Phylogeny
Tonic immobility renders an animal unconscious or puts it in an "out-of-body" state. The animal becomes completely immobile, as the name suggests, and might look dead to a passing predator.
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It's possible that tonic immobility evolved as a form of thanotosis in chickens, guinea pigs, snakes, and other prey animals that exhibit the behavior. (Hoagland 1928)
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In today's world, sharks are considered apex predators, and should have no need to play dead as they are not victims of predation. So why do sharks exhibit tonic immobility?
1) Not all sharks are apex predators. In fact, many smaller sharks are prey to more animals than they are predator. For these sharks, tonic immobility may be a useful tactic.
2) Even those sharks that are apex predators today were not always so. As such, tonic immobility may be a vestigial characteristic leftover from when Dimetrodon and Dunkleosteus preyed on primitive sharks. (Withers 2015)
Humans
Sharks
Whales
Dolphins
Orcas
Turtles
Large Fish
Small Sharks
Small Fish
Key:
points from predator to prey
points from attacker to victim
(not necessarily predator/prey relationship
Figure 3. A diagram of the modern day food chain surrounding sharks.
"Dimetrodon shed teeth throughout its life, and the lost crowns are like bullets at a crime scene...This is CSI, sharks were eaten by Dimetrodon in great numbers." - Robert Bakker
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"There was life before the dinosaurs, and it was different and interesting, you could even argue that without sharks we could not have evolved." - Robert Bakker
An even more compelling reason for the phylogeny of tonic immobility in sharks is its usefulness in mating. Shark mating is often a very violent and traumatizing experience for females. It involves heavy biting and scratching, which often leave deep and long lasting scars. (One explanation for why female sharks have thicker skin than males). After a male's initial attack on a female, she must decide whether to resist and try to escape, or submit. If she submits, penetration ensues, but the biting and scratching continues throughout copulation.
If the male flips the female, inducing tonic immobility, the mating process becomes much easier. Researchers have found through their use of tonic immobility that female sharks are far more receptive to it than males, lending more weight to this phylogenetic hypothesis. (BEC 2015)
