Chimpanzees and Bonobos Have a Human-Like Understanding of Death

Our understanding of what death and dying entail has long been viewed as one of the characteristics that makes humans unique [1- 2]. This understanding is termed a “Concept of Death” (CoD). It is unknown how early in human evolution the CoD arose — whether it is restricted to our species or more widely present in primates [3-4]. I was interested in whether a comparative evolutionary perspective could shed light on this question. In a biological anthropological context, a comparative approach means utilising observation and analysis of living non-human primates to help differentiate between biological and cultural drivers of human behaviour. Since we are the only remaining member of our genus, Homo, comparative primatology helps determine what traits stem from our shared primate heritage and what is uniquely human. Suppose the CoD evolved early in our lineage. This could help contextualise ancient hominin behaviours, offer alternate explanations for findings in the fossil record, or even spur a rethink of the possibility of pre-Homo sapiens burials.

Chimpanzee mothers with infants. Image by Suju-foto from Pixabay

As part of a supervised research project, I utilised this comparative approach to investigate the CoD in our closest living relatives: the two members of the genus Pan. A CoD in the chimpanzee (P. troglodytes) and the bonobo (P. paniscus) would increase the probability that a CoD was also possessed by their last common ancestor with humans. This would then imply an early origin in our evolutionary story — perhaps associated with adaptations to increasing group size. Due to several factors, the most challenging being that death does not happen on command when you have a good project idea, I could not collect my own primary data. Instead, I was restricted to other researchers’ opportunistic observations of Pan behaviours surrounding death. I thus collated and systematically reviewed decades of these videographic, written, and oral records. I analysed behaviours through a methodological framework I adapted from studies of the CoD in human infants and children. I found that chimpanzees and bonobos appear to have a simple but multifaceted CoD, including clear comprehension of death’s biological characteristics and some understanding of its more metaphysical aspects.

Chimpanzee and Bonobo Social Behaviour

Sociality and relationships are intimately connected to aspects of social cognition such as the CoD. It is thus essential to have some basic knowledge of relevant chimpanzee and bonobo social structure and behavioural flexibility. The chimpanzee and bonobo both form large multi-male and multi-female groups that occupy specific territories [5-6]. Their everyday relationships can reach a depth of “bondedness” only found in reproductive pair bonds in other species [7]. Chimpanzees and bonobos regularly show intense interest in the genitals of their group members, with interactions often involving mutual genital inspection, smelling, and grooming [5,8]. Their interest in genitals is second only to their interest in each other’s faces [9]. Both species have extremely hierarchical societies, although chimpanzees are more likely to reinforce their hierarchies with aggression and dominance displays [5,9]. Bonobos maintain more tolerant societies and utilise sex as a social tool for conflict resolution [10]. Mothers of both species are known to continue to carry their infants after death [8].

Subcomponents of the Concept of Death

The CoD in non-human animals has often been contested due to a lack of consistent definitions [2]. However, research on the CoD in children has a long and consistent history [11]. When assessing the development of the CoD in children, researchers break the CoD into seven subcomponents: 1) non-functionality (death means the cessation of bodily and mental functions); 2) irreversibility (once an organism is dead, it cannot be returned to life); 3) universality (death happens to, and only to, living things); 4) inevitability (death happens to all living things); 5) personal mortality (death will happen to me); 6) causality (what causes death); and 7) unpredictability (the timing of death cannot be known in advance) [12].

I consider inevitability and mortality to be sub-aspects of universality, as understanding death’s universality implicitly comprises understanding that this includes yourself, a living thing, and excludes inanimate objects. I also consider causality and unpredictability a cognitive step beyond the fundamental CoD. Therefore, to investigate the CoD in genus Pan, I collapsed these seven subcomponents into only three: 1) non-functionality, being the understanding that death results in the complete cessation of bodily and mental functions; 2) irreversibility, being the understanding that once an organism is dead, it cannot be returned to life; and 3) universality, being the understanding that death also happens to others — this includes only living things and all living things, including oneself.

Behavioural Indicators of the Concept of Death

Research into the pace and pattern of the development of the CoD in human children relies on language and interviews [12-13], so I had to create non-linguistic behavioural equivalents for each criterion.

I recorded individuals as understanding non-functionality when they treated deceased group members’ bodies in ways they never would when alive. These included incidences of post-mortem cannibalism and cases where mothers carried deceased infants in atypical positions that would cause injury if the infant was still alive, such as gripping in the mouth or dragging by a limb [16-20]. I also recorded individuals as understanding non-functionality when they performed deliberate checks for functionality, such as hitting the body [21], lifting and dropping limbs [8,16,19], sniffing genitals [22-23], or prying open the mouth to check for signs of breathing [8,21]. It must be noted here that an organism’s understanding of non-functionality can only be as complex as their understanding of functionality, e.g., a chimpanzee cannot be expected to check for cessation of brain activity, as they do not understand this to be a necessary part of life.

That chimpanzees and bonobos ceased their efforts to wake or revive dead group members after receiving no response indicated that they understood death, unlike sleep, is irreversible. I also recorded individuals as understanding irreversibility when they exhibited strong emotional responses after receiving no indications of life. I observed a variety of such responses, including whimpering [24], screaming [25], rocking back and forth [8], tearing out hair [25], disturbed sleep [21], and refusal of food [8]. Some older female chimpanzees had gentler, although still emotional, reactions, such as grooming and cleaning the body or keeping overnight protective vigils [8, 21-23]. One unique indication of universality was seen after a group of chimpanzees who had earlier killed a rogue group member returned to the scene to find the body removed by human researchers [19]. When the group discovered the disappearance, they showed fear and made alarm calls, indicating they understood both that the dead cannot move and that this non-functionality is irreversible—the dead should not suddenly return to life, get up, and walk away.

I found behavioural indicators of universality much harder to identify, as this subcomponent is less about an organism’s immediate reaction to a death, which can be observed, and more about a mental transference of that death’s implications to future situations. One incident that may indicate a rudimentary understanding of universality occurred after a mother chimpanzee lost an infant to illness [26]. She became overly attached to her remaining child, a six-year-old, and began treating him like a baby — carrying him on her back, hand-feeding him, and sharing her night nest with him — as if she were afraid he too might die. Reacting to the deaths of other species can also indicate some degree of universality, as the individual is showing they can apply their understanding of death more broadly. A group of chimpanzees who encountered a dying baboon became very agitated — making alarm calls and sniffing, stroking, and grooming the body [9, 24]. Both chimpanzees and bonobos were also observed acting differently towards snakes after their death. They let infants and juveniles use the bodies as toys, rather than exhibiting their usual fear and avoidance [8-9]. I was hoping to find evidence of individuals who witnessed an accidental death becoming increasingly cautious when later navigating the same dangerous environment, indicating an understanding that the same death could happen to them. However, the opposite was observed: after seeing a group member fall and break his neck, a second chimpanzee almost fell himself when vines gave way beneath him [24]. He showed no extra caution despite his group member’s death just hours earlier.

Chimpanzees and Bonobos Understand Death

When I brought these disparate incidents and behaviours together, it became clear that chimpanzees and bonobos have a complex CoD, including a cogent understanding of the biological subcomponents of non-functionality and irreversibility and at least some degree of comprehension of the more metaphysical subcomponent of universality. There is abundant evidence for non-functionality and irreversibility: chimpanzees and bonobos deliberately examine bodies for signs of life and have strong emotional reactions, analogous to grief in humans, upon receiving no response. In no case did I note a chimpanzee or bonobo continuing their efforts to wake or revive a body for any significant period after receiving no response. Adolescents and juveniles were seen to investigate bodies the longest, whereas older group members, who have likely encountered death before, interacted with the dead for a far shorter time. This difference suggests that the Pan CoD is learnt, rather than innate, and thus close in nature to the human CoD, which is developed via experience and teaching. However, I did not find satisfactory evidence of universality in chimpanzee or bonobo behaviours. This was unexpected, as universality is the first of the three core subcomponents to develop in human children [12]. It is possible that universality may have been absent from my data due to behaviours imperfectly reflecting underlying thought processes and not due to an absence in cognitive capacities.

Image by Sasint from Pixabay.

Shared Origins of the Human and Pan Concept of Death

One common thread in my research was that the individuals most affected by each death were those emotionally closest to the deceased. One chimpanzee, who died of illness, was a highly social individual who spent time with many different subgroups — accordingly, most of the group was interested in and interacted with his body [23]. Even then, the two individuals most affected were his closest friend, who visited his body more than any other male, and his adoptive aunt, who groomed his body, cleaned his teeth, and kept vigil after everyone else had long since left. Conversely, after the death of a low-ranked and socially peripheral female, the only group member to spend any time near the body was her daughter [27]. Infants are also socially peripheral, having not yet formed any social networks. Unsurprisingly, in cases of infant death, only the mothers had any noticeable emotional response [8,16,22]. Pan behaviours around death appear to be simply a translation of the bonds created in life.

The Pan CoD also appears to be more of an emotional reaction than categorisable behaviour. Many scholars criticise anthropomorphism, but I believe that anthropodenial, or the rejection of similarities between humans and our close relatives to keep us on an evolutionary pedestal [8], is worse. If two closely related species act similarly under similar circumstances, it is reasonable to theorise that they are similarly driven. Therefore, I describe this emotional response to death as grief, and the behaviours that stem from it, such as grooming and keeping vigil, as mourning. In humans, grief is an emotion, a feeling of sorrow caused by distress over a loss, with mourning then being the social behaviours exhibited in response to that grief [28]. As the Pan CoD appears rooted in grief and mourning, it is reasonable to term it a socially driven phenomenon. This may help to contextualise behaviours throughout the hominin family tree. The socially driven CoD seen in chimpanzees, bonobos, and humans likely evolved as an adaptation to protect against destabilisation caused by death. The more communal a species, the more effort is needed to protect against social destabilisation. Chimpanzees and bonobos, like humans, are highly social animals to whom a defined hierarchy is vital for stability. Death impacts social groups by severing bonds, thus creating a rupture in the social fabric: the most gregarious animals have the most mourners as they had numerous strong bonds in life. The CoD evolved because it is needed to function as a social stabiliser. If a species develops the ability to understand death, then they can feel grief. If a species can feel grief, then they can begin to mourn. If a species can mourn, then they can more quickly re-categorise the living to dead, reform the social structure, and shape a new dominance network after death has left a hole in the hierarchy.