A Boa constrictor consumes a white rat

A Dumeril's boa, a relative of Boa constrictor, consumes a rodent. Image © NikonD810/Shutterstock

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Mystery of how Boa constrictor breathes while crushing prey solved

The puzzle of how snakes can keep breathing when squeezing their prey to death has been resolved.

With the ability to control their body in sections, snakes can move their body like a long set of bellows to continue sucking air through their lungs.

New research has found that Boa constrictors can move their ribs independently of one another, allowing the snakes to digest prey much larger than their mouths.

The work, published by American and Australian scientists, reveals the snakes can move different ribs to allow them to keep breathing while other parts of the body squeeze their prey to death. 

Their findings also explain why a particular section of their lungs is seemingly useless for breathing, as instead it acts to draw air through the functional areas of the lungs when they are compressed during constriction.

The research could also have implications for our understanding of some of the world's largest snakes, with the green anaconda known to eat prey as large as a jaguar. It also points to how snakes may have evolved to eat such a large range of prey in the first place.

The study was published in the Journal of Experimental Biology

A Boa constrictor with a rat tail hanging out of its mouth

Boa constrictor are generalists which will eat a large range of prey. Image © Oleg Blazhyievskyi/Shutterstock

What is Boa constrictor?

The boas are a genus of snake found in Central and South America. The most well-known species is Boa constrictor, which was for a long time the only species of its genus.

Some scientists now suggest there could be as many as five separate species, such as Boa imperator, although this is still debated.

The snakes eat a range of animals, from fish and small birds to monkeys and other reptiles. To catch their food the snakes wrap their body around the prey and squeeze. As the snakes get larger they can tackle bigger prey, with wild pigs among the largest eaten by Boa constrictor

It was originally thought that the snakes killed their prey by cutting off its air supply and suffocating the animals. But more recent research has shown that instead the snakes cut off the blood supply to organs including the heart and brain, causing rapid unconsciousness and death.

Once their prey is dead, Boa constrictor consumes it whole. Unlike mammals, the jaw bones of snakes are unfused and are instead connected by elastic tissue. As a result, their jaw can expand to allow them to swallow significantly larger prey than may otherwise be expected.

Their body can also expand to accommodate large food as snake ribs are only attached to the spine. They also lack a diaphragm, which is usually used to draw air into the body. Instead, they move their ribs in and out to achieve the required changes in pressure to breathe.

However, when the ribs are being used for compression or expanding to swallow, the snakes can't also move them to keep the snake breathing. Breathing is particularly important during these activities, which can increase the amount of oxygen the animal needs by up to 17 times its normal level.

As Boa constrictor's lungs are also being compressed when it is squeezing prey to death, the survival of the snake has remained something of a mystery. This new study may now have resolved it. 

A photo annotated to show the different rib areas of Boa constrictor

Different rib sections are used to breathe and constrict depending on the situation. Image © Scott Boback

How does Boa constrictor breathe while squeezing and swallowing?

The researchers used x-rays to examine how the ribs of captive Boa constrictor moved in different areas of the body during different behaviours. This included defensive hissing, which saw some of the largest rib movements across the length of the body.

An expandable cuff used to measure blood pressure was then wrapped around the snakes to allow the researchers to see how the snakes responded when the animals couldn't move certain areas of their body, as if they were constricting prey.

Electrical measuring was also conducted to see if levator costae muscles for different sections of the ribs could activate independently, or if all the ribs were moved at once.

The scientists found that the different sections of the ribs could move independently, with the muscles only activated in areas required to keep the snake breathing. Even when ribs which are normally used for breathing are constricting, posterior ribs can use the non-breathing section of the lungs to suck air in and allow the snakes' survival.

As these muscles are found in all snakes, it suggests that they could have been a crucial development in the evolution of these animals, and perhaps possessed by the ancestors of the group

Among the squamate reptiles, the levator costae are only found in limbless reptiles, suggesting it may be an adaptation that is crucial to a long, legless body form.

The researchers add that this adaptation probably had to develop before snakes gained the ability to eat large prey, or constrict them, as the metabolic costs are otherwise too high to sustain. 

Aside from its evolutionary implications, research to find out just how snakes are able to control their ribs is proposed to further uncover just how these animals perform these extraordinary feats.