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Imagine watching your favourite nature documentary. The predator lunges rapidly from its hiding place, jaws wide open, and the prey … suddenly goes limp. It looks dead. For some animals, this freeze response – called"tonic immobility" – can be a lifesaver.
Possums famously"play dead" to avoid predators. So do rabbits, lizards, snakes, and even some insects., we explored this strange behaviour in sharks, rays and their relatives. In this group, tonic immobility is triggered when the animal is turned upside down – it stops moving, its muscles relax, and it enters a trance-like state.Despite being well documented across the animal kingdom, the reasons behind tonic immobility remain murky – especially in the ocean. It is generally thought of as an anti-predator defence. But there is no evidence to support this idea in sharks, and alternative hypotheses exist.We tested 13 species of sharks, rays, and a chimaera – a shark relative commonly referred to as a ghost shark – to see whether they entered tonic immobility when gently turned upside down underwater. Seven species did, but six did not. We then analysed these findings using evolutionary tools to map the behaviour across hundreds of million years of shark family history.Anti-predator strategy –"playing dead" to avoid being eaten Reproductive role – some male sharks invert females during mating, so perhaps tonic immobility helps reduce struggleOur results don't support any of these explanations.And the sensory overload idea? Untested and unverified. So, we offer a simpler explanation. Tonic immobility in sharks is likely an evolutionary relic.Our evolutionary analysis suggests tonic immobility is"plesiomorphic" – an ancestral trait that was likely present in ancient sharks, rays and chimaeras. But as species evolved, many lost the behaviour. In fact, we found that tonic immobility was lost independently at least five times across different groups. Which raises the question: why? In some environments, freezing might actually be a bad idea. Small reef sharks and bottom-dwelling rays often squeeze through tight crevices in complex coral habitats when feeding or resting. Going limp in such settings could get them stuck – or worse. That means losing this behaviour might have actually been advantageous in these lineages.Rather than a clever survival tactic, tonic immobility might just be"evolutionary baggage" – a behaviour that once served a purpose, but now persists in some species simply because it doesn't do enough harm to be selected against.Our work helps challenge long-held assumptions about shark behaviour, and sheds light on the hidden evolutionary stories still unfolding in the ocean's depths. Next time you hear about a shark"playing dead", remember – it might just be muscle memory from a very, very long time ago.
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