Tonic immobility in sharks: an exciting phenomenon that intrigues scientific and passionate oceans for decades. This mysterious “trance”, triggered when these marine predators find themselves on the back, causes a state of temporary paralysis. A recent study questions our received ideas about this enigmatic behavior.
Tonic immobility, this reaction which transforms formidable predators into frozen creatures, raises many questions in the scientific community. Often observed in various animal species as a survival strategy in the face of predators, its presence in sharks remains paradoxical.
These great seas hunters do not seem to have an obvious evolutionary advantage. Researchers have recently studied this behavior by testing several marine species, offering a new interpretation of this biological mystery published in Reviews in Fish Biology and Fisheries.
When marine predators freeze
Tonic immobility in sharks manifests when returned to the of theof the. Their muscles are relaxed, their movementsmovements stop and they enter a state close to the trance. This phenomenon, long documented, is even used by certain scientists to manipulate these animals safely during studies.
In -depth research made it possible to test this behavior on 13 species of sharks, rays and chimeraschimeras (these “ghost sharks” related). The results are eloquent: seven species demonstrated this immobilization reaction when they were delicately returned underwater.
Opossums, rabbits and lizards use this technique to “play the dead” against their predators, thus increasing their chances of survival. But among sharks, this explanation seems unconvincing. On the contrary, some predators such as orcas exploit this vulnerability by deliberately returning the sharks to immobilize them before attacking their foiefoie rich in nutrientsnutrients.
Theories that fall into the water
Three main hypotheses have long tried to explain this behavioral particularity:
- An anti-pride strategy: immobility would make the shark “invisible” to eyeseyes predators.
- A reproductive mechanism: males would return the females during mating to reduce their résistancerésistance.
- A response to a sensory overload: an “emergency stop” in the face of a stimulationstimulation excessive.
None of these explanations, however, resists scientific analysis. The reproductive hypothesis comes up against that tonic immobility does not vary according to sex. It could even be disadvantageous for females, making them vulnerable to forced or harmful couplings.
As for the theory of sensory overload, it remains speculative and not verified. Faced with these dead ends, researchers offer an alternative explanation more consistent with the data collected.
The inheritance of a distant past
Evolutionary analysis suggests that tonic immobility probably represents an ancestral line present in common ancestors of sharks, rays and chimera. This behavior would then have been lost at least five times in different lines during evolution.
This selective loss is logically explained in certain environmental contexts. For small reef sharks and lines living on seabed, freezing could be fatal. These species regularly seek narrow crevices in coral reefs to eat or rest. Sudden immobility in these confined spaces would risk stuck them, compromising their survival.
Tonic immobility therefore appears as a “evolutionary background”, a behavioral vestige which may have served in the past but which persists today simply because it is not sufficiently harmful to be eliminated by the natural selectionnatural selection in some species.
Lighting on marine scalable history
This discovery reminds us that all the traits observed in nature are not necessarily adaptive. Certain behaviors simply represent the oddities of evolutionary history, silent witnesses of the past which continue to influence the biology of contemporary species.
The tonic immobility of sharks perfectly illustrates how the in -depth study of apparently inexplicable behavior can reveal fascinating chapters of marine evolutionary history. These predators, often perceived as perfectly adapted killing machines, carry in them the traces of a complex biological heritage shaped by hundreds of millions of years of evolution.
