Boa constrictors, snakes that coil around prey and squeeze it to death before swallowing it whole, prevent suffocating themselves during the process by adjusting the sections of the ribcage they use to breathe, a new study has found.
Unlike humans who have diaphragm muscles beneath the rib cage, the snakes rely entirely on the fine control they have on different sections of their rib cage when constricting prey and digesting dinner, noted the study, published on Thursday in the Journal of Experimental Biology.
When the snakes are fully squeezing prey to death and the ribs can no longer move, the hind section of the lung pulls air into it, working like bellows, say the scientists, including those from Brown University in the US.
Researchers attached tiny metal markers to two ribs in each reptile – one a third of the way down the snake’s body and another halfway along – to visualise how the ribs moved using X-rays.
A blood pressure cuff was placed over the ribs in both regions, and the pressure was gradually increased to immobilise the snakes.
“Either the animals did not mind the cuff or became defensive and hissed to try to get the researcher to leave... This was an opportunity to measure some of the biggest breaths snakes take,” study co-author John Capano said, adding that the reptiles filled their lungs in when hissing.
Reconstructing the rib movements of the snakes, scientists found that boa constrictors could control the movements of ribs in different portions of the rib cage independently.
When the snakes were gripped by the blood pressure cuff a third of the way along the body, they breathed using the ribs further back, swinging the ribs backward while tipping them up to draw air into the lungs.
However, when the ribs toward the rear of the lung were constricted, the snakes breathed using the ribs closer to the head.
Scientists said the ribs at the far end of the lung only moved when the forward ribs were gripped, drawing air deep into the region, even though it has a poor blood supply and does not provide the body with oxygen.
Based on the analysis, researchers said the far end of the lung was behaving like bellows, pulling air through the front section of the lung when it could no longer breathe for itself.
Scientists also filmed and recorded nerve signals controlling the rib muscles when constricted by the blood pressure cuff, which revealed that the ribs were not simply being held immobile.
In the sections of the snake constricted by the cuff, researchers could not find any nerve signals, suggesting the snakes had shifted to breathing by activating a different set of ribs further along the body.
Since subduing and digesting prey is one of the most energy-intensive activities for boas, scientists said it was likely essential that they evolved the ability to adjust where they breathe before adopting this lifestyle.
“It would have been difficult for snakes to evolve those behaviours without the ability to breathe,” Dr Capano said.
“This study provides a new perspective on snake evolution and suggests that modular lung ventilation evolved during or prior to constriction and large prey ingestion, facilitating snakes’ remarkable radiation (species diversity) relative to other elongate vertebrates,” scientists wrote.
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