Boa Constrictors Kill By Stopping Blood Circulation

The popular belief that boas and other constricting snakes deal death by suffocation seems to be a flawed assumption

Boa constrictors seem to deliver death not through suffocation, but by cutting off blood flow to the heart and brain. Photo: Ivan Kuzmin/imageBROKER/Corbis

Snakes aren’t kind killers, often pumping prey full of toxins or swallowing still-living victims whole. Boas and other constrictors, however, prefer a more intimate approach: They lock their victims in a deadly embrace, crushing the life out of them before feeding. Popular lore says that constrictor victims succumb to death by suffocation, but while this theory has been questioned as far back as the 1920s, the assumption hasn’t been verified in scientific tests.

Now, lab experiments reveal that constrictors most likely dole out death by stopping their prey's blood flow, depriving the heart and brain of that vital fluid. Animals trapped in such a death grip would pass out and die within minutes, according to a study published this week in The Journal of Experimental Biology

The death-by-suffocation hypothesis probably stems from the fact that a rat or rabbit caught in a constrictor’s coils looks as if it is gasping for air. But the speed at which its life is snuffed out tells a different story, says lead author Scott Boback, a herpetologist at Dickinson College.

Boback and his colleagues anesthetized 24 rats, which they offered up to 9 boa constrictors—some wild-caught in Belize, and others captive-bred. Before sacrificing the rodents, the researchers inserted ECG electrodes and catheters into the animals’ bodies so they could monitor heart rate and blood pressure data throughout the crushing process.

The researchers were surprised to observe that blood circulation dropped by half within six seconds of the snakes wrapping their coils around the rats. Over the next minute, the rats’ hearts began beating erratically, causing “severe impacts on cardiovascular function,” the researchers write.

By the end of a six-minute period, more than 90 percent of the rats suffered likely irreversible heart damage and died. The rats, however, would not have known this. Had they been conscious, Boback thinks they would have passed out moments after the squeezing began due to lack of blood flow to the heart and brain.

The results show that constrictors' prey die much too quickly for suffocationwhich likely takes minutes, not seconds, to be the culprit. "The interesting thing about our findings is that there were a number of physiological failures that were all occurring simultaneously in constricted rats," Boback says in an email. These include decreasing pressure in the animals' arteries, increasing pressure in their veins and blood that was high in potassium and acidity. "Each one of these failures could have caused death in the animals," he continues. "The fact that all of them were occurring at the same time is pretty remarkable and significant for the rats."   

Still, if your fate is to be a snake's dinner, the relatively quick death delivered by constriction seems almost preferable to being swallowed alive or injected with lethal venom, which starts digesting an animal’s tissues before it is dead, or else causes uncontrolled internal bleeding or clotting. As Boback says in a statement: “By understanding the mechanisms of how constriction kills, we gain a greater appreciation for the efficiency of this behavior.”

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