{"id":211928,"date":"2025-06-25T02:00:10","date_gmt":"2025-06-25T02:00:10","guid":{"rendered":"https:\/\/www.europesays.com\/uk\/211928\/"},"modified":"2025-06-25T02:00:10","modified_gmt":"2025-06-25T02:00:10","slug":"sharks-freeze-when-you-turn-them-upside-down-and-theres-no-good-reason-why","status":"publish","type":"post","link":"https:\/\/www.europesays.com\/uk\/211928\/","title":{"rendered":"Sharks freeze when you turn them upside down\u2014and there’s no good reason why"},"content":{"rendered":"

\"Sharks<\/p>\n

A neonatal sicklefin lemon shark (Negaprion acutidens) being dorsoventrally inverted to induce TI in Moorea, French Polynesia. Credit: Rachel Moore, from Reviews in Fish Biology and Fisheries (2025). DOI: 10.1007\/s11160-025-09958-3<\/p>\n

Imagine watching your favorite 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\u2014called “tonic immobility<\/a>“\u2014can be a lifesaver. Possums famously “play dead” 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 behavior in sharks, rays and their relatives. In this group, tonic immobility is triggered when the animal is turned upside down\u2014it 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<\/a>.<\/p>\n

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

The mystery of the ‘frozen shark’<\/p>\n

Despite being well documented across the animal kingdom, the reasons behind tonic immobility remain murky\u2014especially in the ocean. It is generally thought of as an anti-predator defense. 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 chimera\u2014a shark relative commonly referred to as a ghost shark\u2014to see whether they entered tonic immobility when gently turned upside down underwater.<\/p>\n

Seven species did, but six did not. We then analyzed these findings using evolutionary tools to map the behavior across hundreds of million years of shark family history.<\/p>\n

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

\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tThree main hypotheses<\/p>\n

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

    \n
  1. Anti-predator<\/a> strategy\u2014”playing dead” to avoid being eaten<\/li>\n
  2. Reproductive role\u2014some male sharks invert females during mating, so perhaps tonic immobility helps reduce struggle<\/li>\n
  3. Sensory overload response\u2014a kind of shutdown during extreme stimulation.<\/li>\n<\/ol>\n

    Our results don’t support any of these explanations.<\/p>\n

    There’s 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 immobilize them and then remove their nutrient-rich livers\u2014a deadly exploit.<\/p>\n

    The reproductive hypothesis also falls short. Tonic immobility doesn’t 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 “plesiomorphic”\u2014an ancestral trait that was likely present in ancient sharks, rays and chimeras. But as species evolved, many lost the behavior.<\/p>\n

    \n Discover the latest in science, tech, and space with over 100,000 subscribers<\/strong> who rely on Phys.org for daily insights.
    \n Sign up for our     free newsletter<\/a> and get updates on breakthroughs,
    \n innovations, and research that matter\u2014daily or weekly<\/strong>.\n <\/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\u2014or worse. That means losing this behavior 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 “evolutionary baggage”\u2014a behavior that once served a purpose, but now persists in some species simply because it doesn’t do enough harm to be selected against.<\/p>\n

    It’s 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 behavior, 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\u2014it might just be muscle memory from a very, very long time ago.<\/p>\n

    More information:<\/strong>
    \n\t\t\t\t\t\t\t\t\t\t\t\tJoel H. Gayford et al, Tonic immobility in cartilaginous fishes (Chondrichthyes): function, evolutionary history, and future directions, Reviews in Fish Biology and Fisheries (2025).     DOI: 10.1007\/s11160-025-09958-3<\/a><\/p>\n

    \n\t\t\t\t\t\t\t\t\t\t\t\t\tProvided by
    \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t    The Conversation<\/a>
    \n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t    <\/p>\n

    \t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/a>\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t<\/p>\n

    \n\t\t\t\t\t\t\t\t\t\t\t\t This article is republished from The Conversation<\/a> under a Creative Commons license. Read the original article<\/a>.\"The\n\t\t\t\t\t\t\t\t\t\t\t <\/p>\n

    \n\t\t\t\t\t\t\t\t\t\t\t\tCitation<\/strong>:
    \n\t\t\t\t\t\t\t\t\t\t\t\tSharks freeze when you turn them upside down\u2014and there’s no good reason why (2025, June 24)
    \n\t\t\t\t\t\t\t\t\t\t\t\tretrieved 24 June 2025
    \n\t\t\t\t\t\t\t\t\t\t\t\tfrom https:\/\/phys.org\/news\/2025-06-sharks-upside-good.html\n\t\t\t\t\t\t\t\t\t\t\t <\/p>\n

    \n\t\t\t\t\t\t\t\t\t\t\t This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
    \n\t\t\t\t\t\t\t\t\t\t\t part may be reproduced without the written permission. The content is provided for information purposes only.\n\t\t\t\t\t\t\t\t\t\t\t <\/p>\n","protected":false},"excerpt":{"rendered":"A neonatal sicklefin lemon shark (Negaprion acutidens) being dorsoventrally inverted to induce TI in Moorea, French Polynesia. Credit:…\n","protected":false},"author":2,"featured_media":211929,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3847],"tags":[75,76,74,71,70,72,53,73,16,15,1717],"class_list":{"0":"post-211928","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-wildlife","8":"tag-materials","9":"tag-nanotech","10":"tag-physics","11":"tag-physics-news","12":"tag-science","13":"tag-science-news","14":"tag-technology","15":"tag-technology-news","16":"tag-uk","17":"tag-united-kingdom","18":"tag-wildlife"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@uk\/114741538158644102","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/211928","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/comments?post=211928"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/211928\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media\/211929"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media?parent=211928"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/categories?post=211928"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/tags?post=211928"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}