How Boa Constrictors Can Breathe Even When They Crush Their Prey

Boa constrictor science GettyImages 1128449635


Watching a boa constrictor catching and consuming its prey is quite something. First the snake strikes the prey with its teeth and grabs it, then it wraps its body tightly around the poor creature and slowly squeezes the life out. The constrictor cuts off blood supply to the heart and brain. Then the boa loosens its jaw and swallows the prey whole. The boa uses its muscles to move its prey the length of its body to its stomach, where the unfortunate vermin is digested over the next four to six days.

Boa constrictors usually consume several medium-sized rodents, lizards, and birds. They are also known to feed on even larger prey, including monkeys, wild boars, and ocelots. Regardless of what’s on the menu, how do the snakes manage to breathe while crushing an animal, since that constriction also uncomfortably squeezes the boas’ own ribs? Unlike mammals (including humans), boa constrictors do not have a separate diaphragm. They rely entirely on the movement of their ribs to breathe.

Biologists from Brown University and Dickinson College conducted a series of experiments to find out more, and they described their results in a new paper published in the Journal of Experimental Biology. Boa constrictors, they found, have a remarkable ability to selectively use different parts of their ribcage to breathe during constriction. Whenever the ribs closest to the head are obstructed, the lungs essentially serve as a bellows to draw in air so that the snake can still breathe.

The team used a combination of techniques for their research to collect critical data on airflow, muscle activation and rib movement in vivo. All but one of the snakes used in the experiments were captive born, bred from boa constrictors captured in Belize. The lone outlier was purchased from a reputable reptile breeder, according to the authors.

Co-author John Capano of Brown University conducted the X-ray experiments, using a technique known as XROMM (X-ray Reconstruction of Moving Morphology) to create X-ray films of the snakes. He also took CT scans and used that data to reconstruct the movements of the rib and vertebrae in a computer model. Capano first attached small metal tags to two ribs in each of three adult female boa constrictors. One marker was placed about a third of the body length and the other halfway.

Then Capano placed blood pressure cuffs over the ribs in those two spots and gradually increased the pressure to immobilize the snakes — essentially simulating what would happen if they crushed their prey. Some snakes didn’t seem to mind the cuff, per Capano, while others hissed. The latter reaction turned out to be ideal for the experiments, because the snakes have to fill their lungs with air before hissing. Therefore, the hissing snakes produced the largest breaths Capano could measure.

The team used pneumotachography (often used to study sleep apnea and related conditions in humans) to monitor airflow in five boa constrictors, making small lightweight masks for the tubes from plastic bottles. The hose breathes through with a PVC tube that has a fine metal mesh inside to provide some resistance to the airflow. The pressure difference across that fixed resistance produces the flow rate.

The authors acknowledged that these results were inconsistent, especially as the snakes continued to take off their masks. (Even humans find the procedure uncomfortable, so one can hardly blame the hoses.) However, the method provided reliable data on pressure variation and volume changes as the hoses inhaled and exhaled, and the biologists were able to visually confirm that data in the X-ray videos in different cases.

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