Sharks “freeze” when they are turned upside down. No one knows why

When they are turned up, several species of sharks “freeze” and pretend to be dead. A new research points out that this behavior is pleosiomorphic.

Imagine seeing your favorite documentary. The predator quickly throws himself from the hiding place, with his jaws wide open, and the prey… suddenly is inert. It looks dead.

For some animals, this immobilization response – called “tonic immobility” – can be a board of salvation. The skunks are famous for “pretending to be dead” to predat. The same make rabbits, lizards, snakes and even some insects.

But what happens when it is a shark to do so?

In a recent review in the Reviews in Fish Biology and Fisheries, the researchers explored this strange behavior in sharks, rays and their close relatives. In this group, tonic immobility is triggered when the animal It is turned on with your stomach – Stop moving, the muscles relax and enters a state similar to the trance. Some scientists even use this technique to handle certain species of shark safely.

But why does it happen? And does this behavior really help these marine predators survive?

The mystery of the “frozen shark”

Despite being well documented in the animal kingdom, the reason behind the tonic immobility remains unclear – especially in the ocean. It is usually seen as a defense against predators. But in sharks there is no evidence that supports this idea, and There are alternative hypotheses.

The authors tested 13 species of sharks, rays and a chimera – a relative close to the sharks, commonly called “ghost shark” – To see if they went into tonic immobility when they were gently turned by stomach up, under water.

Seven species had this behavior, but six no. Then these data were analyzed using evolutionary tools to map the behavior Over hundreds of millions of years of the evolutionary history of sharks.

So why do some sharks “freeze”?

There are three main hypotheses to explain tonic immobility in sharks:

1. Anti-Predator Strategy -“pretend dead” to avoid being eaten

2. Reproductive function – Some males saw females during mating, and immobility could reduce resistance

3. Answer to Sensory Overload – A kind of “dismissal” before extreme stimuli

The results do not support any of these explanations.

There is no evidence that sharks benefit from being real estate when attacked. In fact, predators like orcas exploit this answer by turning the sharks of belly up to immobilize them and then remove their livers rich in nutrients – a deadly strategy.

The reproductive hypothesis is not sustained either. Tonic Immobility does not differ between sexesand remaining immobile can make females vulnerable to forced or aggressive mating.

And the sensory overload? It has not yet been tested or confirmed. Therefore, a simpler explanation is proposed: tonic immobility in sharks is probably a evolutionary trace.

An inheritance of evolution

The analysis suggests that tonic immobility is “pleasure” – An ancestral trait that was probably present in the old sharks, rays and chimeras. But as species evolved, many lost this behavior.

In fact, tonic immobility has been independently lost at least five times in different groups. What raises a question: why?

In certain environments, “freeze” can be a bad idea. Small sharks of reef and rays that live in the bottom of the sea usually sneak into narrow slits in coral reefs when feeding or resting. Being inert in these spaces can make them stuck-or worse. This means losing this behavior may have been advantageous for these species.

What does all this mean?

Instead of an ingenious survival tactics, tonic immobility can only be “evolutionary baggage” – A behavior that has been useful, but now persists in some species simply because it does not cause bad enough to be eliminated by natural selection.

It is a good reminder that not all characteristics of nature are adaptive. Some are only historical curiosities.

The new work helps challenge old assumptions about shark behavior and sheds light on evolutionary stories hidden in the depths of the ocean. The next time you hear about a Shark to “pretend to be dead”Remember-it can only be muscle memory of a very, far away past.