Boa constrictors move ribs to avoid suffocating when they kill prey

The boa constrictor has developed a way to control air movement in its lungs using ribs in the lower part of its body, so it can breathe while it squeezes the life out of its prey

By Christa Lesté-Lasserre

The boa constrictor can move different parts of its ribcage independently, allowing it to breathe even while it crushes prey using its body.

The mammals, reptiles and birds that this snake hunts can survive for several minutes as the constrictor squeezes the life out of them using the upper third of its body. But that part of its body also contains the lungs, so at first glance this might suggest these snakes risk suffocating themselves while killing prey.

John Capano at Brown University, Rhode Island, and his colleagues have worked out how the snakes cope. Capano says they draw air into their constricted lungs by selectively rotating ribs farther down the body.

“They seem to have very refined control over which segment of their body they actually are ventilating with,” he says. “This explains how it is that they’re using a ribcage to ‘kill’ another ribcage, but that they themselves are fine.”

Snakes lack diaphragms, which means they have to move their ribs in order to inhale and exhale. While they have very long lungs that extend beyond the bodily region used for constricting, the bottom part of the lungs is “literally just like a balloon” and can’t actually respire, says Capano.

Like many biologists, Capano and his colleagues wondered if this so-called “saccular” lung region played a role in helping boas breathe during the relatively long constriction process. They placed a human blood pressure cuff over different parts of three boa constrictors (Boa constrictor) and observed the body movement of each during hissing – which requires deep breathing.

X-ray videos revealed that when any one section of ribs was constricted by the cuff, the snakes moved other ribs elsewhere in the body to compensate. In particular, when the cuff covered the region over the upper lungs, the snakes would start moving the ribs around the saccular lung region. And when the higher ribs weren’t constricted, the saccular region ribs stayed still, says Capano.

The researchers realised that when the upper lung region was constricted, the snakes were using these lower ribs to inflate and deflate the saccular lung area, says Capano. This would draw air through the upper lung region, where oxygen could exchange with carbon dioxide so that true respiration could continue to occur.

This activity was a “wilful” choice by the snakes, he adds. Using a technique to study muscle activity revealed that the boas can control which rib muscles to move.

“These snakes can very discreetly shift which subset of ribs they are using to ventilate, at will,” Capano says. “That means turning on two sets of ribs, or turning on nothing, or turning on another set in the hind and then immediately switching it back to the front.”

Journal reference: Journal of Experimental Biology, DOI: 10.1242/jeb.243119

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