Sharks are captivating animals and rank among the most powerful predators in the sea. Nonetheless, they display an unusual behavior termed tonic immobility, wherein they become rigid when flipped onto their backs. This reaction parallels the freeze response observed in creatures such as rabbits, lizards, snakes, and possums. In spite of their daunting reputation, sharks can be rendered inert merely by being turned upside down, a method often employed by researchers for secure handling during studies.
Tonic immobility in sharks is a thoroughly studied occurrence, yet its function continues to baffle scientists. Various theories have been suggested, including it acting as an anti-predator mechanism, a mating behavior, or a reaction to sensory overload. Nevertheless, recent research indicates that none of these theories withstand closer examination. The anti-predator hypothesis seems improbable as freezing presents no clear survival benefit, and predators like orcas could take advantage of this weakness. The reproductive hypothesis is also debatable, given that tonic immobility is observed in both genders, and the sensory overload theory has yet to be validated.
In a research study examining 13 species of sharks, rays, and a chimaera, only seven species demonstrated tonic immobility when inverted. This variability adds to the complexity of understanding this behavior. Despite considerable research efforts, the precise reason for sharks undergoing tonic immobility remains undefined, underscoring the necessity for additional exploration into this fascinating feature of shark biology.
