{"id":10339,"date":"2025-06-24T09:09:10","date_gmt":"2025-06-24T09:09:10","guid":{"rendered":"https:\/\/www.europesays.com\/us\/10339\/"},"modified":"2025-06-24T09:09:10","modified_gmt":"2025-06-24T09:09:10","slug":"sharks-freeze-when-you-turn-them-upside-down-and-theres-no-good-reason-why","status":"publish","type":"post","link":"https:\/\/www.europesays.com\/us\/10339\/","title":{"rendered":"Sharks freeze when you turn them upside down \u2013 and there\u2019s no good reason why"},"content":{"rendered":"

Imagine watching your favourite nature documentary. The predator lunges rapidly from its hiding place, jaws wide open, and the prey \u2026 suddenly goes limp. It looks dead.<\/p>\n

For some animals, this freeze response \u2013 called \u201ctonic immobility\u201d \u2013 can be a lifesaver. Possums famously \u201cplay dead\u201d to avoid predators. So do rabbits, lizards, snakes, and even some insects. <\/p>\n

But what happens when a shark does it?<\/p>\n

In our recent study<\/a>, 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 \u2013 it stops moving, its muscles relax, and it enters a trance-like state. Some scientists even use tonic immobility<\/a> as a technique to safely handle certain shark species.<\/p>\n

But why does it happen? And does it actually help these marine predators survive?<\/p>\n

The mystery of the \u2018frozen shark\u2019<\/p>\n

Despite being well documented across the animal kingdom, the reasons behind tonic immobility remain murky \u2013 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.<\/p>\n

We tested 13 species of sharks, rays, and a chimaera \u2014 a shark relative commonly referred to as a ghost shark \u2014 to see whether they entered tonic immobility when gently turned upside down underwater. <\/p>\n

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.<\/p>\n

So, why do some sharks freeze?<\/p>\n

\"Sharks<\/a><\/p>\n

Tonic immobility is triggered in sharks when they are turned upside down.
\n Rachel Moore<\/p>\n

Three main hypotheses<\/p>\n

There are three main hypotheses to explain tonic immobility in sharks:<\/p>\n

    \n
  1. Anti-predator strategy \u2013 \u201cplaying dead\u201d to avoid being eaten<\/li>\n
  2. Reproductive role \u2013 some male sharks invert females during mating, so perhaps tonic immobility helps reduce struggle<\/li>\n
  3. Sensory overload response \u2013 a kind of shutdown during extreme stimulation.<\/li>\n<\/ol>\n

    Our results don\u2019t support any of these explanations. <\/p>\n

    There\u2019s no strong evidence sharks benefit from freezing when attacked. In fact, modern predators such as orcas can use this response against sharks<\/a> by flipping them over to immobilise them and then remove their nutrient-rich livers \u2013 a deadly exploit. <\/p>\n

    The reproductive hypothesis also falls short. Tonic immobility doesn\u2019t differ between sexes<\/a>, and remaining immobile could make females vulnerable to harmful or forced mating events. <\/p>\n

    And the sensory overload idea? Untested and unverified. So, we offer a simpler explanation. Tonic immobility in sharks is likely an evolutionary relic.<\/p>\n

    A case of evolutionary baggage<\/p>\n

    Our evolutionary analysis suggests tonic immobility is \u201cplesiomorphic\u201d \u2013 an ancestral trait that was likely present in ancient sharks, rays and chimaeras. But as species evolved, many lost the behaviour.<\/p>\n

    In fact, we found that tonic immobility was lost independently at least five times across different groups. Which raises the question: why? <\/p>\n

    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 \u2013 or worse. That means losing this behaviour might have actually been advantageous in these lineages.<\/p>\n

    So, what does this all mean?<\/p>\n

    Rather than a clever survival tactic, tonic immobility might just be \u201cevolutionary baggage\u201d \u2013 a behaviour that once served a purpose, but now persists in some species simply because it doesn\u2019t do enough harm to be selected against.<\/p>\n

    It\u2019s a good reminder that not every trait in nature is adaptive. Some are just historical quirks.<\/p>\n

    Our work helps challenge long-held assumptions about shark behaviour, and sheds light on the hidden evolutionary stories still unfolding in the ocean\u2019s depths. Next time you hear about a shark \u201cplaying dead\u201d, remember \u2013 it might just be muscle memory from a very, very long time ago.<\/p>\n","protected":false},"excerpt":{"rendered":"Imagine watching your favourite nature documentary. The predator lunges rapidly from its hiding place, jaws wide open, and…\n","protected":false},"author":3,"featured_media":10340,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[27],"tags":[159,67,132,68,837],"class_list":{"0":"post-10339","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-wildlife","8":"tag-science","9":"tag-united-states","10":"tag-unitedstates","11":"tag-us","12":"tag-wildlife"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@us\/114737562516609562","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/10339","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/comments?post=10339"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/10339\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media\/10340"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media?parent=10339"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/categories?post=10339"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/tags?post=10339"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}