Thanatosis: what is it and which animals practice it?

Thanatosis is a curious survival mechanism used as a last resort in the face of predators.

In the wild, all living beings are subject to a constant evolutionary force: natural selection. Thus, each animal must balance a delicate balance at the individual level: to reproduce as much as possible without dying in the attempt.

Predation is an essential part of this whole process of loss and gain, for in response to this constant threat, many living things have adopted morphological, physiological and behavioral variations in order to avoid being a carnivore's dish at the end of the day.

From fascinating colors to unusual behaviors, the tools present in nature to avoid death will never cease to amaze us. Today we introduce you to thanatosis, undoubtedly 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 life of many living beings in extreme situations.

What is thanatosis?

The thanatosis, from a general point of view, can be defined as a behavior in which the threatened animal appears to be dead.. The more correct term to refer to this event is tonic immobility (TI), since it corresponds to a muscular paralysis during which the animal does not respond to external stimuli, rather than an act of apparent death itself.

To understand the reason for such curious defense mechanisms, it is necessary to understand, even if only briefly, the continuous arms race between prey and predators. Let's get down to 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 on, they are slowly selected, as the animals possessing the most valid adaptations will transmit their genes to the following generations.

Thus, these mechanisms do not respond to perfection, these mechanisms do not respond to absolute perfection or to a conscious selection of the most valid adaptations.. The evolutionary pressures to which animals are subjected show that, in this world, "everyone does what he can with what he has".

Having made this necessary clarification, it is also necessary to emphasize that the predator-prey relationship has been extensively studied by hundreds of ethologists and behavioral ecologists, as it is the clearest evolutionary driver next to the need for reproduction. In general, the contact between the two components (predator and prey) can be broken down into several separate events:

• The two individuals are in close proximity in a three-dimensional space.
• Detection and identification occurs, first by one or both parties at the same time.
• A reduction of separation occurs, as one of the elements actively tries to approach the other.
• There is contact, subjugation of the prey by the predator and, finally, consumption.

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 marvelous camouflage. On the other hand, a gazelle runs with all its being to avoid a reduction of separation between prey and predator.

Thanatosis, the term we are concerned with today, is a mechanism that can be deployed up to the a mechanism that can be deployed up to the final phase, i.e. the subjugation of the prey itself, as the animal can remain completely immobile.The animal can remain completely immobile 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.. As defined by a review article in the journal Behavioural ecology and sociobilogywe can enumerate certain defining characteristics of this phenomenon:

• Tonic immobility (TI) is the adoption of a posture without movement of a prey as a response to physical contact or very close proximity to a predator.
• This is an unlearned adaptation.
• The phenomenon of thanatosis does not diminish the visibility of the prey or its degree of protection against possible attacks.
• Motor inhibition is maintained for a variable interval of time after the interaction with the predator.
• Once the threat is over (and if the animal has not been harmed), the prey can return to its previous physiological state.

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 inordinate eye opening. and a disproportionate 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 several appreciations in reference to this term. For example, when a hedgehog curls into a ball and remains still, this is not 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 defending itself from an attack with a specific posture.

On the other hand, when a stick bug stops moving in response to a vibration, it is not practicing thanatosis either. This evolutionary mechanism of "standing still" is aimed at a greater degree of camouflage with the environment.. Thus, the animal is not playing dead, but mimicking. As we can see, this adaptation is much more limited than one might at first think, so what animals practice thanatosis?

Some practical examples

In general, thanatosis is triggered when all the active mechanisms to avoid predation fail. It is a "last bullet", as the prey expects the predator to lose interest in it by the act of playing dead. This makes all the evolutionary sense in the world because, in many cases, dead animals are sources of viruses, bacteria and many other pathogens. Despite the energy it may have cost 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, in particular, in species such as Phyllomedusa burmeisteria tree frog. This group of vertebrates usually does not have any kind of weapon to counteract the action of a predator (usually 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 exhibition of certain ventral colors of aggressive tones, in short: I'm rotten, don't eat me.

Another particularly striking example is that of the collared snake (Natrix natrix). This completely harmless little snake defends itself against predators by coiling its body and snorting, just as a viper would. If this behavior is unsuccessful, the desperate animal plays dead, coiling on itself and showing its open mouth and hanging tongue..

There is an even more dramatic variation of this technique, as the snake may 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 resort to these behaviors after being handled by humans.

In spite of having given you two examples within the group of herps, it is necessary to stress that tanatosis also occurs in birds, teleost fish, invertebrates and even in some mammals such as rabbits and opossums.. It is because of the behavior of the latter animal that the term "playing possum" has become popular in the English language, since when someone ignores a person's calls or requests, he is "playing dead" in front of him.

Summary

As we have seen in these lines, thanatosis is an antipredator strategy of great interest and, of course, very useful in several animal taxa.. Although most of these behaviors are correlated with defense methods, it is also necessary to point out that there can be episodes of tonic immobility for reproductive or even predatory purposes.

If anything is clear to us after learning about these types of animal behaviors, it is that natural selection provides living beings with the most fascinating tools to endure over time and to be able to transmit their genes to future generations. Of course, nature will never cease to surprise us.

Bibliographical references:

• Coutinho, C., Ayres-Peres, L., Araujo, P. B., Jara, C. G., & Santos, S. (2013). Thanatosis in freshwater anomurans (Decapoda: Aeglidae). Journal of Natural History, 47(41-42), 2623-2632.
• Hansen, L. S. S., Gonzales, S. F., Toft, S., & Bilde, T. (2008). Thanatosis as an adaptive male mating strategy in the nuptial gift–giving spider Pisaura mirabilis. Behavioral Ecology, 19(3), 546-551.
• Humphreys, R. K., & Ruxton, G. D. (2018). A review of thanatosis (death feigning) as an anti-predator behaviour. Behavioral ecology and sociobiology, 72(2), 22.
• Rogers, S. M., & Simpson, S. J. (2014). Thanatosis. Current Biology, 24(21), R1031-R1033.
• Valdivieso, J. R. (2019). El viejo truco de “hacerse el muerto” en ranas. Revista de Biología Tropical, Blog-Blog.