A boa constrictor's squeeze seems to work by stopping blood flow

Anesthetized lab rats, wired with pressure sensors and heart monitors, were offered to captive boa constrictors in a 2015 study. Within six seconds of the coils closing, blood pressure in an artery of a rat’s leg fell to half its starting value. Pressure in the big veins near the heart rose sixfold. The heartbeat slowed. None of that sounds like an animal running out of air. It sounds like a plumbing problem.

That is the main idea of this article: constriction is a squeeze that stops circulation. We walk through what the researchers measured, why the older suffocation story hung on, how a boa seems to tell when its meal has stopped, and what the study could not show. We also say plainly what we did and did not find about bones. Most of the numbers come from one research group and from rats, so we mark where the evidence ends. Some sources disagree with each other, and we point that out too.

Meet the boa constrictor

According to the Animal Diversity Web (ADW), a University of Michigan database, the boa constrictor lives only in the New World, from northern Mexico through Central and South America, plus nearby islands. It is found in rainforest clearings and edges, woodland, dry forest, thorn scrub and farmland, and it is semi-arboreal, meaning it spends part of its time in trees. It is an ambush hunter that feeds mainly on small mammals, including bats, and on birds. The Reptile Database lists six subspecies but notes that the status of several forms is unresolved, so even the name on the label is still under discussion among specialists.

This article is not a guide to handling boas. They are large snakes, and they are best watched from a distance.

What the squeeze does to blood

The 2015 study, by Scott Boback and colleagues in the Journal of Experimental Biology, describes itself as the first to document how the prey’s body responds. The team measured anesthetized rats before, during and after constriction by boas. Peripheral arterial pressure is the blood pressure in a limb artery. Central venous pressure is the pressure in the large veins that return blood to the heart. Systemic perfusion pressure is the difference between them, roughly the push that drives blood through the tissues.

Within 6 seconds, arterial pressure in the leg fell to half of baseline while central venous pressure rose six times. Within 60 seconds the heart rate had dropped to nearly half. The squeeze lasted a mean of 6.5 minutes. By the end, arterial pressure was down 2.9-fold, heart rate 3.9-fold and perfusion pressure 5.7-fold. Ten of 11 rats showed abnormal electrical activity in the heart. Right after the squeeze, the blood’s potassium had nearly doubled and its pH had fallen from 7.4 to 7.0, which means the blood had turned more acidic. The authors read this as support for the idea that constriction causes rapid death through circulatory arrest, meaning the circulation stops.

Science News describes the result as blood backing up behind the coils until the heart cannot pump effectively. National Geographic’s account is similar: arterial pressure falls, venous pressure rises and blood vessels close.

Suffocation, crushing and what was not measured

The suffocation idea was not silly. The Christian Science Monitor says many people assumed it because a rat in a boa’s coils looks as if it is gasping. The same report says a 1928 study and a 1994 study by the anesthesiologist David Hardy pointed the other way, and that Hardy read the speed of death as a sign of circulatory or cardiac arrest. The 2015 work tested the idea directly. Boback told the reporter that suffocation can take minutes to kill a rat, while circulatory arrest can kill within about 60 seconds.

What about bones? Here we have to be careful. The study measured pressures, heart signals and blood chemistry. It did not report broken bones, and none of the pages we read describes bone-crushing as how boas kill. National Geographic reports that the squeeze pressures were not especially high, comparing the pressure on a small rat to what would stop blood flow in a human arm. That comparison is the reporter’s account of the researchers’ remark, not a number from the paper. We would not say bones can never break in any constriction by any snake, only that this research does not point to crushing.

The limits are real. The rats were anesthetized, which is not how prey behaves in the wild. The researchers did not measure brain activity, according to the Monitor, and the heart explanation is their interpretation of the readings. National Geographic adds a puzzle: in Honduras, a boa constricted a spiny-tailed iguana for about an hour. The team assumed it was dead, and the iguana was alive and apparently unharmed the next morning. They could not explain it, and the article suggests boas may have more trouble with cold-blooded prey. That is one observation, not a finding.

How a boa knows when to let go

A boa that squeezes too briefly lets dinner walk away. One that squeezes too long wastes energy. A 2012 study in Biology Letters asked how boas decide. The researchers fitted warm, dead rats with a simulated heart. The boas constricted longer and with greater total pressure when the heart was beating, and when the heartbeat was stopped partway through, the snakes released shortly afterward. Snakes that had never met live prey did the same, which suggests the behavior is at least partly innate. Dickinson College’s account of the work describes the same pattern: more pressure and longer squeezes with a heartbeat, shorter and weaker ones without.

So a boa’s squeeze is not a fixed vise. It is a feedback loop that reads the prey. The published abstract does not give the number of snakes, the length of the squeezes or the pressure values, so we have none to report.

What this looks like in practice

Here are the main numbers from the 2015 study, as stated in its abstract.

Common mistakes and myths about constriction

Myth: the snake suffocates its prey. The 2015 researchers say the rats were not dying from lack of air. The ADW page still describes a squeeze that ends in suffocation, so you will find both stories in print.

Myth: the snake crushes bones. We found no source in our reading that describes crushing as the method. See the section above for what was and was not measured.

Mix-up: the squeeze is a constant vise. Tightening and release follow the prey’s heartbeat, according to the 2012 study.

Mix-up: a snake’s jaw comes unhinged to swallow. Science News calls the idea that snake jaws separate at the back a myth. It credits movable head bones and a stretchy dimple at the chin, which it describes as cartilage. Some anatomy references describe the joint between the two lower jaws as an elastic ligament instead, so we do not name the tissue ourselves.

Mix-up: constriction and venom do the same job the same way. The two methods differ. ADW calls the boa non-venomous, while the King cobra uses a neurotoxin that, ADW says, affects the brainstem’s breathing centers.

Mix-up: only big snakes constrict. The Corn snake, a much smaller eastern U.S. species, also kills prey by constriction, according to ADW. Our sources do not say whether its squeeze works the way the boa’s does.

Conservation status

The boa constrictor is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category. One of our sources, the Animal Diversity Web, lists the species as Not Evaluated, which shows how reference pages can lag behind the IUCN.

Least Concern is not the same as no problems. ADW names overcollection for the pet trade, direct persecution, harvest for skins and meat, habitat loss and road deaths, and says some island populations are endangered. It also lists most boas under CITES Appendix II, a list that controls international trade, with one subspecies in Appendix I. We have no confirmed population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the IUCN assessment, so we do not state one.