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 of which may present phylogenetic continuity with human thanatological responses. 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 and used Bayesian phylogenetic regressions with an information-theoretic approach to test hypotheses proposed to explain between- and within-species variation in ICC. We found that ICC was more likely when the infant's death was non-traumatic (e.g. illness) versus traumatic (e.g. infanticide), and when the mother was younger. These results support the death detection hypothesis, which proposes that ICC occurs when there are fewer contextual or sensory cues indicating death. Such an interpretation suggests that primates are able to attain an awareness of death. In addition, when carried, infant age affected ICC duration, with longer ICC observed for younger infants. This result suggests that ICC is a by-product of strong selection on maternal behaviour. The findings are discussed in the context of the evolution of emotion, and implications for evolutionary thanatology are proposed.

Dead infant carrying by chimpanzee mothers in the Budongo Forest

“…We found no seasonal effect on dead infant carrying, but we do show a possible age effect, in which – amongst infants who died at under 1-year old – very young infants were more likely to be carried. Our observations also support the suggestion that infants may be less likely to be carried following traumatic death (Fernández-Fueyo et al 2021). Most cases of infant carrying were relatively short (minimum confirmed carry length of a few days); however, we also reported three prolonged cases of extended infant carrying.…”

“…Cases of extended carrying by parous mothers in other groups also suggest limited support for the 'learning to mother hypothesis' (Matsuzawa 1997;Biro et al 2010;Biro 2011;Lonsdorf et al 2020). However, in line with the fact that younger primate mothers are more likely to carry dead infants (Fernández-Fueyo et al 2021), our three observations support the suggestion that rare instances of particularly extended carrying across several months might be more frequent in young mothers. Nevertheless, repeated prolonged carries (as seen in UP)…”

“…Dead infant carrying (also referred as infant corpse carrying) is the most frequently reported thanatological behaviour and shows substantial variation in how it is expressed across primate species (Fernández-Fueyo et al 2021; Watson and Matsuzawa 2018), 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) among others (Fernández-Fueyo et al 2021), while failed apparent attempts at carrying have been observed in ring-tailed lemurs (Nakamichi et al 1996) and marmosets (Thompson et al 2020). Several hypotheses have been put forward in an attempt to explain the function of and motivation for this puzzling behaviour (for a review see: Watson and Matsuzawa 2018; Gonçalves and Carvalho 2019).…”

Maternal caretaking behavior towards a dead juvenile in a wild, multi-level primate society

“…Describing and studying these responses can clarify adaptive and possible maladaptive behaviors in various species, as well as the underlying biological (including psychological) mechanisms in death-related responses 1 – 3 . The comparative thanatology literature includes many reports of responses toward corpses in nonhuman primates: prosimians, monkeys and apes 2 – 4 . In particular, care and transport of dead infants by their mother and sometimes other individuals has been described in a range of primate species including, among others, capuchin monkeys ( Sapajus sp.)…”

Staring death in the face: chimpanzees' attention towards conspecific skulls and the implications of a face module guiding their behaviour

“…Several published reports describe chimpanzees' reactions towards their dead, ranging from affiliative to aggressive and from quiet/passive to loud/expressive [15][16][17][18][19][20][21][22]. Indeed, chimpanzee mothers have been observed carrying their dead infants for days, weeks or months [23], a pattern commonly observed in many females across the primate order [24,25]. While these publications have contributed to our knowledge of Pan thanatology, we still know very little about how chimpanzees engage with skeletons in their natural environments.…”