Snakes that constrict their prey and squeeze them to death have an interesting challenge to face. How does a snake squeeze its prey so hard that it stops breathing while, at the same time, ensuring it doesn’t squash its own lugs and suffocate itself? The latest study on this topic concludes that snakes, such as boa constrictors, are able to breathe with different parts of their ribcage, depending on whether they are throttling a prey, eating a meal or digesting their most recent dinner.
“With no diaphragm, they rely entirely on motions of their ribs,” said John Capano from Brown University, USA. He added that the earliest snake ancestors must have overcome the challenge of breathing while squeezing and digesting dinner, but it was not clear how modern snakes prevent themselves from suffocating while constricting their victims.
The researchers considered the possibility that boa constrictors adjust the region of the ribcage they use to inhale, depending on whether they are resting, constricting a prey animal or digesting. But no one had monitored in detail the breathing patterns of snakes in the act of subduing their dinner to check whether the animals can indeed adjust which section of the ribcage they use.
Capano and co-author Elizabeth Brainerd (Brown University) investigated the way in which boa constrictors breathe by first attaching small metal markers to two ribs in each of the study snakes. One marker was a third of the way down the snake’s body, while the second marker was halfway along the body. They then used X-rays to monitor how the ribs moved while the snake carried out various activities.
The researchers placed a blood pressure cuff around the ribs of the snakes in the same regions where the rib markers had been placed. As the cuffs were inflated, the increasing pressure immobilized the snake’s ribcage and breathing became more difficult. What they discovered was that the snakes simply used different sections of the rib cage with which to breathe when the ribs were prevented from moving freely by the blood pressure cuffs. In this way, the hind section of the snake’s lung works like a bellows, to pull air into the lung when the ribs further forward can no longer move, as would be the case when squeezing a prey item to death.
The findings of the study, published today in the Journal of Experimental Biology, clearly show that when a boa constrictor is squeezed by the blood pressure cuff a third of the way along the body it breathes using the ribs further along, while when the ribs are constricted halfway along the body, the snake breathes using ribs closer to its head. Breathing is thus compartmentalized along the body, with different regions being able to move independently.
“Either the animals did not mind the cuff or became defensive and hissed to try to get the researcher to leave,” recalls Capano, explaining that the reptiles really fill their lungs when hissing: “this was an opportunity to measure some of the biggest breaths snakes take.”
In addition, Capano, Scott Boback and Charles Zwemer (both from Dickinson College, USA), filmed and recorded the nerve signals controlling the rib muscles when constricted by the blood pressure cuff. Boback also filmed a snake with a GoPro as it dined, which revealed that the ribs were not simply being held immobile. There were no nerve signals in the constricted muscles; the snakes had shifted to breathing by activating a different set of ribs further along the body.
Subduing and digesting a victim are two very energetic things that boa constrictors do and it would have been crucial for them to evolve the ability to breathe using different regions of the rib cage before they adopted the strategy of squeezing their prey to death. If this had not happened, they would have suffocated themselves each time they tried to constrict a prey. “It would have been difficult for snakes to evolve those behaviors without the ability to breathe,” concluded Capano.