ScienceShot: I Can Feel Your Heartbeat ... Stop  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