Behind the Strange Behaviors of Animals Lies a Natural Selection Pressure for Evolution.
Whenever an opossum feels threatened, it plays dead. The opossum rolls over on the ground, curling up in a fetal position. It opens its eyes wide, gapes its mouth, and sticks out its tongue. The only task left is to remain motionless, unresponsive to anything happening in the world around it.
During this time, a more subtle process of death-like state occurs within the opossum’s body. Its body temperature drops, and both its breathing and heart rate decrease significantly. The opossum’s tongue, which is usually a light pink, turns a shade of bluish-green.
The sphincter muscles in its urinary and anal tracts relax, expelling urine and feces from its anus to amplify a foul odor. At this point, just by looking from a distance where the stench prevents you from approaching the opossum, it truly appears to be a dead body.
However, inside that façade, the opossum is still wide-eyed and patiently waiting.
All its senses remain alert to perceive the surrounding environment, directed towards a coyote scavenging for food. It turns out that the presence of this coyote is precisely the reason the opossum has played dead.
The characteristic of the coyote is that it only eats fresh meat and never touches decayed corpses. The opossum’s act has ultimately fooled the predator. Once the coyote moves far away, the opossum begins to stir, tidies itself up, and resumes its daily activities.
Once again, the play-dead strategy has proven effective.
Are Animals Aware of Death?
Despite the dramatic performance, no one can know from the opossum’s perspective whether it truly intends to play dead. If the opossum has no concept of death, its behavior might just be an automatic instinct. Who knows, in its mind, the opossum may simply be rolling over and pretending to be a rock.
While this assumption might be true, we have been mistaken in underestimating the awareness of death in animal species. In fact, this little performance of the opossum is one of the best pieces of evidence scientists have regarding the concept of death within the brains of mammals.
When this opossum plays dead, it truly deserves to be called an artist.
Humans have long considered themselves the only species with a concept of death. Death is equated with culture, reason, language, and morality – traits that only humans possess. These characteristics separate us from the natural world, justifying our rights to exploit and utilize nature without limits.
However, elevating our own species may simply be a consequence of complicating concepts, a process occurring in our brains that we think other animal species cannot comprehend.
For instance, regarding death. We assume that an animal that does not understand artificial respiration, does not know how to mourn, does not hold funerals, and does not soothe its pain by imagining afterlife realms, cannot have a deep awareness of death.
Nevertheless, the human concept of death is not necessarily the only concept of death present in nature, which other animal species must also share. At its simplest level, death consists of just two components: inactivity and irreversibility.
An animal only needs to understand these two concepts to have the capacity to comprehend death. Inactivity refers to the state of being motionless, unable to walk, move, or engage in activities like foraging or mating.
Irreversibility means that once inactivity occurs, it will persist indefinitely. Even if the animal waits for a while, for a few sunrises and sunsets, it cannot move again.
This minimal concept of death requires very little cognitive complexity, so it is likely that many animal species in the natural world also understand it. For example, the opossum’s play-dead behavior is an excellent testament to this.
Scientists call it thanatosis, which in Greek means “death“. However, the fact that the opossum plays dead does not mean it knows what death is; in a more convincing sense, it suggests that predators, such as coyotes, mastiffs, dogs, foxes, birds of prey, bobcats, and large snakes, are the ones that truly understand what death is.
Professional and Amateur Performers
To see this clearly, we need to start by distinguishing thanatosis from a similar behavior in the animal world known as tonic immobility. Tonic immobility is also utilized by many species, from insects to humans, when they feel threatened.
In a state of tonic immobility, you stop all muscle movement and remain still in a certain position. It is akin to pretending to be an inanimate object. However, unlike thanatosis, in tonic immobility, you can stand, raise your arms and legs, and tense your muscles as long as you do not move them.
On the other hand, animals that exhibit thanatosis usually lie flat, relax all their muscles, and display many other characteristics of playing dead. The opossum may be the species with the most complex thanatosis behavior in nature, but it is not the only one.
An opossum in tonic immobility rather than thanatosis.
Some species of frogs also exhibit thanatosis behavior. They lie still and stop responding to all external interactions. The frog opens its eyes, stretches all four legs out limp. Its tongue also hangs out of its mouth while it exhales breaths that smell like ammonia.
Hognose Snakes also have a dramatic and theatrical display of playing dead. Initially, the animal will thrash about, writhing erratically and aggressively. Eventually, it pretends to weaken and rolls over, motionless. The snake’s mouth is slightly open, and its tongue falls to one side of its jaw. It even holds its breath completely.
Some snakes will continue the act by secreting blood from their mouths. You can poke the snake with a stick or lift it into the air, but the snake will not react.
In contrast, tonic immobility is often just an amateur performance. You can see it in sharks and rays, where the animals lie belly up, motionless but still breathing deeply.
Some species, like insects and lizards, also enter tonic immobility, but in reality, you can still see their hearts beating, sometimes even faster than normal.
A Hognose Snake begins its performance.
You can even pick up the snake; it performs so deeply that it bleeds from its mouth.
Both tonic immobility and thanatosis are defensive mechanisms protecting prey from predators. However, they are fundamentally different, not only in appearance but also in the deeper mechanisms they employ against predators.
To see this, consider the four main stages in a typical animal predation:
- (1) detecting the prey;
- (2) recognizing the prey as suitable to eat;
- (3) subduing the prey;
- (4) consuming the prey.
While tonic immobility primarily targets stages (1) and (4), thanatosis aims at stages (2) and (3). We can see that remaining still and motionless helps the prey blend into the environment, reducing the predator’s attention, especially when the prey has an additional layer of camouflage.
Some predators have developed vision that responds only to movement, so tonic immobility works by eliminating a crucial stimulus that could attract the attention of the predator.
This is even more beneficial if the prey lives in groups. Remaining motionless allows it to wait for a conspecific to be captured by a predator, leading the predator to abandon the next target because it has already eaten.
In some animal species, tonic immobility serves as an anti-consumption mechanism, meaning it targets step 4. For example, some frog species will shrink their bodies and remain motionless, just waiting for the predator that has eaten them to regurgitate them after their skin secretes unpleasant-tasting toxins.
A similar mechanism is applied by some species of grasshoppers, but in contrast to frogs. If a grasshopper tries to expand its body, spreading all its sharp legs and arms and remaining motionless, it will turn into a spiny dart that the frog cannot swallow. At this point, the frog will have to spit the grasshopper out, allowing it to escape.
Now let’s talk about the mechanism of thanatosis, a behavior that targets the second and third stages of the predation process. This means it helps prey to present themselves as an unappetizing meal while simultaneously escaping the violent subjugation behaviors of predators.
For the first purpose, it is clear that prey have employed chemical measures to trigger the initial sense of unpalatability in the predator. Animals that exhibit thanatosis often accompany this behavior with the expulsion of feces and urine. For instance, frogs will exhale a breath that smells like ammonia.
However, if that were all, it would not be enough to explain the masquerade behavior of the opossum, with its greenish tongue and limp, corpse-like immobility. Scientists say that disgust is not the only factor that these animals in thanatosis want to activate. They also want the predator to be horrified by their death.
Many, if not most, animal species, like us, are obsessed with death. This is referred to as the fear of “necrophobia“, similar to how one might fear blood, coffins, and the sound of funeral trumpets. Predators also fear animals that are motionless, foul-smelling, and with tongues hanging out for mysterious reasons, not due to their own claws and fangs.
Fresh carnivores feel disgusted by the smell or appearance of corpses, even if they do not understand what a corpse is. This aversion serves an important protective function for the organism, as corpses are a breeding ground for pathogens. If an animal eats or even approaches a corpse, it can become infected.
You can induce a chicken into a state of muscle rigidity by drawing a straight line in front of it.
For the second purpose, thanatosis aims to resist the subjugation of predators. Imagine an opossum trying to escape from a coyote; rolling over and playing dead is a better strategy than attempting to run and realizing its speed is not enough to outrun the coyote.
At this point, the coyote might tear into it to subjugate the opossum. This poses a greater risk of leaving a significant wound compared to lying still and pretending to be dead. Some predators tend to treat their prey relatively gently, even releasing and playing with them before delivering the final blow.
Some may remember the location of the dead animal and continue hunting other animals before returning to collect them. Therefore, the thanatosis strategy is beneficial in these situations. The prey can preserve the integrity of their bodies while also hiding or escaping when the predator is not paying attention.
Under the Hand of Evolution
We return to the minimal definition of death that an animal can perceive: inactivity and irreversibility. When a predator sees prey in a state of thanatosis, it may mistakenly believe that the prey is dead based on:
Silence, reduced physiological function, lack of response, and similar behaviors create an illusion of the absence of vital body functions. This means the animal is indeed inactive.
Meanwhile, blood, foul odors, a green tongue, and other signs indicate that this inactivity cannot be reversed, unlike what we might find in a person who is sleeping or unconscious. This means that the animal is dead, and it is dead permanently.
Behind many behaviors exhibited by animals, there must be at least one pressure of natural selection, which facilitates their evolutionary process. Sometimes, these selection pressures lie in how their predators perceive the outside world. This, in turn, determines the chances of whether a prey can survive long enough to pass on its genes.
So here, it could be that the opossums themselves do not know they are playing dead. The very ability of their predators, such as coyotes, to recognize death is the selective pressure that drives the opossum’s thanatosis behavior.
Stick insects are similar; they do not need to look in a mirror to know that they resemble a twig. They simply need to heed the ancestral instinct to stand still when encountering a predator. Subsequently, the appearance passed down from their ancestors will help the stick insect survive.
However, if stick insects have evolved to possess the appearance and instinct of muscle rigidity, then their predators must have also developed a misconception that these insects are real twigs, and they do not want to eat a dry branch.
Throughout their evolutionary history, the ancestors of these twig-like insects would have faced less risk of being eaten by enemies. Thus, the more they resemble twigs, the greater their reproductive opportunities. Ultimately, the genes that express twig-like appearances will be passed down to all their descendants.
Returning to the masters of thanatosis in the animal kingdom, regardless of the specific evolutionary history that has given rise to this behavior, it opens a window for us to glimpse into the minds of predators.
Thanatosis shows us that for whatever reason, predators do not want to eat animals that they think are dead. Throughout their evolutionary history, opossums that can mimic dead bodies more closely will have a higher chance of survival. Thus, when they reproduce, their acting genes will also be passed down to the next generation.
Therefore, any explanation regarding the protective function of thanatosis will lead to one conclusion: that animals have the ability to recognize death, if not the opossums themselves, then the coyotes hunting them.
Besides thanatosis, there are other reasons that show that carnivores understand what death is. They have a strong mental motivation to pay attention to the moment a prey dies. In that hunt, the death of the prey represents two extremes:
One is that it has become edible food; the other is that it is no longer alive to pose a danger (as many animals are equipped with sharp horns or hooves, or will react violently to a predator’s attempts to subdue them).
Throughout the life of a predator, it has witnessed hundreds, if not thousands, of prey deaths to understand what irreversible inactivity is and the basic characteristics of death.
In summary, the concept of death is not just a unique achievement of humanity. It may be a fairly common trait in the animal kingdom. We humans often think of ourselves as a unique species, but now is the time to rethink that.
The exceptionalism of humanity is what separates us from nature, allowing us to exploit it for our own benefit. Once we eliminate that mindset, we can develop a sense of protecting nature and respecting the habitats of other animal species.
