In keeping with the recent Shark Week mania, I am writing a post semi-focused on sharks. The main topic that I wanted to hit on was tonic immobility (TI), a temporary type of paralysis. This behavior is not unique to sharks. Rays, lobsters, mice, chickens, and trout are just a few of the other animals that implement this strategy in their lives. I liked this short clip by BBC Earth because it shows TI in a few different animals (i.e. pigeons, alligators, and chickens). Researchers sometimes take advantage of TI to help collect data on live animals. On large, dangerous animals, such as sharks, this technique comes in rather handy. Check out this NatGeo video showing TI in tiger sharks while a researcher attaches an accelerometer tag. Researchers studying sea turtles also benefit from TI when they need to weigh sea turtle hatchlings quickly and safely: Watch a baby sea turtle being hypnotized so we can weigh it.
As mentioned in the shark video, the stimulation of the rostrum/snout of the shark overwhelms the animal temporarily and induces a trance-like state. This is because of the electroreceptors on the rostrum, the Ampullae of Lorenzini. To learn more about this sensory system, check out BloggerheadSeaTurtle’s Great White Shark necropsy post! One of the cool things about Nature is that humans are not the only ones who have figured out how to take advantage of sharks and rays possessing TI tendencies. Orcas have discovered that if they grasp a shark or stingray and quickly flip it upside-down, the animal will go into TI, thereby giving the orca an easy meal (see Killer whales attack and eat shark and Tonic Immobility in Sharks). The duration of TI varies. From what I found, it can range from a few seconds to over three hours (chicken). The issue with animals like sharks is that they have to move in order to breathe. If their TI periods end up going too long, they can suffocate to death, which is apparently also a hunting strategy used by orcas. Opinions seem to vary on the “Why” behind TI. The main theories center on fear and predation as “playing dead” is a viable option for a multitude of animals for survival and motionless objects are more difficult to perceive. One suggestion for sharks, in particular, is mating because the effect seems to be stronger in females. I would be really interested in reading some more articles on this topic in the future, so I hope more research is in the works!
The most recent articles that I found on TI were actually related to humans. Last month, an article about TI and rape/sexual assault victims was published: Many rape victims experience involuntary paralysis that prevents them from resisting. It outlined that the majority of rape victims experience some degree of TI. This research could have dramatic judicial consequences, as well as alter the future medical and psychological treatments for victims (see Tonic Immobility: Study Shows Why Rape Victims Can’t Just Fight Back). When I was growing up, almost all of my biology courses beat “Fight or Flight” into my brain as the only survival tactics animals use. However, TI emphasizes that “Freeze” is also a viable option, including for humans.
I really enjoyed learning more about this topic. I wish I was able to find more concise articles about the mechanism behind TI and what is actually happening in the body to cause TI. The majority of articles that I found just summed up TI as a “trance-like” state but did not elaborate in much detail, and I did not have the necessary time needed to research the topic in great detail, unfortunately. The best resource that I found was from another blogger’s page: Zombies of the Ocean: The Mechanism behind Shark Tonic Immobility. Interestingly, in terrestrial animals, it seems that muscle contraction is used during TI, whereas in aquatic animals, like sharks, muscle relaxation is a defining characteristic of TI. The blogger also included personal correspondence with shark experts and gave a great summary of what is known about how/why sharks exhibit TI. I do not want to just copy/paste their article, but I would recommend y’all checking it out for yourself! The take home message is that there is still a lot to learn about TI! I hope this post sparked your interest though! Have a great weekend!