In order to relax or fall asleep, most people’s first step is to lay down. Whether that be on one’s back, side, or stomach, it allows for maximum muscle relaxation and reduces workload on the heart. Now imagine entering a REM like state as soon as you lay on your back, no matter the original intention. This is known as tonic immobility (TI). 

Tonic immobility is a reversible, temporary state of inactivity similar to hypnosis that often occurs in predator-prey contexts. This mechanism is an automatic paralysis that triggers a deep and rhythmic breathing with a steady heart rate. However, TI is not to be confused with death-feigning. Where an animal will pretend to be incapacitated in order to protect itself from predators due to a built in physiological defense. Vertebrates and invertebrates alike have tonic immobility, one of the most popular examples being sharks. 

Sharks were discovered to have the TI mechanism as early as 1997, where scientists in the Farallon Islands observed an Orca flipping over and holding a White Shark.15 minutes later, the defenseless and increasingly immobile shark suffocated and then torn apart by the Orca. TI is now used by researchers when handling sharks in order to subdue them. A hypnotized shark reduces the possibility of injury for the scientist and the shark.

Although TI is common among elasmobranchs , scientists have a hard time determining the original usage of this adaptation. One hypothesis scientists propose is that the shark enters tonic immobility as a state similar to death-feigning in order to escape predators. However, this idea is considered relatively weak since predators like orcas exploit the shark’s TI response to their advantage. A more believable theory is that some Elasmobranchs will undergo TI in order to protect themselves from overwhelming sensory input, as well as serving a role in courtship and copulation. It has been observed that the male Gray nurse sharks and male Tiger sharks induce TI on the females to invoke easier mating.

Despite scientists not being able to agree on a theory, they have deduced that TI most likely evolved in certain lineages but was lost in others. It would have been lost if it no longer provided an advantage, or never evolved at all if their environment didn’t favor it. Even though there is no clear-cut answer for the purpose of tonic immobility, it is still an interesting phenomenon that encourages new scientific discoveries every day. 

Featured image: nurse shark (Image credit: NOAA)