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Susana Monsó: What animals think of death

It is time to rethink human exceptionalism. We now have solid evidence of culture, morality, rationality, and even rudimentary forms of linguistic communication. The concept of death should also be counted among those characteristics to which we can no longer resort to convince us of how very special we are.

SUSANA MONSO: When the Virginia opossum feels threatened, she plays dead… To all outward appearance, she is no different from a corpse. In this deathly state, she waits. The opossum is aware of her surroundings, monitoring the present danger: a coyote in search of food… The danger passes. Then the opossum springs back into action, unscathed and unfazed, and goes about the rest of her day. The trick worked…

Despite the persuasive performance of death, no one would assume that the opossum herself believes that she’s playing dead. Her behaviour is most likely automatic, and the opossum no more knows that she’s disguising as a corpse than a stick-insect knows that she looks like a stick. While this presupposition is probably right, it would nevertheless be wrong of us to assume that there’s nothing to learn about animals’ concept of death from the opossum’s display. In fact, her little show is one of the best pieces of evidence we have of how widely distributed in nature the concept of death is likely to be…

Humans have long thought of themselves as the only animal with a notion of mortality. Our concept of death is one of those characteristics, like culture, rationality, language or morality, that have traditionally been taken as definitional of the human species – setting us apart from the natural world and justifying our boundless use and exploitation of it. However,… the widespread notion that only humans can understand death stems from an overly complex view of this concept. The human concept of death is not necessarily the only concept of death.

Understanding death does not require grasping its inevitability or its unpredictability, nor does it require understanding that death applies to all living things or being familiar with its underlying physiological causes. In minimal terms, the concept of death is simply made up of two notions: non-functionality and irreversibility.

This means that all an animal needs to grasp in order for us to be able to credit her with some understanding of death is that dead individuals don’t do the sorts of things that living beings of her kind usually do (ie, non-functionality) and that this is a permanent state (ie, irreversibility). This minimal concept of death requires very little cognitive complexity and is likely to be very widespread in the animal kingdom.

The opossum’s death display, also known as thanatosis, is an excellent demonstration of this, not because of what it tells us about the opossum’s mind, but because of what it shows us about the minds of her predators: animals such as coyotes, racoons, dogs, foxes, raptors, bobcats and large snakes… Many animals, when they feel threatened, go into a kind of paralysis that reduces the probability of being preyed upon. This is known as tonic immobility and can be found in a wide range of species, from insects to humans…

Although thanatosis might have evolved from tonic immobility, it’s much more than a mere paralysis: the animal is feigning death. The Virginia opossum is probably the animal with the most elaborate thanatosis display (hence the expression ‘to play possum’), but not the only one to exhibit a behaviour deserving this label. Some frogs engage in thanatosis, whereby they stop responding to all interactions while lying still, with their eyes open and their limbs extended and flaccid… She even secretes blood from her mouth. You could poke her with a stick or lift her up and she would not react…

Thanatosis as an anti-recognition mechanism works by making the prey appear unpalatable to the predator. In principle, this could ride on a simple disgust mechanism, since thanatosis is often accompanied by urination and defecation, or by other chemical defences, such as the frogs’ ammonia-like breath, which the predator might find yucky…

Why the need, then, to stay still, reduce her vital functions, display a blue tongue, and so forth? The opossum’s thanatosis doesn’t appear to be for generating disgust, but for generating the appearance of being dead. It’s true that many, if not most, animals come with a hard-wired tendency towards necrophobia, which is a stereotypical aversion to corpses that is not mediated by a concept of death. This means that most animals are disgusted by the smell or the appearance of corpses, even if they do not understand what a corpse is.

This unlearned aversion serves an important protective function for the organism, since dead bodies are a paradise for pathogens. Could it be the case, then, that thanatosis works by activating the predators’ necrophobia? This would again mean that it would not give us any evidence of the predators’ possession of a concept of death, since it would simply exploit predators’ non-conceptual and innate aversion to corpses…

The complexity of thanatosis, and its clear resemblance to the characteristics of corpses, therefore makes it unlikely that no cognitive processing of the prey’s corpse-like appearance on behalf of the predator plays a role. If thanatosis evolved to exploit predators’ necrophobia, it is probably a necrophobia that is mediated or supported by a concept of death.

The opossum’s trick would work not just because she looks disgusting, but because she looks disgusting because she appears dead (and the predator has learned that dead prey taste bad or upset the stomach). In such a case, the predator’s concept of death would be involved in the success of the mechanism…

My point is not to settle the question of the concrete reasons why natural selection has favoured the evolution of thanatosis as a defence mechanism. What I want to argue is that, if thanatosis has evolved, it’s because there are some advantages – regardless of the concrete form they take – to appearing specifically as though one were dead…

This does not mean that opossums themselves necessarily have a concept of death, or that they behave this way with the intention of being mistaken for a corpse. On the contrary, it appears to be a genetically inherited behaviour that does not require any learning and that is triggered automatically upon the detection of certain stimuli. What this does mean, however, is that the predators’ concept of death was the likely selection pressure that shaped these displays.

Maybe opossums lack a concept of death, but we can be pretty sure that the animals who intended to feed on them throughout their evolutionary history did tend to have one… Regardless of the concrete evolutionary history that gave rise to thanatosis, this behaviour provides us with a window into the minds of predators. Thanatosis shows us that, for whatever reason, predators are less likely to eat animals that they themselves conceive as dead.

Throughout their evolutionary history, those opossums who were better capable of imitating a corpse, and thus of fooling predators into thinking they were dead, would have higher chances of surviving until reproduction, thus favouring higher levels of sophistication and complexity in this behaviour in successive generations. Any explanation of the defence function of thanatosis must necessarily incorporate the concept of death…

That thanatosis is indicative of a concept of death in predators is further supported by the fact that this defence is unlikely to work against specialised predators, for these would have evolved an appropriate response. Instead, we expect it to work against generalist predators who don’t encounter this prey too often and are not familiar with their little trick.

For it to work against generalist predators, they have to have a concept of death, that is, not just the capacity to react to certain stimuli that are associated with death, but a concept that can be applied to different species. Only with a concept can a predator mistake for dead an animal that she has never encountered before. The distribution of thanatosis in the animal kingdom points to how extended the concept of death is likely to be in nature. We find elaborate forms of thanatosis in some species of amphibians, reptiles, birds and mammals…

The concept of death, far from being a uniquely human feat, is a fairly common trait in the animal kingdom. We humans like to think of ourselves as a unique species. However, little by little, all those traits that we have been relying on to ground this uniqueness have been falling, as the science advances and reveals the staggering diversity and complexity of animal minds and behaviour.

We now have solid evidence of culture, morality, rationality, and even rudimentary forms of linguistic communication. The concept of death should also be counted among those characteristics to which we can no longer resort to convince us of how very special we are. It is time to rethink human exceptionalism, and the disrespect for the natural world that comes with it. SOURCE…

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