How Do Constrictors Kill Prey? Not by Crushing Bones

How constriction works, step by step

Constriction follows a similar pattern in boas, pythons and other constricting snakes:

1. The strike. The snake lunges and grabs the prey with its mouth. Rows of small, backward-curving teeth give a firm grip.
2. The coils. Almost at the same moment, the snake throws one or more loops of its body around the prey’s chest and body.
3. The squeeze. The snake tightens its coils and keeps the pressure on, adjusting its grip whenever the prey moves or breathes out.
4. The check. The snake senses the prey’s struggles and heartbeat through its body and keeps squeezing until they stop.
5. The meal. It releases the coils, finds the head and swallows the prey whole, usually head first.

The boa constrictor is the classic example, but the same method is used by snakes as different as the Burmese python of Southeast Asia and the corn snakes and kingsnakes of North America.

What actually kills the prey?

For a long time, many books said that constrictors crush their prey or suffocate it. The traditional idea was that the snake tightens its coils each time the prey breathes out, until the animal can no longer fill its lungs. Breathing really is blocked during constriction, but this would take several minutes to kill.

In 2015, a team led by biologist Scott Boback at Dickinson College in Pennsylvania tested what happens inside prey during constriction. They monitored the heart and blood pressure of anesthetized rats while boa constrictors constricted them. Within about 6 seconds, blood pressure in the arteries of the leg had dropped to half its normal value, while pressure in the large veins near the heart rose about six-fold. Within a minute, the heart rate had fallen to about half.

By the end of constriction, which lasted on average about 6.5 minutes, blood pressure and heart rate had collapsed, and 10 of the 11 rats showed abnormal electrical activity in the heart. Their blood had also become acidic and high in potassium, signs of a failing circulation. The researchers concluded that constriction kills quickly by circulatory arrest: the pressure of the coils stops blood from flowing, so the brain and heart are starved of oxygen.

Why don’t constrictors crush bones?

Crushing bones would be risky and unnecessary for the snake. The pressure needed to stop blood flow is far lower than the force needed to snap ribs or limbs. Broken bones are rarely reported in prey killed by constriction. A whole, intact animal is also easier to swallow than one with sharp, broken bones.

The myth probably comes from the dramatic appearance of a large snake wrapped around its prey and from old travelers’ tales of pythons and anacondas. Read How do snakes swallow large prey? to see what happens after the coils loosen.

How do snakes know when the prey is dead?

Constriction costs a snake a lot of energy, and while it squeezes, it is exposed to danger. So it pays to stop as soon as the prey is dead, but not too soon.

A 2012 study by the same research group found that boa constrictors can feel the heartbeat of their prey. The boas were offered warm rats that were already dead but fitted with a small simulated heart. When the “heart” was beating, the snakes constricted longer and with more total pressure than when it was still. When the heartbeat was stopped midway, the snakes relaxed their coils soon afterward. Even young boas that had never caught live prey responded to the heartbeat, which suggests the behavior is at least partly inborn.

Which snakes use constriction?

Constriction is one of the most widespread ways snakes kill. It is used by:

• Boas and anacondas of the Americas, including the boa constrictor and the green anaconda.
• Pythons of Africa, Asia and Australia, such as the Burmese and reticulated pythons.
• Many colubrid snakes, including rat snakes, corn snakes, kingsnakes, milk snakes, gopher snakes and bullsnakes.

Some constrictors are specialists. North American kingsnakes regularly eat other snakes, including rattlesnakes, and studies have found that they constrict more strongly than rat snakes of similar size. That extra strength helps them overpower prey that is long, muscular and able to fight back. Our article Are corn snakes venomous? explains how one of the most familiar small constrictors subdues mice without venom.

Constriction lets a snake kill prey large enough to be dangerous to it without needing venom. Scientists consider it one of the key innovations that helped snakes spread into so many habitats.

Myths about constrictors

Myth: constrictors crush their prey to a pulp. The prey’s bones normally stay intact; the coils stop the circulation instead.

Myth: constrictors squeeze only until the prey cannot breathe. Breathing is blocked, but experiments show circulation fails first and much faster.

Myth: pet pythons lie stretched out beside their owners to “measure” them for a meal. This viral story has no scientific basis. Snakes stretch out for many reasons, such as warming up or looking for a way out, and they do not size up prey this way.

Myth: all large constrictors are a danger to people. Wild boas are far too small to eat an adult. Only the very largest pythons and anacondas have ever been known to kill people, and such cases are extremely rare; see Can a snake eat a human?

Respecting constrictors in the wild

Constrictors are nonvenomous, and most species are shy animals that avoid people. If you see one in the wild, watch it from a distance and let it go on its way. Never try to pick up or handle a wild snake. If anyone is seriously injured by any animal, seek emergency medical help immediately: call 911 in the US, 112 in the EU or 999 in the UK.