Locomotion inside-out: Swallowing in snakes

Imagine eating a 50 pound hamburger in one single bite--most snakes can do something like that! They can swallow food items much bigger than their own head and even bigger than their whole body. Snakes do this by expanding the jaws at the tip and at the articulation with the skull, and then by moving the jaws in small steps over the food item.

There have been many good research studies on how the jaws and skulls of snakes work during feeding, but they only tell part of the story about how snakes feed. Even after food gets past the snake's jaws, it still has a long way to go--maybe half the length of the body--to get to the stomach. Snakes that feed on large food items swallow the food by using a kind of inside-out locomotion. Once the food is in the throat, it is pushed down by two kinds of movements. One of the swallowing movements seems to be the same as in other animals. Muscles in the walls of the esophagus contract to squeeze the esophagus and push the food down. The second swallowing movement is high specialized in snakes, and involves undulations of the backbone and body. The spine and body bend behind (on the side toward the snake's mouth) the food, which makes the snake's ribs push inward against the food. These spinal bends are pushed along the body like the undulations of locomotion, and so they push the food back along the snake toward the stomach. This all makes it look like the snake is crawling forward over the food inside it. In fact, sometimes the snakes really do crawl forward as they swallow.

The undulations that snakes use for swallowing are similar to the ones they use for the most common kind of locomotion (lateral undulation). But the undulations of locomotion push outward against objects on the ground whereas swallowing undulations push inward against the food. Also, swallowing undulations are sometimes less regular and have more starts and stops than locomotor undulations. Some of the same spinal muscles are used to undulate the body during swallowing and locomotion, but the speed of undulation and the waves of muscle contraction along the body are slower in swallowing than in most locomotion.

The spinal bends and muscle contractions of swallowing and locomotion show how some of the same muscles are used for different behaviors, and how different mechanical demands (such as inward versus outward force exertion) relate to different patterns of muscle activity. By being able to bend the body in different ways for different reasons, snakes have solved many (maybe even most) of the potential problems imposed by not having legs and by eating food items whole that are often bigger than themselves.

Selected References

© 2001 Brad Moon, All Rights Reserved

Questions or comments? Send me e-mail: BradMoon(at)louisiana.edu

Main page
Prospective students