees have higher levels of infant mortality compared to captive conspecifics (Courtenay and Santow 1989; Hill et al 2001), which provides increased opportunities to observe mothers’ responses to their infants’ death (Watson and Matsuzawa 2018).…”