Thanatosis: What Is It And What Animals Practice It?
In the wild environment, all living beings are subject to a constant evolutionary force: natural selection. Thus, each animal must balance a delicate scale on an individual level: reproduce as much as possible without dying in the attempt.
Predation is an essential part of this entire process of losses and benefits, because in response to this constant threat, many living beings have adopted morphological, physiological and behavioral variations in order to avoid being a carnivore’s plate at the end of the day.
From fascinating colors to unusual behaviors, the tools present in nature to avoid death will never cease to surprise us. Today we introduce you to thanatosis, without a doubt, one of the most curious behaviors that can be observed in the animal kingdom Stay with us, because you will discover that this “last bullet” can save the lives of many living beings in extreme situations.
Thanatosis, from a general point of view, can be defined as a behavior in which the threatened animal appears to be dead The most correct term to refer to this event is tonic immobility (TI) for its English translation, since it corresponds to a muscular paralysis during which the animal does not respond to external stimuli, rather than an act of apparent death. in itself.
To understand the reason for this type of curious defense mechanisms, it is necessary that we understand, even briefly, the continuous arms race that occurs between prey and predators. Go for it.
Phases of the interaction between predator and prey
First of all, it is necessary to emphasize that natural selection is not a conscious and completely effective mechanism. Characteristics appear in populations in the form of spontaneous mutations or as a combination of previous characters, and from here, they are slowly selected, since the animals that have the most valid adaptations will transmit their genes to the following generations.
So that, These mechanisms do not respond to absolute perfection or conscious selection The evolutionary pressures to which animals are subjected show that, in this world, “each one does what he can with what he has.”
Once this necessary clarification has been made, it is also necessary to emphasize that the predator-prey relationship has been widely studied by hundreds of ethologists and behavioral ecologists, since it is the clearest evolutionary driver along with the need for reproduction. In general, The contact between both components (predator and prey) can be broken down into several separate events :
There are both morphological and behavioral barriers for each of these phases, for example, a leaf bug prevents a bird from finding it during the detection period, thanks to its wonderful camouflage. On the other hand, a gazelle runs with all its being to avoid a reduction in separation between prey and predator.
Thanatosis, the term that concerns us today, is a mechanism that can be deployed until the final phase, that is, the subjugation of the prey itself since the animal can remain completely motionless for minutes or hours after the predator has released it.
What characterizes the phenomenon of thanatosis?
It is common for various behaviors in the animal world to be confused with thanatosis, but Not all immobility corresponds to playing dead According to the definition provided by a review article in the journal Behavioral ecology and sociobiologywe can list certain defining characteristics of this phenomenon:
Studies have even recorded that in the state of tonic immobility some vertebrates show reduced respiratory rate, bradycardia (the heart beats slower than normal), protrusion of the tongue and an excessive eye opening. Thus, in many cases it is not only a postural adaptation, but also a physiological one.
In any case, it is necessary to make various assessments in reference to this term. For example, when a hedgehog curls up into a ball and stays still, we are not dealing with a phenomenon of thanatosis. In this case, the animal is bristling its quills and protecting its vital organs, which is why it is not “playing dead,” but rather defending itself from an attack with a specific posture.
On the other hand, when a stick bug stops moving when faced with a vibration, it is not practicing thanatosis either. This evolutionary mechanism of “staying still” is intended to a greater degree of camouflage with the environment Thus, the animal is not playing dead, but rather camouflaging itself. As we see, this adaptation is much more limited than one might initially think, so: what animals practice thanatosis?
Some practical examples
In general, thanatosis It is launched when all active mechanisms to avoid predation fail It is a “last bullet”, since the prey hopes that the predator will lose interest in it by playing dead. This makes all the evolutionary sense in the world since, in many cases, dead animals are sources of viruses, bacteria and many other pathogens. Despite the energy it took for the predator to corner the prey, in many cases it is not in its best interest to take the risk of consuming a supposedly decomposing living being.
A clear example of this is found in various groups of amphibians, specifically, in species such as Phyllomedusa burmeisteri, a tree frog. This group of vertebrates does not usually have any type of weapon to counteract the action of a predator (generally larger and more lethal) and, therefore, sometimes the only option is to play dead. Some amphibians accompany this act with the emission of nauseating substances or the exposure of certain ventral colors of aggressive tones, in short: I am rotten, don’t eat me.
Another especially striking example is that of the collared snake (Natrix natrix). This small, completely harmless snake defends itself against predators by coiling its body and snorting, just like a viper would do. If this behavior does not work, the desperate animal he plays dead, curling up on himself and showing his open mouth and hanging tongue
There is an even more dramatic variation of this technique, as the snake can choose to show its belly and relax its muscular surface, giving a vision of an animal in full decomposition. It is estimated that approximately more than 66% of the specimens of this species engage in these behaviors after being handled by humans.
Despite having given you two examples within the group of herpets, it is necessary to emphasize that Thanatosis also appears in birds, teleost fish, invertebrates and even in some mammals, such as rabbits or opossums It is because of the behavior of this last animal that the word “playing possum” has become popular in the English language, since when someone ignores a person’s calls or requests, they are “playing dead” in front of them.
Summary
As we have seen in these lines, thanatosis is an antipredatory strategy of great interest and, of course, very useful in various animal taxa Although most of these behaviors are correlated with defense methods, it is also necessary to highlight that episodes of tonic immobility can occur for reproductive or even predatory purposes.
If something is clear to us after learning about this type of animal behavior, it is that natural selection gives living beings the most fascinating tools to endure over time and be able to transmit their genes to future generations. Of course, nature will never cease to surprise us.
