In the summer of 2018, a baby albino chimpanzee was spotted in the Budongo Forest Reserve in Uganda, the first to be seen in the wild. With his white fur and pale skin, the chimpanzee provoked an immediate rousing in the rest of the group. Other chimps made alarm calls and “waa barks,” noises that usually signal an encounter with a potentially dangerous animal.
On July 19, adult chimpanzees killed the baby. It was a tragic incident, recalled Susana Monsó, a philosopher at the Universidad Nacional de Educación a Distancia in Madrid and the author of Schrödinger's Possum. But what she found most striking was how the chimps behaved after the baby had died.
Though the primates had exhibited fear calls when the albino was alive, once he died, they stopped. Then they readily approached him, inspected his fur and body, and groomed his back.
For Monsó, who has been studying whether animals have a concept of death, this incident provides a clue that animals have some notion that death means, at the very least, “he's not going to move anymore.”
“When they first saw the baby, they expected something scary to happen,” she said. “Then, at the moment it died, they weren't scared by it at all. This means their expectations have shifted.”
We know that animals often behave in particular ways toward dead members of their own species. Ravens and crows gather and make loud calling noises. Chimpanzees in the Taï Forest in Africa have been seen covering dead bodies with leafy branches. In 2015, when a wild female chimpanzee died, the male she had been in a relationship with for three and a half years prevented young individuals from approaching her while he “performed several close-contact and caretaking behaviors.” Some primate mothers carry the body of their dead infant for days or weeks, or eat parts of the mummified corpse. Elephants have been seen gathering around, interacting with, or carrying the bodies of their babies. Dolphins sometimes keep dead bodies afloat, and in 2011 a beluga whale mother carried her dead calf for around a week.
A field called comparative thanatology documents these practices, and compares how different species interact with death and the dying. Hanging over this research are more philosophical questions: What do these behaviors really mean? Are animals acting in instinctive, hormonal, and unaware ways? Or, when they interact with their dead, do they have some level of understanding of the concept of death?
When interpreting animal behavior, there's always the risk of anthropomorphism, or projecting human-like emotions and thoughts onto nonhuman animals. But there could still be ways to probe whether animals have a concept of death with philosophy's help, by defining what a concept of death is at a bare minimum, and combining observations of animals in the wild with experiments in the lab.
Learning whether animals can grasp such concepts will help us to better understand their minds, and it could have important implications for the ways we treat them. But grappling with the concept of death is a trait long considered to belong to humans alone. Showing that animals can grasp it too—even on a smaller scale—would mean we’re not alone in engaging with our mortality.
There is nothing more human than being anguished by death—or asking, as Leo Tolstoy did, “Is there any meaning in my life that wouldn’t be destroyed by the death that inevitably awaits me?”
But from the ancient world to the Enlightenment and onward, philosophers and scientists have had mixed views on whether we share this trait with nonhuman animals, since having a concept of death is tied up with larger questions around animal consciousness.
Aristotle thought that humans were different from other animals because we have a “rational soul,” whereas animals had “sensitive souls,” which could respond to sensory impressions but not have the capacity for rational thought. René Descartes was less generous: He believed that animals were just “mechanisms” or “automata,” not much different than a complex cuckoo clock. “There is none that leads weak minds further from the straight path of virtue than that of imagining that the souls of beasts are of the same nature as our own,” he wrote.
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Italian philosopher Giambattista Vico wrote that a human custom that separates humans from animals is burial of the dead, Baron de Montesquieu wrote animals can suffer from death but don’t know what it is, and Arthur Schopenhauer claimed animals live in the present and only "know" of death when it happens to them, while humans reminisce about the past and anticipate the future with the knowledge of their own mortality. As Jean-Jacques Rousseau wrote, “An animal will never know what it is to die, and the knowledge of death and its terrors is one of the first acquisitions that man has made in moving away from the animal condition.”
“The list goes on into the 20th century with philosophers such as Martin Heidegger, anthropologists like Ernest Becker, or biologists like Theodosius Dobzhansky making similar claims,” said André Gonçalves, a researcher at the Primate Research Institute at Kyoto University in Inuyama, Japan. “The history of how animals respond to death is a long one, albeit scattered and mostly confined to footnotes, from Aristotle to Darwin to the present.”
This history likely influenced those who later observed animals responding to death. For most of the 20th century, behaviors like dead-infant carrying were viewed as animals not being able to tell the difference between the living and the dead, and not worth investigating. “Monkeys and apes do not recognize death, for they react to their companions as if the latter were alive but passive,” wrote the primatologist Solly Zuckerman in 1932.
Because of this view, there was little attention paid to what animals did with their dead until 2010, when a publication described the death of an elder female chimpanzee. Humans observed pre-death care of the chimpanzee, other chimps testing for signs of life at the moment of death, the female chimpanzee’s adult daughter staying by her all night, her corpse being cleaned, and, later, the place where she died being avoided.
“Without death-related symbols or rituals, chimpanzees show several behaviors that recall human responses to the death of a close relative,” wrote professor of psychology James Anderson and his colleagues at Kyoto University. “Are humans uniquely aware of mortality? We propose that chimpanzees’ awareness of death has been underestimated.”
For the past 15 years, the field of comparative thanatology has taken up this investigation in earnest. (In Greek mythology, Thanatos was the personification of death.) It has focused on cataloging exactly how animals respond to death, and comparing between species, and being open to the idea that these responses aren’t just “automata.”
A paper from 2019 described how humans removed a dead infant bonnet macaque from its mother, who then regularly visited its burial spot for at least two days. Chimpanzees have been observed in what’s called “stunned silence,” when their usual calling noises stop after the death of a chimp.
After the death of the adult female chimp that Anderson observed, he wrote, “The next day, the three surviving chimpanzees were profoundly subdued. From the day area they watched silently as two keepers lowered Pansy from the platform, carried her into the exit corridor, placed her in a body bag, and loaded her into a vehicle that was then driven away. They remained subdued the following day as the night area was cleaned.”
Many agree that great apes and also monkeys show compassionate care for the dying, but whether they have an understanding of death is uncertain. Plenty of thanatologists have instead come to the opposite conclusion: that the animals they observe do not have a concept of death. As Charles Darwin wondered in The Descent of Man, “Who can say what cows feel, when they surround and stare intently on a dying or dead companion?”
“I hold a semi-agnostic position in relation to other species having a concept of death,” Gonçalves said. “Nonhuman social animals are not always wholly indifferent to death; they have reactions and perform all sorts of behaviors surrounding it, and I think these merits further investigation.”
Understanding the “concept” of death is different from being able to classify or distinguish the dead from the living. Ants perform “necrophoresis,” which is when they remove dead ants from their nests—meaning they can tell which ones are dead and which are alive. What the ants are detecting is not the concept of death but a chemical called oleic acid that dead ants produce. It’s been shown that if you put oleic acid on any object in the nest, other ants will remove it.
Other animals have similar discrimination skills, which are not a conceptual understanding. This is where philosophy can provide guidance, according to Monsó. To ask whether animals have a concept of death, it first requires defining what a “minimal concept” of death would be—or what are the minimum requirements an animal would need to meet for us to conclude they know what it is.
Humans have a complex concept of death, weighed down by cultural baggage and myriad emotional responses. This is part of why many academics may not believe that animals can understand the concept of death, said Jennifer Vonk, a comparative psychologist at Oakland University. There hasn’t been much evidence that nonhuman animals can represent abstract, unobservable constructs.
But just like when we try to assess whether animals possess some sort of language or communication skill, we don’t start by asking whether they can write sonnets. We break language down into its fundamental parts and ask if animals have a cognitive grasp on those first.
Monsó started with building blocks of death that come from developmental psychology studies where human children are interviewed about death. Those subcomponents of death are: non-functionality, irreversibility, universality, personal mortality, inevitability, causality, and unpredictability. Some elements, like inevitability and personal mortality, are certainly part of a human’s concept of death, but Monsó argued that the essence of a rudimentary concept of death doesn’t need to include them. At its core, Monsó proposed, just non-functionality and irreversibility are fundamental. This would mean that an animal understands that death makes an individual not functional, and that its non-functioning is permanent.
Proposing a definition for exactly what a minimal concept of death is from a philosophical perspective could help those who do comparative thanatology be specific about what they’re looking for, she said. Monsó thinks it’s likely that this bare minimum could be achieved in many species. After all, death is common in nature, and there could be evolutionary advantages to understanding what it means to die, or to know another is dead.
But this is far from agreed upon. In a paper from last year on the behavior of animal mothers toward the body of their dead offspring, research scientist Arianna de Marco and her co-authors pushed back against the animals having a concept of death per se, instead suggesting that animals like great apes can understand something more vague: that “something serious has happened.”
They wrote that a great ape could understand that another animal has entered a state of “dormancy,” or is unlikely to regain wakefulness. Recognizing that another animal is “dormant” and won’t wake up can still elicit a powerful emotional response or behavior.
“However, there is no evidence that any nonhuman primates are aware of mortality,” they wrote.
Gonçalves and Vonk agreed that non-functionality and irreversibility are important components of the human concepts of death, and also that the concept of death is likely a continuum, with nonhuman animals finding themselves somewhere along it. But just because death is everywhere doesn’t mean it’s necessarily an advantage for animals to recognize it. In fact, humans’ recognition of their own mortality has led to psychological coping strategies, called Terror Management Theory.
“Nonhumans may recognize when an individual is no longer a functioning agent interacting with the world, but I would be surprised if they appreciated an end of consciousness or mental life in the same way that adult humans do, or if they recognize that all living beings die and that death is irreversible,” Vonk said. “That does not mean that they do not have a concept of death; it simply means that their concept of death may be more limited and less abstract than the human concept.”
If a mother chimp finally leaves her baby’s body behind, does that mean she understands irreversibility? If a group of elephants leaves their dead behind, does it mean they understand that it is dead forever and won’t be coming back? Or is it just that they’re frustrated and giving up?
“Like chimpanzees, elephants will often return to the corpse; how do we interpret this?” Gonçalves said. “Do they realize their group member is dead? Were they just passing by and happened on it by chance? Are they paying respects, not unlike humans do in funerals? Are they checking in to see if their group member recovered? While I'm more inclined to believe the last explanation, the truth is we can only guess what's going on in their minds.”
Outside of guessing, there are ways to try to test for a minimal concept of death. One is by observational studies: watching what animals do in response to the dead and making interpretations. The other is in the lab: setting up experiments that test how they either respond to the dead, or looking for cognitive abilities that might imply that could understand the concept of death—like the ability to recognize non-functionality and irreversibility.
Some studies like this have been done before. In 1973, an experiment showed mother squirrel monkeys with the dead bodies of their own and other infants. The mothers who had offspring that had died at an older age reacted more to the corpses. One study from 1964 tested the reaction of Rhesus monkeys to “fear-provoking stimuli,” including live snakes; an awake and alive monkey of their species; an anesthetized monkey; and a dead monkey that had been decapitated, holding its head in its hands. The results were unclear: The decapitated monkey did get more looks than the live one, but the overall looking time was higher for the live monkey. Since the study design didn't allow touching either the dead or alive monkey, it’s hard to make sense of it. There are obvious ethical dilemmas around such experiments, and Monsó said she wouldn't encourage such studies being done today.
Instead, Monsó proposed testing animals for being able to understand non-functionality and irreversibility through stand-ins like tools or machines that break irreversibility. One such study has just started using Goffin’s cockatoos at the University of Veterinary Medicine in Vienna, said Alice Auersperg, a cognitive biologist who heads the cockatoo lab.
“They are highly intelligent and have strong social bonds between individuals that can last for multiple years or even decades,” Auersperg said. “Moreover, they are able to use several types of tools which are very rare in animals yet important for our test setups.” In recent work, they showed that the cockatoos could use composite tools in an experimental setup inspired by the game of golf. The experiments won’t test for the concept of death directly but rather for the cognitive capacities that Monsó theorized are necessary to understand death.
Not everyone is convinced that this kind of study can tell us much about death. David Peña-Guzmán, a philosopher at San Francisco State University, agreed that non-invasive studies should be done, but he doesn’t think that animals would respond to machine or tool stand-ins in the same way as other animals.
“Animals don’t develop emotional attachments to the machines they are exposed to in a laboratory; they don’t incorporate them into their social dynamics or care economy; neither do they treat them as purposive social agents,” he said. “ In short, animals are not confused about the difference between the [mechanical] and the living.”
Monsó agreed that something needs to be alive before you can conceive of it to be dead, so a tool doesn’t fall into that category. But if combined with observational evidence of animals in the wild, it could make for a compelling case.
“Even if we're not talking about living functions, we are still in the neighborhood of the cognition you would need for the concept of death,” Monsó said.
Monsó believes that an outright assumption that animals can’t have even a minimum concept of death at all is a byproduct of anthropocentrism, or the centering of human thoughts and feelings and experience. She thinks there’s been too much of a focus on grief as a reaction to death, and that it clouds our interpretation of animals’ behavior.
When the research chimpanzee Washoe’s baby died, its body was removed. Washoe then signed to a researcher, “Baby?” The researcher signed back, “Baby dead, baby gone, baby finished.” According to the researcher, “Washoe dropped her cradled arms to her lap. She moved over to a far corner and looked away, her eyes vacant.”
It’s hard not to project feelings onto a scene like this. For humans, death is often paired with grief, and grief is distracting. Additionally, a fear of death and dying has led humans to ruminate on complex metaphysical themes, said Peña-Guzmán, like the directionality of time, the immortality of the soul, and reincarnation.
“Because of this, we tend to assume that only creatures who engage in such fancy philosophizing possess a death concept,” Peña-Guzmán said. “It is almost as if in thinking about death we automatically conjure up an image of a dejected human pondering the meaning of life, as in Vesalius’s sketch of a human skeleton gazing at a skull in De humani corporis fabrica.”
Peña-Guzmán agreed that researchers should try to look for the “core” of the concept of death, since the concept as we know it could include components that make sense to us as humans but are not essential.
If we are interested in animals’ relationship to death as a topic on its own, and not only in relation to humans, we have to also look way beyond practices that we can identify with. One example is when pets feed on their owners after they die. “This is an extremely common phenomenon, much more common than we want it to be,” Monsó said. Even with dogs, who have strong bonds with their owners, “we’ve seen examples of dogs eating their owners 45 minutes after the owner died and with food in their bowl.”
Monsó said the pattern of eating is also different than when a dog would be scavenging; when dogs scavenge, they usually eat the abdomen area first, but in these cases dogs focus on the face. “It’s a very disturbing behavior, but I think it's a super interesting one,” Monsó said. “But it's only discussed in forensic science papers. I think one of the reasons may be why it hasn't been deemed relevant until now has to do with the fact that it's not a behavior that we can really relate to.”
Gonçalves doesn’t agree that comparative thanatologists are conflating grief with the concept of death. “In 2013, Barbara King wrote in her book How Animals Grieve that grief does not presuppose a concept of death and has been reiterated many times since,” Gonçalves said. He said we shouldn’t look away from interesting phenomena out of a fear for anthropomorphism either, just as we shouldn’t ignore behaviors that don’t look or feel like grief to us.
Still, Gonçalves has seen articles that describe animals as having mourning rituals and understanding death (and said the Wikipedia page on animal grief is “absolutely dreadful”), and he thinks there's reason to be careful.
“There's no evidence currently that they do have anything that counts as a ritual,” he said. “If you ask any researcher dedicated towards the study of cultural aspects in nonhuman animals, I don't think you'll find any saying they do have so-called mourning rituals.”
Gonçalves advocated for field researchers using cameras to more objectively record entire interactions around death, and then making interpretations after the fact. Vonk and Georgia State University psychologist Sarah Brosnan, have proposed that a data repository be created where all responses to death could be recorded, and in 2020 anthropologist Alecia Carter created the “ThanatoBase,” where researchers can add their observations on primate death.
While Gonçalves doesn’t agree with many of Monsó’s claims, he does think she “explored more thoroughly the question of the concept of death in nonhuman animals than anyone that came before, and in doing so has perhaps uncovered a need for more careful delineations into what should count or not as good evidence for said concept.”
What if animals do know what it means to die? Does it change the way we should treat them? It might shift some of our responsibilities with the animals under our care. For instance, we could ask what are the cases when we should allow them to learn about death, and when we should give them an opportunity to understand what happens when another animal has died.
“Perhaps we have an ethical obligation to at least prevent animals in factory farms and laboratories from seeing or hearing other animals being killed, seeing dead bodies lying around, or experiencing markers of death,” Peña-Guzmán said.
Monsó also thinks we should allow animals their full reactions without interference. “I think that monkey mothers who want to cling to their babies should be allowed to do so for as long as they need,” she said. “This might conflict with the interest of a zoo, for instance, because it might be disturbing for the visitors to see the mother holding onto a decomposing corpse. But I think that the interests of the monkey should be weighed here.
For the pets in our homes, it could mean we have a “moral duty to ‘show up’ for animals when they experience death, to help them mourn when they are bereaved and to be at their side to reassure them when their own time has come,” Peña-Guzmán said.
Ben Bradley, a philosopher at Syracuse University, said there have been some philosophers who argue that the concept of death is necessary in order for death to be bad for you. As long as an animal’s life is painless, killing them is no harm since they don’t know what death means.
“If you can’t conceptualize something, then you can’t care about it, and so it can’t be bad for you,” he explained. “If this is right, then if animals don’t have a concept of death, their deaths aren’t bad for them. This would have important implications for how we treat animals, because it would imply that it is morally permissible to kill them for food, unless it were wrong for some reason other than being bad for the animals.”
Bradley thinks we should reject the claim that nothing can be bad for you unless you care about it. He wrote a book chapter on this called “Death Is Bad for a Cow,” and also a song of the same name, with the lyrics:
Listen to me and I will tell you how
When you take that cow to the butcher's knife
You deprive the cow of the goods the goods of her future life
Don't need to have a sense of self over time
Or know what it means to reach the end of the line
Death is a serious harm
Even if, even if you live on a farm.
Gonçalves said we shouldn’t wait until the concept of death is proven to try to treat animals in ethical ways. “We should prevent the infliction of unnecessary pain and suffering regardless of them having a concept of death or not,” Gonçalves said.
On a larger level, Monsó sees this work, and question, as continuing to chip away at the idea of human cognitive superiority over animals in all domains. “Whenever we can prove that there is continuity in a particular aspect of our mental lives in the mental lives of other animals,” she said, “it undermines any claims of human superiority that we use to justify our boundless exploitation of nature.”
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