As boa constrictors tighten their bodies around their prey, the snakes sense the dying animal's heart rate to determine how much pressure to apply and when to stop squeezing.
Scott Boback and his colleagues at Dickinson College in Carlisle, Pennsylvania, implanted simulated hearts into warm, dead rats and presented them to 16 snakes. In the presence of a heartbeat, the boas constricted for nearly twice as long and with more than double the pressure than in the absence of a heartbeat. When the heart stopped midway through constriction, the snakes released their prey.
The researchers speculate that the ability to detect a heartbeat co-evolved with constriction (pictured) to help snakes identify when death occurs and minimize energy expenditure.
Biol. Lett. http://dx.doi.org/10.1098/rsbl.2011.1105 (2012)
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Snakes strangle with feeling. Nature 481, 412 (2012). https://doi.org/10.1038/481412a