Rather than envenomating their prey, many snakes get a toothy hold on their next meal, coil their serpentine body around it, and squeeze. They soon asphyxiate their prey. Anacondas, pythons, boa constrictors, as well as less-fearsomely sized king, gopher, and bull snakes all prepare dinner this way.

These facts have caused inquiring minds to wonder, “Why don’t snakes asphyxiate themselves with the pressure exerted on their own lungs during this embrace?” Scientists now have the answer. To understand it, here’s a bit of snake anatomy.

Snakes ribs, as many as 200 pairs, float. In other words, the ribs attach at only one end, to the spine. Therefore their skeletons readily adapt to changing body girth, for instance when an anaconda swallows a pig. For comparison, the top ten pairs of human ribs attach to the breastbone as well as the spine, and only the lower two sets are floating.

Additionally, constrictors have a special muscle attached to each rib and to the spine. Known as the levator costa, this muscle’s contraction lifts the rib upward and outward to expand the body cavity. That is important, because snakes do not have diaphragms, so the only way they can pump air in and out of their lungs, which extend back about a third of their body length, is to contract and relax their levator costa muscles. That works fine when the snake is resting or on the move.

When it embraces its prey, however, the snake also equally embraces itself and therefore cannot breathe, at least in the portions of its body performing the deadly hug. Can constrictors activate their levator costa muscles selectively, breathing with ones not involved in the squeeze? That would be efficient but would require sophisticated neural control—somewhat analogous to humans breathing in and out of just one lung. Enterprising investigators found the answer by applying standard, human blood pressure cuffs around the bodies of boa constrictors and placing masks over their faces (the snakes’, not the investigators’). Then the researchers studied rib motions in both constricted and unconstructed portions by recording electrical activity and video X-ray images. By placing the blood pressure cuff in different locations along the snake’s trunk and repeating the pressurizations, the investigators determined that constrictors exclusively use levator costa muscles only in free areas and do not even attempt to breathe using the muscles in the pressurized areas. They (the investigators, not the snakes) call the phenomenon modular lung ventilation, which allows constrictors to squeeze and breathe simultaneously. Bones, muscles, and nerves working together. This amazing efficiency of nature will definitely provide me some solace if I ever encounter a python’s embrace.    

This time last year, I posted A Plea for Anatomical Correctness at Halloween. Apparently the plea went unheeded, because I have recently seen these frightful creatures. Rest assured, the Bone Police are on their way. Let me know if you spot other cases of osseous infractions, and I will report them to the authorities.

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