Sharks Do Something Strange When Upside-Down, and Scientists Aren’t Sure Why

Flip certain sharks upside down, and they’ll suddenly go limp. Their bodies slacken, their fins stop moving, and for a moment, it’s like they’ve completely checked out. It looks like they’re faking death. But scientists still don’t know what they’re doing or why it happens.

The reaction is called tonic immobility, and it’s not unique to sharks. Possums do it. So do rabbits, snakes, and even insects. But a new study published in Reviews in Fish Biology and Fisheries found that only some sharks and rays respond this way. Researchers tested 13 species and found that seven entered the trance-like state, while six did nothing at all.

The instinct to freeze has been widely assumed to help animals avoid predators. That theory falls apart when it comes to sharks. Orcas, for example, have learned to use it against them by flipping them over, waiting until they’re helpless, and then removing their livers with horrifying precision. Not exactly a survival strategy.

Do You Know Why Sharks Do This When Upside-Down? Neither Do Scientists.

Other theories suggest it might play a role in mating since male sharks sometimes invert females. But the study found no difference between sexes, and being immobile mid-ocean doesn’t exactly scream safety. A third theory points to sensory overload, like the shark equivalent of shutting down under stress. That idea hasn’t been tested yet.

So what’s actually going on? The researchers think tonic immobility might just be leftover evolutionary software. Something ancient sharks evolved millions of years ago that doesn’t really serve a purpose anymore. It still pops up in some species, mostly because it doesn’t do enough damage to get weeded out.

When the team mapped this behavior across hundreds of millions of years of shark evolution, they found that tonic immobility was likely inherited from early ancestors but has been lost at least five times in different lineages. For reef sharks and bottom-dwellers that squeeze through tight coral structures, freezing might be more of a liability than an advantage.

Which means this weird, glitchy behavior might not be a survival tactic at all. It might just be a fossil of instinct. A muscle memory that never fully disappeared.

So, the next time you see a shark flip belly-up and go still, don’t assume it’s playing dead. It might just be following ancient code written long before we had a name for it.