This Unusual Shark Behavior Might Ultimately Go Unused

tonic immobility. One of the most intuitive suggests that it would be a defense strategy in the face of predators. Like certain opossums or lizards that “make it dead” to escape the threat, the sharks could also enter a form of feigned paralysis. However, nothing proves that this behavior improves their chances of survival. On the contrary, it can sometimes be diverted by other predators, such as orcas, who precisely know how to take advantage of them.

A second track, equally widespread, evokes a role in reproduction. We know that some males return the females during mating. Tonic immobility could then facilitate the act by reducing the movements of the female. But this hypothesis does not resist analysis. The reflex affects both males and females, which weakens the idea of ​​a mechanism exclusively linked to copulation. In addition, staying frozen could expose the female to forced behaviors, expensive in energy and risky for its survival.

Finally, a third theory suggests that this reaction would be triggered by a sensory overload. Faced with a brutal influx of information, the shark's brain would opt for a form of pause, a kind of neurological restart. Here again, this idea remains speculative. In a study published in Reviews in Fish Biology and Fisheries, no experimental test has confirmed this scenario. Thirteen species of chondrichthyans – sharks, rays and chimeras – were returned in a controlled frame. Result, seven of them remained motionless, six not, without obvious correlation with their lifestyle or their physiology. None of the three classic hypotheses manages to explain this heterogeneous distribution.

Tonic immobility in sharks does not follow any known adaptive logic

In most cases, behaviors inherited by a species improve its chances of survival, reproduction or adaptation to its environment. This is the very principle of natural selection. However, in the case of tonic immobility among sharks, this logic seems to collapse. Far from offering any advantage, this reflex could on the contrary endanger them.

Orcas have thus been observed in the process of deliberately returning sharks to immobilize them, facilitating the extraction of their liver rich in nutrients. This hunting technique, reported by Popular Science, perfectly illustrates how supposedly defensive behavior becomes an exploitable weak point. This paradox is not limited to a few isolated cases. He questions the reason for being of this reflex.

The data collected by researchers at James Cook University, published in The Conversation, show that tonic immobility follows no uniform adaptive logic. Some species are therefore sensitive, others not. And this variation does not seem to be linked to sex, size, habitat or position in the food chain. Sharks living in congested environments, such as coral reefs, have even interest in not freezing. Sudden paralysis in a crevasse could cause a deadly tangle. In these contexts, losing this behavior undoubtedly represented an evolutionary advantage.

What this impasse reveals about our understanding of evolution

The study of tonic immobility in sharks calls into question a deeply rooted idea. That according to which each biological trait must necessarily have a useful function. Modern science tends to complexify this vision. Certain behaviors persist not because they are beneficial, but simply because they do not cause enough wrong to be eliminated.

In the language of biologists of evolution, tonic immobility is described as a plesiomorphic trait. In other words, a vestige inherited from a common ancestor, transmitted over time as an unconscious habit. If this behavior is not harmful in a given environment, it can last without being actively selected or deleted.

Phylogenetic analyzes have revealed that this reflex was lost at least five times in distinct lines. This suggests that he has not been actively preserved, but simply tolerated. This perspective obliges to rethink certain foundations of evolutionary biology. Not all behaviors are the result of perfect optimization. Some, like the latter, could only be behavioral residues, indifferent to the passage of time.

A scientific enigma still far from being resolved

Despite the advances of recent years, tonic immobility in sharks remains a largely unexplained phenomenon. Current research, although more rigorous and diversified than before, do not yet manage to establish a precise cause, nor a convincing function for this reflex.

Scientists, like those of the study, plead for a standardization of observation methods. Even today, the results vary depending on whether the shark is returned, applying pressure on a precise area of ​​your body, or stimulates certain sensory sensors. This lack of uniformity complicates comparisons between species and night to the robustness of the conclusions.

Other tracks are starting to emerge, especially around the broader evolving history of cartilaginous fish and their jaw cousins. Understand why this reflex appeared, in what context it could have been useful in the past, could make it possible to better understand why it remains today. Because behind this strange behavior perhaps hides a story old of several hundred million years, of which only a small part is still accessible to us.