The heart wants what the heart wants, and boa constrictors want the hearts in their prey to stop beating. New research reveals that the snakes stop squeezing when they sense their prey's heart stop. In the study, scientists inserted a tiny, water-filled bulb next to the hearts of dead rats, and pumped water in and out of it to mimic heartbeats. The researchers also warmed the rodents to their living body temperature of 38°C. Then they let the snakes at them. Although wild-caught boas squeezed harder than captive-bred snakes, both stopped constricting shortly after the fake rat heart stopped beating, the team reports online today in Biology Letters. The authors speculated that boas evolved their sensitivity to heartbeats early on, when they went after cold-blooded animals that could survive for hours on little oxygen; just because their prey stopped struggling didn't necessarily mean they were dead. Constriction requires an enormous amount of energy—seven times what the snake spends while resting—so once an animal is deceased, the reptiles can save their strength for eating it.
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