Fainting Gators? Tonic Immobility in the Alligator

If you roll an alligator on its back and hold it there, it will exhibit a strange behavior. After 15 or 20 seconds, the alligator will go limp and become unresponsive. It’s called tonic immobility and, for a phenomenon so widespread in the animal kingdom, we know surprisingly little about it.

“You can take an amazing variety of organisms—invertebrates as well as vertebrates—and induce this almost trance-like state by rolling them over,” says Bruce Young, a professor of anatomy at Kirksville College of Osteopathic Medicine, A. T. Still University, and co-author of a new paper on tonic immobility in the alligator.

Young says there are two big competing theories to explain tonic immobility. One is that the animal is afraid and the other is that it is triggered by an inner ear reflex. However, neither theory can explain all the phenomena associated with tonic immobility in every animal that exhibits it.

Young’s unique background led him to think about tonic immobility in alligators from a different perspective. He teaches anatomy in a medical school, but he has worked on reptiles, including alligators, for most of his career. He knew what all alligator wrestlers know: that turning the animals on their bellies will result in the animals going unresponsive. But he also possessed knowledge of the strange circulatory system of the alligator.

Unlike humans, alligators have two aortae coming out of their hearts—a left aorta and a right aorta—with an opening between them. Young thought that turning an alligator over might cause gravity to influence the blood flow between the aortae. That could be significant because in alligators, the main blood supply to the head, the carotid vessel, only arises from the right aorta. So holding an alligator upside-down might result in less blood getting to its brain.

An inadequate blood supply to the brain can cause syncope, or fainting, in humans. Could tonic immobility in the alligator be due to the same mechanism?

Wrestling Alligators for Science

The first thing Young and his colleagues did was get some alligators and induce them into tonic immobility. This was not a trivial task. As the methods of the paper understate, “These trials involved a great deal of writhing on the part of the alligators.”

Since all his colleagues were clinicians with zero alligator wrestling experience, Young found himself uniquely qualified to be the one to grapple with the 4.5 – 6-foot beasts.

“You hold them on their backs for a short period of time and then you just feel them go limp,” says Young. “And then maybe 20 seconds later their legs will start twitching, they will roll over, and they are ready for a fight.”

The duration of tonic immobility in alligators is on the short side; some animals, like green iguanas, can remain unresponsive for up to an hour. Young says a full minute of immobility would be a lot for an alligator.

Once they had documented that alligators actually do go into tonic immobility, the researchers explored why. Young would strap an alligator to a long plank that was placed over sawhorses. With the help of a colleague, he could turn the plank over quickly, eliciting tonic immobility. At the same time, the researchers observed what was happening in the circulatory system using MRI and Doppler ultrasonography.

Blood to the Brain

Young and his colleagues found that when you roll over an alligator, organs including the heart shift within the body. Doppler ultrasonography revealed further details about blood flow during inversion, showing that the pattern of blood flow through the heart and between the two aortae is significantly different when the animal is turned over. And when the researchers quantified the blood flow through the carotid vessel, they found that an upside-down alligator experiences a nearly 30% reduction in flow velocity through the carotid.

“Rolling the alligators onto their backs creates this abnormal physiological state, reducing blood flow to the brain,” says Young. “We think that, as a consequence, the animal enters into syncope, which manifests in the alligator as this tonic immobility.

“That’s a fancy way of saying something that alligator wrestlers have known for a long time: If you put the animal in this posture and maintain it, it will go into this state and give you a lot more freedom to do something reckless.”

Attributing tonic immobility to a blood flow issue means that in some ways, tonic immobility, at least in the alligator, shares features with reflex syncope in humans. This is what happens when you’re sitting down for a long period of time and you stand up too quickly, resulting in feeling light-headed or fainting. Young says it’s the same thing in alligators.

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“There’s a quick period of vascular insufficiency caused by the influence of gravity on blood flow that decreases blood flow to the brain and produces fainting,” he says. “Some people manifest fairly dramatic fainting that looks somewhat like tonic immobility, with a true flaccid paralysis, unresponsiveness, and then waking up.”

Tonic Immobility: A Spectrum of Behaviors?

Young notes that to produce tonic immobility in alligators, he had to force them into an unnatural position; although alligators do turn upside down during a death roll maneuver, the timing is much faster (they don’t stay on their backs for 30 seconds).

“Alligators are the only predatory terrestrial vertebrate to display tonic immobility, and we don’t really understand how this relates to nature,” says Young. “It could simply be a consequence of their unusual circulatory systems.”

This also means that this explanation is not likely to be the mechanism behind tonic immobility in other animals. For most of the organisms in which tonic immobility has been described, the physiological basis is unknown. It could be that the label “tonic immobility” has been applied to a suite of different behaviors for which we mostly do not know the cause.

For Young, this line of research has demonstrated that both novelty and connections can be found in surprising places.

“We found a behavior in the alligator that has links to medical conditions in humans and we also found ways of connecting this behavior to the unusual cardiovascular system of the alligator,” says Young. “Plus, we have described and demonstrated a phenomenon that alligator wrestlers have known for generations, providing a scientific basis for a long-standing roadside attraction.”