It’s pretty obvious that boa constrictors squeeze the life out of their prey.

But how, exactly? Do they suffocate them?

That explanation, long the conventional wisdom, had been challenged but not tested, according to Scott M. Boback, a herpetologist at Dickinson College in Pennsylvania.

He joined with his colleague Charles F. Zwemer and several undergraduates to do just that. Their conclusion, reported in The Journal of Experimental Biology, is that blocking blood flow — not stopping the breath — is what kills the prey.

For anyone who studies snakes, the evolution of constriction is important. This method of killing appeared very early among snakes, and it also evolved independently among different snakes. Dr. Boback said that snakes called black racers, common in the Southeast, are constrictors, but evolved separately from boas, and more recently.