Constriction in snakes

Many snakes constrict prey to kill it before swallowing it. Constriction involves wrapping loops of the body around a prey animal to restrain it so that it cannot escape or injure the snake, and to kill it before ingestion. Constriction was probably a key innovation in the evolution of snakes and was associated with the diversification of snakes.

Snakes constrict many different kinds of prey. Some snakes constrict lizards and other snakes, but most snakes use constriction to kill mammals from mice to antelopes. Even some venomous snakes (particularly some Australian snakes) use constriction to restrain prey.

The postures of constriction vary among snakes. For example, some boas and pythons bend the neck vertically around prey whereas many colubrid snakes, such as gopher and king snakes, bend the body sideways around the prey. Constriction postures seem to be more stereotyped in boas and pythons than in colubrids, which have highly variable constriction postures.

The epaxial muscles (the same muscles that are active during locomotion and other movements) are highly active during striking and coil formation, but only intermittently active during sustained constriction. This suggests that the epaxial muscles participate in constriction but are not the only muscles used in constriction. Although these long muscles and tendons are probably not the most important constricting muscles, they do not interfere with constriction. Which other muscles are involved in constriction is not yet known.

Constriction appears to involve continuous squeezing, but often is actually intermittent. For example, gopher snakes (Pituophis melanoleucus) and king snakes (Lampropeltis getula) squeeze mice continuously whenever they struggle, but hold the constriction posture without actually squeezing whenever the prey is still. Muscles use a lot of energy when they exert force, so by squeezing intermittently only when necessary, the snakes probably save a lot of energy. Holding the constriction posture even when not squeezing allows a snake to squeeze again very quickly if the prey starts to move again.

Constriction is usually assumed to kill prey by suffocation because the forces exerted appear to interfere with breathing in the prey. However, constriction often kills small mammals faster than would be expected if suffocation were the immediate cause of death. In many snakes, constriction may be strong enough to collapse the blood vessels in small prey, which in turn would quickly stop the heart from working and lead to heart attack and stroke. The postures, muscle activity patterns, and forces of constriction need to be studied in more species and with different kinds of prey, but we are already learning that constriction can be stronger than we used to think and that it does not necessarily involve tradeoffs with locomotor speed.

Selected References

© 2001 Brad Moon, All Rights Reserved

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

Main page
Prospective students