Like boa constrictors, they swallow everything and don’t choke

New research shows how boa constrictors can adjust which region of the thorax they use to breathe in depending on whether they’re resting, contracting, or digesting.

Dinner puts the snake in a bind. The mechanics of squeezing a creature to death and then swallowing it whole puts pressure on the boa’s ribs and lungs, limiting its ability to fill its lungs with oxygenated air.

The new findings show that snakes have clearly overcome — some might even say, overcame — the challenges of breathing while eating.

“By showing how snakes were able to circumvent the mechanical constraints of constriction and the ingestion of large prey – the very things that helped distinguish them from other limbless, elongated animals – this study offers a new perspective on snake evolutionsays John Capano, a postdoctoral researcher in the lab of Matt Fuxjager, associate professor in the Department of Ecology, Evolution, and Organism Biology at Brown University and lead author of the paper in the Journal of Experimental Biology.

Snakes don’t have a diaphragm, so they rely entirely on the movement of their ribs to breathe, Capano says. To quantify individual rib movements, Capano and Professor of Biology and Medicine Elizabeth Brainerd used a 3D imaging technology called X-ray Reconstruction of Moving Morphology (XROMM), which can show the rapid skeletal movement of living creatures.

XROMM combines 3D models of bone morphology with motion data from X-ray videos to create high-precision reanimation of bones moving in space (for example: flying birds, frogs jumpand snakes breathe).

In the experiments, Capano and Brainerd placed a blood pressure cuff around the ribs of boa constrictors to restrict their movements. Capano attached tiny metal markers to two ribs in each reptile — one set of markers on a third of the snake’s body and another halfway down — to use X-rays to visualize how the ribs were moving. He then placed a blood pressure cuff over the ribs in both regions and gradually increased the pressure to prevent the ribs from moving.

Some of the snakes responded to the feel of the cuff by hissing defensively, filling their lungs with air, and expanding their ribs. “The hissing gave us an opportunity to measure some of the largest breaths snakes take,” says Capano.

When reconstructing the rib movements of the boa constrictors, it was clear that the animals were able to independently control the movements of the ribs in different sections of the thorax. When the blood pressure cuff covered the boa constrictor for one-third of its body length, the animals breathed with the ribs that were further back.

However, when the ribs were constricted at the back of the lungs, the snakes breathed with the ribs closer to the head. In fact, the ribs at the far end of the lungs only moved when the front ribs were grasped, sucking air deep into the back section of the lungs, even though that more back region has poor blood supply and cannot take oxygen into the body.

The researchers discovered that the back end of the lungs (known as the saccular region) acted like a bellows, pulling air through the front part of the lungs (the vascular region) when that area wasn’t doing its job. In this way, they could continue to supply the body with oxygen even when the vascular region was no longer able to ventilate itself. In control experiments where no pressure was applied to the vascular region, snakes continued to breathe with rib movements in this area.

In addition, Capano, along with Dickinson College’s Scott Boback and Charles Zwemer, recorded the nerve signals that control the muscles that move the ribs during various behaviors. Boback also filmed a snake with a GoPro camera as it ate. The researchers found that the snake could turn off nerve signals to muscles in both the front and back of the lungs; The snakes could shift where they were breathing by activating a different set of ribs farther along the body.

The researchers found several lines of evidence to support their hypothesis that boa constrictors actively modulate the trunk segments and ribs used for lung ventilation in response to impeded rib movements. They also confirmed that during prey ingestion, boa constrictors breathed using segments that were not filled with food, shifting the sections used as prey moved through their digestive system.

Because subduing and digesting a victim requires a lot of energy (and therefore oxygen), the researchers concluded that the snakes likely adapted their breathing style before evolving into larger prey.

The findings help explain how snakes were able to adapt to a wide variety of habitats, evolve into myriad species, and reproduce, Capano says.

“Snakes are by far one of the most prevalent and adaptable predators on the planet, but the fact that they have managed to become so successful within the confines of an elongated, limbless body is quite remarkable – especially considering that this body type has limited the diversity and range of other similarly shaped vertebrates.” says Capano.

The evolution of the ability to constrict and ingest large prey drastically expanded snakes’ foraging options beyond the small insects consumed by most elongate vertebrates. However, Capano says, “We hypothesize that constriction and feeding on massive prey could not have evolved to such extreme and effective levels without modular lung ventilation.”

Additional co-authors are from the University of the Sunshine Coast in Australia and Brown. The National Science Foundation and Sigma Xi funded the work.

Source: University of Brown Like boa constrictors, they swallow everything and don’t choke

Dais Johnston

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