Gray nurse sharks, or sand tiger sharks, are a critically endangered species that can be found in the waters of Japan, Australia, South Africa, and the east coasts of North and South America. Given their conservation status, it's always an exciting moment when these sharks breed. A video shot by Australia's Wolf Rock Diving Centre is making the rounds online and has people fascinated by the behavior of these sharks during that process. That's because they display a behavior known as tonic immobility (TI).
Colloquially known as “playing dead,” this temporary state of paralysis occurs in many different animal species. This trancelike state involves a decrease in heart rate and limp muscles that can last anywhere from one to 15 minutes if left undisturbed. Often, it's a defensive strategy to fool a predator into leaving the animal alone. But for gray nurse sharks and a few other shark species, TI is also an important part of the courtship and mating process.
Mating in the shark world can be rough business. Male sharks will bite the female on her fins and gills to turn her over into a position suitable for mating. This also induces temporary paralysis, which is useful for the male as it's believed that they can better position their sperm to enter into the female's uterus if she's not moving. Researchers believe that this behavior developed over time as a way for sharks to protect themselves from injury during the mating process. And, in fact, gray nurse sharks aren't the only ones who behave in this manner.
Tonic immobility has also been observed during mating in other shark species, including the zebra shark. So, while it may appear strange to us, it's all in the norm of shark behavior. Once the mating is finished, the female shark will come out of the trance and continue on her normal routine.
Interestingly, tonic immobility is also used by shark conservationists, who use the phenomenon to handle or subdue sharks safely for breeding purposes or health checkups.
Divers in Australia captured gray nurse sharks going into tonic immobility during mating.
This paralysis is induced by male sharks who bite the females and then flip them over. It's believed the behavior helps keep the animals stay safe during mating.
Luckily, the trancelike state is only temporary, and the sharks return to normal shortly after.
h/t: [reddit]
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