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.

We searched the scientific literature for cases of primate mothers responding to the corpse of their dead infant. Cases were cross-referenced using three published reviews [5,7,28]. We included only events in which there was enough opportunity for the mother to carry the corpse [5]. Specifically, we recorded a case of ‘corpse not carried’ if the mother was in the vicinity of the infant when the death occurred and the corpse was not consumed or monopolized by other individuals or removed by observers after the death, but the mother did not carry it. Additionally, we classified attempted but unsuccessful lifting (e.g. [30,31]) as ‘corpse not carried’ to avoid interpretation of underlying motivation. Our definition thus does not differentiate between mothers who are unable or unwilling to carry their young. For each case, we recorded 10 variables where possible: (1) the species; (2) the site where the case was reported; (3) whether the corpse was carried or not; if carried, (4) the carry duration (in days); the mother’s (5) parity, (6) age and (7) rank; (8) the infant’s age; (9) the cause of the death; and (10) the living condition (wild, provisioned, laboratory or captive). In cases where the exact duration was not known, we used the minimum (where > N) or maximum (when <N) confirmed carrying days or the mid-point of a stated range. We also included the minimum carry duration in cases where the corpse was removed after the mother had carried. We classified maternal age in two categories (young versus old) to make them consistent across studies. In the few cases in which infant age was not reported precisely, we took the mid-point of a range, or N + 1 or N−1 if it was reported as >N or <N days, respectively. Infant ages were divided by the species weaning age to make them comparable across species. We also compiled data on additional variables to test further hypotheses. These additional variables included information about the species or the site. Specifically, we recorded the: (1) daily travel distance for the species at the site; species’ (2) degree of terrestriality, (3) body mass and (4) level of maternal investment; and the site’s (5) maximum temperature and (6) climate type. See Electronic Supplementary Material, §2.1, of this paper (https://doi.org/10.1098/rspb.2021.0590) for details of how these variables were measured and of resources from which they were obtained.

The readme file contains a description for each of the variables in the thanatobase, their units, and the allowable values. NA =  value not available. The readme file also includes a description of other files that can be found here and that were used in the analyses. Finally, there is an specific readme file for the additional varibles (variables used to calculate some variables of the thanatobase).