Evolutionary thanatology

Prairie voles as a model for adaptive reward remodeling following loss of a bonded partner

Loss of a loved one is a painful event that substantially elevates the risk for physical and mental illness and impaired daily function. Socially monogamous prairie voles are laboratory-amenable rodents that form life-long pair bonds and exhibit distress upon partner separation, mirroring phenotypes seen in humans. These attributes make voles an excellent model for studying the biology of loss. In this review, we highlight parallels between humans and prairie voles, focusing on reward system engagement during pair bonding and loss. As yearning is a unique feature that differentiates loss from other negative mental states, we posit a model in which the homeostatic reward mechanisms that help to maintain bonds are disrupted upon loss, resulting in yearning and other negative impacts. Finally, we synthesize studies in humans and voles that delineate the remodeling of reward systems during loss adaptation. The stalling of these processes likely contributes to prolonged grief disorder, a diagnosis recently added to the Diagnostic and Statistical Manual for Psychiatry.

Dead-infant carrying by chimpanzee mothers in the Budongo Forest

It has been suggested that non-human primates can respond to deceased conspecifics in ways that suggest they experience psychological states not unlike humans, some of which could indicate they exhibit a notion of death. Here, we report long-term demographic data from two East African chimpanzee groups. During a combined 40-year observation period, we recorded 191 births of which 68 died in infancy, mostly within the first year. We documented the post-mortem behaviour of the mothers and describe nine occasions where Budongo chimpanzee mothers carried infants for 1-3 days after their death, usually until the body started to decompose. We also observed three additional cases of extended carrying lasting for more than 2 weeks, one of which was followed by the unusual extended carrying of an object and another which lasted 3 months. In each case, the corpses mummified. In addition, we report four instances of recurring dead-infant carrying by mothers, three of whom carried the corpse for longer during the second instance. We discuss these observations in view of functional hypotheses of dead-infant carrying in primates and the potential proximate mechanisms involved in this behaviour.

Putrescine--a chemical cue of death-is aversive to chimpanzees

As in many other species of nonhuman primates, chimpanzee mothers with a dead infant may continue to care for and transport the infant for days, weeks, or even longer. The bereaved females do this despite what humans perceive as the foul odour from the putrefying corpse. Putrescine is a major contributor to the "smell of death," and it elicits behaviours aimed at getting rid of the source of the smell, or escape responses in mammals including humans. However, it has never been shown that the odour of putrescine is aversive to chimpanzees. To address this question, we visually presented six adult chimpanzees with the corpse of a small bird, or a stuffed glove, in association with putrescine, ammonia, or water, and recorded the chimpanzees' reactions. The apes spent significantly less time near the object when it was paired with putrescine than the other substances, although they showed no signs of increased arousal or anxiety. We interpret the findings as evidence of an aversion to the smell of death in chimpanzees, discuss the implications for understanding the continued maternal-like behaviour of bereaved female chimpanzees, and suggest future research directions for the field of comparative evolutionary thanatology.

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

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.

Issues of theory and method in the analysis of Paleolithic mortuary behavior: A view from Shanidar Cave

Mortuary behavior (activities concerning dead conspecifics) is one of many traits that were previously widely considered to have been uniquely human, but on which perspectives have changed markedly in recent years. Theoretical approaches to hominin mortuary activity and its evolution have undergone major revision, and advances in diverse archeological and paleoanthropological methods have brought new ways of identifying behaviors such as intentional burial. Despite these advances, debates concerning the nature of hominin mortuary activity, particularly among the Neanderthals, rely heavily on the rereading of old excavations as new finds are relatively rare, limiting the extent to which such debates can benefit from advances in the field. The recent discovery of in situ articulated Neanderthal remains at Shanidar Cave offers a rare opportunity to take full advantage of these methodological and theoretical developments to understand Neanderthal mortuary activity, making a review of these advances relevant and timely.

Responses to death and dying: primates and other mammals

Although some definitions of thanatology-broadly definable as the study of death and dying-exclude nonhumans as subjects, recognition of the scientific value of studying how other species respond to sick, injured, dying and dead conspecifics appears to be growing. And whereas earlier literature was largely characterized by anecdotal descriptions and sometimes fanciful interpretations, we now see more rigorous and often quantitative analysis of various behaviors displayed towards conspecifics (and sometimes heterospecifics) at various stages of incapacitation, including death. Studies of social insects in particular have revealed chemical cues that trigger corpse management behaviors, as well as the adaptive value of these behaviors. More recent research on other taxonomic groups (including aquatic and avian species, and mammals) has sought to better document these animals' responses to the dying and dead, to identify influencing factors and underlying mechanisms, and to better understand the physiological, emotional, social and psychological significance of the phenomena observed. This special issue presents original short reports, reviews, and full research articles relating to these topics in New World monkeys, Old World monkeys and apes, as well as equids and proboscids. The range of events, data, hypotheses and proposals presented will hopefully enrich the field and stimulate further developments in comparative evolutionary thanatology.

Anthropomorphism in comparative affective science: Advocating a mindful approach

Anthropomorphism is the attribution of human-like capacities and traits to non-human entities. Anthropomorphism is ubiquitous in everyday life and in scientific domains, operating both implicitly and explicitly as a function of the human lens through which we view the world. A rich history of work in psychology, animal behavior, cognitive science, and philosophy has highlighted the negative and, to a lesser degree, the positive implications of anthropomorphism. In this article, we aim to provide a nuanced perspective of how anthropomorphism impacts the work of comparative affective science. Specifically, we discuss three domains of empirical inquiry in which lessons can be drawn about the benefits and pitfalls of anthropomorphism: responses to death, inequity aversion, and prosocial behavior. On balance, we advocate a mindful approach to anthropomorphizing in comparative affective science, and comparative science more generally.

Callitrichid responses to dead and dying infants: the effects of paternal bonding and cause of death

Many primates show responses to dead infants, yet testing explanations for these behaviors has been difficult. Callitrichids present a unique opportunity to delineate between hypotheses, since unlike most species, male caretakers form closer social bonds with infants than mothers. Callitrichids are also known to commit infanticide, leaving obvious wounds that may enable them to more readily recognize death. We present: (1) a case study of a wild common marmoset (Callithrix jacchus) group responding to an infant's natural death, and (2) a review of published infant deaths across callitrichids (N = 16), testing for trends in the sex of reacting individuals and cause of death. In our case study, several group members frequently interacted with the dead infant, attempting to carry it. However, the strongest response was from a male that remained with the corpse for ~ 3 h, despite his group leaving the area. Across callitrichid species, corpse interactions were significantly sex-biased: 100% (N = 6) of accidental deaths involved corpse interaction by males (p = 0.007), compared to 60% (N = 3 of 5) by females (p = 0.095). Cause of death also played a significant role, with individuals attempting to carry dead infants in 100% (N = 6) of accidental deaths, but only 11.1% (N = 1 of 9) of infanticides (p = 0.001). Although the available literature is small and potentially subject to publication biases, these data support the idea that visually obvious wounds may influence callitrichids' perception of dead conspecifics. Additionally, male-biased patterns of corpse interaction in callitrichids indicate that social bonds likely shape reactions to the dead, in addition to kinship. While published data on primate thanatology are limited, this study demonstrates quantitative approaches that can provide empirical insights into primates' responses to dead conspecifics.

Baboon thanatology: responses of filial and non-filial group members to infants' corpses

What do animals know of death? What can animals' responses to death tell us about the evolution of species' minds, and the origins of humans' awareness of death and dying? A recent surge in interest in comparative thanatology may provide beginnings of answers to these questions. Here, we add to the comparative thanatology literature by reporting 12 cases of group members' responses to infants' deaths, including 1 miscarriage and 2 stillbirths, recorded over 13 years in wild Namibian chacma baboons. Wild baboons' responses to dead infants were similar to other primates: in general, the mother of the infant carried the infants' corpse for varying lengths of time (less than 1 h to 10 days) and tended to groom the corpses frequently, though, as in other studies, considerable individual differences were observed. However, we have not yet observed any corpse carriage of very long duration (i.e. greater than 20 days), which, though rare, occurs in other Old World monkeys and chimpanzees. We hypothesize this is due to the costs of carrying the corpse over the greater daily distances travelled by the Tsaobis baboons. Additionally, in contrast to other case reports, we observed male friends' 'protection' of the infant corpse on three occasions. We discuss the implications of these reports for current questions in the field.

Primate thanatology and hominoid mortuary archeology

In recent years, a thanatology of primates has become a respectable research topic, and although still sparse, observations among several taxa have shown how complex responses to the dead can be. In human evolutionary archeology, re-analysis of old ‘burial’ sites is slowly revising our view on the development of specifically human responses to the dead. We propose here the means of integrating information from the two disciplines of primatology and archeology, in support of the field of primate thanatology. We propose a terminology and a shared set of research questions, from which we generate a number of observations that can be utilized in the field, in order to establish a working dialogue and foster greater collaboration across the two disciplines.

Do chimpanzees (Pan troglodytes) console a bereaved mother?

Comparative thanatology encompasses the study of death-related responses in non-human animals and aspires to elucidate the evolutionary origins of human behavior in the context of death. Many reports have revealed that humans are not the only species affected by the death of group members. Non-human primates in particular show behaviors such as congregating around the deceased, carrying the corpse for prolonged periods of time (predominantly mothers carrying dead infants), and inspecting the corpse for signs of life. Here, we extend the focus on death-related responses in non-human animals by exploring whether chimpanzees are inclined to console the bereaved: the individual(s) most closely associated with the deceased. We report a case in which a chimpanzee (Pan troglodytes) mother experienced the loss of her fully developed infant (presumed stillborn). Using observational data to compare the group members’ behavior before and after the death, we found that a substantial number of group members selectively increased their affiliative expressions toward the bereaved mother. Moreover, on the day of the death, we observed heightened expressions of species-typical reassurance behaviors toward the bereaved mother. After ruling out several alternative explanations, we propose that many of the chimpanzees consoled the bereaved mother by means of affiliative and selective empathetic expressions.

Reaction to allospecific death and to an unanimated gorilla infant in wild western gorillas: insights into death recognition and prolonged maternal carrying

It is still unclear to what extent animals possess knowledge of death. Primates display a large variety and often contradictory behaviors toward conspecific corpses, particularly those of infants (e.g., prolonged carrying and care). This study reports on reactions in a wild, habituated western gorilla group (Gorilla gorilla, 11-13 individuals) in the Central African Republic to an unanimated conspecific infant, and to an allospecific corpse. Individuals' reactions were compared to their usual behavior using both continuous focal animal sampling and 10-min instantaneous scan sampling. In the first observation, an infant gorilla fell out of a tree and looked dead. The mother retrieved it and remained unusually close to another adult female, until the infant started to move again, almost 1 h later. Cases of infants regaining consciousness after almost-fatal accidents may act as positive reinforcement for continued carrying by mothers, which might be socially learned. In the second case, three immature gorillas reacted to a dead red river hog. For 20 min they stared at the corpse from tree branches above, while chest beating, defecating, and urinating several times. They showed fear and did not approach the corpse. These observations show that non-predatory species, such as gorillas, may be able to acquire and develop some knowledge about death even though they do not kill other vertebrates.

Responses to dead and dying conspecifics and heterospecifics by wild mountain gorillas (Gorilla beringei beringei) and chimpanzees (Pan troglodytes schweinfurthii)

Comparative ["evolutionary")] thanatology is devoted to investigating how animals respond to signs of death and dying, in conspecifics and other species. Responses to corpses often involve fear and confusion, and "deceased infant carrying" by females is widespread in nonhuman primates. Such behavior could result from "animacy detection malfunctions" (Gonçalves and Biro in Philos Trans R Soc (B) 373:20170263, 2018): corpses have attributes of animate agents, but-like objects-they do not act, but instead are acted upon by outside forces. Many or most nonhuman primates have core cognitive mechanisms for detecting animacy, but these might not resolve this paradox. Skeletons of conspecifics, seriously injured or ill individuals behaving oddly and not responding as expected to social acts and signals, and corpses, skeletons, and sick or injured individuals belonging to other species could trigger milder animacy detection malfunctions. A central question is whether any nonhuman primates learn from experience that death involves permanent loss of biological functionality and is universal. The relevant literature is mostly anecdotal or devoted to case studies, and this question is open. In response to calls for more information, I describe 25 cases of responses to corpses, skeletons, and mortally injured or ill individuals, both conspecifics and heterospecifics, seen during fieldwork on mountain gorillas (Gorilla beringei beringei) and chimpanzees (Pan troglodytes schweinfurthii). Observations were generally consistent with the animacy detection malfunction hypothesis, although cases of prolonged deceased infant carrying are problematic. Also, one case in gorillas apparently involved sympathetic concern for a dying individual, and sympathetic concern might have occurred in a chimpanzee case.

Scent of death: Evolution from sea to land of an extreme collective attraction to conspecific death

All living organisms must eventually die, though in some cases their death can bring life-giving opportunities. Few studies, however, have experimentally tested how animals capitalize on conspecific death and why this specialization would evolve. Here, we conducted experiments on the phylogenetically most closely-related marine and terrestrial hermit crabs to investigate the evolution of responses to death during the sea-to-land transition. In the sea, death of both conspecifics and heterospecifics generates unremodeled shells needed by marine hermit crabs. In contrast, on land, terrestrial hermit crabs are specialized to live in architecturally remodeled shells, and the sole opportunity to acquire these essential resources is conspecific death. We experimentally tested these different species' responsiveness to the scent of conspecific versus heterospecific death, predicting that conspecific death would have special attractive value for the terrestrial species. We found the terrestrial species was overwhelmingly attracted to conspecific death, rapidly approaching and forming social groupings around conspecific death sites that dwarfed those around heterospecific death sites. This differential responsiveness to conspecific versus heterospecific death was absent in marine species. Our results thus reveal that on land a reliance on resources associated exclusively with conspecifics has favored the evolution of an extreme collective attraction to conspecific death.