The victims are squeezed until their circulation finally collapses. But not only the prey – the constrictors themselves are also under great pressure with their strangulation technique. A study of the boa constrictor shows how the reptiles can still breathe. The constrictors can therefore independently activate different sections of their long thorax for breathing movements. The experiments show that parts that are not involved in the strangulation can act like a kind of bellows for ventilation. The system also allows the snakes to breathe when a large prey is applying internal pressure after swallowing, the researchers say.
While some species produced fangs, other snakes have weaponized their entire bodies over the course of evolution: Boas and pythons in particular are notorious for their sophisticated strangulation technique, which they can use to overpower even large prey such as caimans and capybaras. After the reptiles have bitten, they wrap their long, muscular bodies around the prey. They then pull the body slings tighter with each exhalation of the victim. This also prevents the prey from breathing, but contrary to popular belief, it does not suffocate. The intense pressure instead leads to fatal circulatory failure, previous studies have shown.
How do they breathe during the grueling exertion?
So far, however, it has remained unclear how the snakes manage to supply themselves with air while the victim is tightly clutched. Because there seems to be little room for breathing movements. That the constrictors stop taking a breath is out of the question, because strangulation is hard work that involves high energy consumption and therefore a need for oxygen. “Because snakes don’t have a diaphragm either, they seem to have to rely entirely on the movement of their ribs for breathing,” says John Capano of Brown University in Providence. But how do they do that? As part of their study, he and his colleagues have now investigated the assumption that the animals can partially use their long lung and rib system to breathe.
They used the Boa constrictor, which is widespread in America, as a test constrictor. For the experiments, the scientists put metal cuffs on their test animals, which could variably restrict their breathing movements in certain areas of the body. The scientists used this to simulate the effect of the pressure on the snakes’ bodies when prey is strangled. Tiny metal markings were also placed on certain ribs of the boas. They were used to track the rib movements during breathing more precisely, which were recorded via X-rays.
Modular lung ventilation
The scientists report that their results show that the animals are able to independently control the movements of the ribs in different parts of the thorax. When the boas were pressurized by the pressure cuff at one-third of their body length, the animals breathed using the movements of more posterior ribs. When these, in turn, were constricted, the snakes breathed with the ribs closer to the head. According to the results, the rear part of the rib and pulmonary system seems to play the decisive role when strangling prey: When the front part is under pressure, this area can act like a kind of bellows and pull air through the pulmonary system, the researchers explain.
They also found indications that the effects are not based on a mechanical suppression of the rib movements, but rather on an active control process: by examining the nerve activity, they were able to show that no muscle contractions are triggered on the ribs that are under pressure. So the snake only triggers the energy-intensive activities for breathing movement where they make sense.
According to the researchers, the modular concept of breathing movement also plays an important role in another ability of many snakes: they can eat prey that is huge compared to their own body size. If, for example, a swallowed caiman presses on the inside of a boa’s chest, it can use its unaffected rib areas to breathe.
The sophisticated breathing technique could thus have become the key to the success of the snakes, which could fan out into many species over the course of evolutionary history, the researchers say. In conclusion, Capano and colleagues write, “This study offers a new perspective on snake evolution and suggests that modular lung ventilation arose during or before constriction techniques and the ingestion of very large prey.”
Source: The Company of Biologists, Article: Journal of Experimental Biology, doi: 10.1242/jeb.243119