Constriction Does Not Crush or Suffocate, and the Green Anaconda Is Not the Longest Snake

The standard description of how a constrictor kills has two versions, and both are wrong.

The first is that the snake crushes its prey, breaking bones and flattening organs. The second, more common in careful sources, is that it suffocates: every time the animal breathes out the snake takes up the slack, the chest cannot expand again, and eventually the prey stops breathing. It is a patient, grim, slow-motion image, and it has been repeated in textbooks for decades.

In 2015 somebody finally put instruments inside the coils.

What the Measurements Showed

Scott Boback and colleagues published the result in the Journal of Experimental Biology. They anaesthetised rats, fitted them with electrocardiogram electrodes and blood pressure catheters in a major artery and a major vein, and then let boa constrictors do what boa constrictors do, recording throughout.

The numbers are not subtle. Within six seconds of the coils closing, arterial blood pressure at the femoral artery had fallen to half its baseline. Over the same few seconds, central venous pressure rose roughly sixfold.

That combination is the whole story. Arterial pressure is what pushes blood out to the brain, heart and organs; venous pressure is what lets it return. Squeeze the whole body at once and you collapse the first while the second backs up. Circulation stops. The prey loses consciousness in seconds, and the heart fails shortly after.

Suffocation would take minutes. This does not.

Why the Distinction Matters

It is tempting to file this under trivia, but it changes what the animal is doing.

A snake that suffocates its prey has to hold on for several minutes, out in the open, wrapped around something that is still struggling. That is dangerous and expensive. A snake that induces circulatory arrest is finished in well under a minute, which is a far better deal for an animal that has to survive whatever the prey does on the way down, and whatever else is watching.

It also explains a detail keepers have long noticed: constrictors tighten in response to the prey's heartbeat, not just its breathing. If circulation is the target, that is exactly the feedback you would expect the snake to be reading.

One honest caveat. The measurements were taken with boa constrictors, because you can work safely with a boa in a lab. The mechanism is expected to hold across constricting snakes generally, anacondas and pythons included, since the physics of squeezing a body does not change with the species. But the direct data is from boas, and it is worth saying so rather than quietly generalising.

The Other Anaconda Myth: Longest Versus Heaviest

The green anaconda is routinely called the biggest snake in the world. That is half right, and the half that is wrong is the half everyone remembers.

For sheer length, the record belongs to the reticulated python. The longest properly documented individual was a python of about 10 metres, taken in Sulawesi in 1912. The longest reliably measured captive snake, a reticulated python named Medusa, reached 7.67 metres and around 159 kilograms.

Green anacondas do not reach those lengths. Females typically run 3 to 5 metres. What they do instead is get thick. An anaconda is built on a completely different cross-section, and a 5 metre anaconda substantially outweighs a 5 metre python. Guinness World Records lists the green anaconda as the heaviest snake species, and the reticulated python as the longest, which is the correct way to split it.

The nine and ten metre anacondas of Amazon expedition literature have never been confirmed. Claims have circulated for well over a century. None has produced a measured animal.

Two Anacondas, Not One

There is a genuinely recent wrinkle here. In February 2024, researchers split what had been a single species into two, on the basis of genetic and geographic evidence: the southern green anaconda, which keeps the name Eunectes murinus, and the northern green anaconda, Eunectes akayima.

They look almost identical. The divergence between them is nonetheless substantial, and it is the sort of finding that quietly invalidates a lot of older population figures, because nobody collecting them knew they were counting two species.

What the Animal Is Actually Like

Strip away the exaggeration and the anaconda is still a remarkable animal. Its eyes and nostrils sit on top of its head so it can lie almost entirely submerged with nothing showing. It is clumsy and slow on land, where its own weight works against it, and transformed in water. Females are far larger than males, which is the reverse of the usual pattern in snakes. And it gives birth to live young rather than laying eggs, sometimes twenty or more at a time, each already able to swim and hunt.

The full profile is in the Beastlypedia green anaconda file.

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