This is why boa constrictors can breathe while squeezing the life out of prey

Watching a boa constrictor eat its prey is quite fascinating. First, the snake strikes the prey with its teeth, then it pulls its body tightly around it, and finally it squeezes the life out of it. The blood flow to the heart and brain is cut off. The boa unhinges its jaw and swallows the prey. The boa uses its muscles to move its prey down the length of its body to the stomach, where the unlucky varmint is able to digest it over the next four to six days.

The boa constrictors mostly eat medium-sized rodents, lizards, and birds. They are known to eat larger prey, including monkeys, wild pigs, and ocelots. Even though the snakes are crushing an animal to death, how do they breathe when they squeeze the boas own ribs? Unlike mammals, boa constrictors don't have a separate body part. They rely on their ribs to breathe.

The results of a series of experiments conducted by biologists at Brown University and Dickinson College were published in the Journal of Experimental Biology. They discovered that boas have a remarkable ability to use different parts of their rib cage for breathing. The far end of the reptile's lungs is used to pull in air when the ribs are not open.

The team used a combination of techniques for their study. All but one of the snakes used in the experiments were born in captivity. The outlier sole was purchased from a reptile breeder.

The x-ray experiments were performed by John Capano of Brown University. He used the data from the scans to create a computer model of the rib and vertebrae movements. Capano attached metal markers to the ribs of three adult female boa constrictors. One marker was placed about a third of the way down the body length, and the other marker was placed halfway down.

Capano placed blood pressure cuffs over the ribs to make it look like the snakes would crush their prey. According to Capano, some snakes did not mind the cuff. Since hissing requires snakes to fill their lungs with air, the latter response was ideal for the experiments. The biggest breaths that Capano was able to measure were produced by the hissing snakes.

The team made small lightweight masks for the boas out of plastic bottles, using pneumoglobinography to study sleep apnea and related disorders in humans. A fine metal mesh is used in the snake breaths to provide resistance to the air flow. The flow rate is determined by the pressure difference over that fixed resistance.

The results were inconsistent because the snakes kept taking off their masks. Humans find the procedure uncomfortable, so one can't blame the snakes. The method provided reliable data on pressure variation and volume changes as the snakes breathed in and out, and the biologists visually confirmed that data in the X-ray videos in several cases.

Scott Boback and Charles Zwemer took on the task of determining whether the snakes were capable of specific patterns of muscle activation. This was done by recording the nerve signals that controlled the rib muscles as pressure was being applied with the cuffs on one adult female boa and one adult male boa. Boback was able to capture one snake using a camera. The snake activated a different set of ribs further down its body to continue breathing, and he found no nerve signals in the constricted muscle.

The authors wrote that they found multiple lines of evidence in support of their hypothesis that boa constrictors actively modify the trunk segments and ribs used for lung ventilation. When pressure was applied to the cuff a third of the length down the body, the snakes responded by moving the ribs further back to breathe. They were tipped up by the snakes to get air into their lungs. When pressure was applied further down the body, toward the far end of the lung, the snakes were able to breathe.

The ability to eat large prey would not have arisen unless the snakes had first gained the ability to sustain high metabolism and regulate their breathing, according to the authors. This trait likely co-evolved with those other two. The ability to breathe while consuming large prey would help conserve energy.

Early snakes would not have been able to circumvent the mechanical and physiological constraints each behavior produced.

The Journal of Experimental Biology was published in 2022. The About DOIs section contains a description of thejeb.243119.