What is tonic immobility
Tonic immobility, also known as (TI) is an unlearned reflex action that leads to a state of paralysis that some animals can enter into involuntarily when they feel threatened and is characterised by a state of immobility. This behaviour has been reported to occur in a variety of animals including insects, reptiles, birds, amphibians, mammals and fish.
How TI affects sharks
Although this phenomenon does not occur in all species of sharks, it does affect most types. In the Great White, the shark’s dorsal fin straightens, its breathing slows down and muscle contractions are lax. For many species in the animal kingdom, it’s a survival strategy; looking dead to a predator is a handy trick.
TI has its advantages
Tonic immobility is extensively used as an aid when handling or tagging wild sharks in the field to minimise struggling by the animal and reduce the possibility of injury. A shark can remain in this state for up to 15 minutes before it corrects itself and gradually swims away. Many anglers also make use of tonic immobility when removing hooks to improve the chances of the shark’s survival after release.
According to sharktrust.org, some scientists have suggested that tonic immobility in sharks may be related to mating rituals. Sharks reproduce through internal fertilisation and numerous reports indicate an immobile state often associated with such activities.
We’re not the first to discover this phenomenon
In 1997 a Great White Shark was attacked and eaten by a Killer Whale in the Farallon Islands off the coast of California.
It was the first ever recorded attack of this nature. Onlookers who were part of a cruise expedition later described the event as a complete surprise as the Killer Whale rammed once into the Great White leaving the shark dazed, it was then quickly flipped upside down and held firmly in the mouth by the Killer Whale, holding it still for fifteen minutes as the shark slowly succumbed to its inevitable death.
Tonic immobility is a phenomenon that is still not fully understood, however, because of it, scientists and divers can gain more knowledge of sharks in a safe and effective manner and essentially help us understand why predators like the shark succumb to such states of calm and relaxation.
Don’t forget to catch Zombie Sharks this Wednesday on the Discovery channel.