Thursday, January 19, 2012

Time to Death - Boas Monitor Prey's Heartbeat During Constriction

A threatening Boa constrictor. JCM
The 19th century literature on constriction by snakes often describes the prey as being crushed. And, for many years it was thought that constricting snakes killed their prey by preventing the prey from breathing. A coil from the snake's body was tightened each time the prey exhaled, gases returning the the prey's lungs would have to be exhaled, so when the prey exhaled the snake tightened its hold, making it impossible for the prey to inhale. There was another hypothesis that was overlooked. In 1912 Frank Wall proposed snakes induce asphyxia by essentially stopping blood flow to the heart.

 A new study by Scott Boback and colleagues suggests that constriction may be more sophisticated than previously thought. Killing prey by constriction is both energy expensive and potentially dangerous to the snake, constriction requires and significant increase in aerobic respiration and the prey may retaliate and injure the snake. The authors tested constricting boa constrictors to see if they adjusted their hold on prey. They developed a method of isolating a rat's heartbeat as a potential cue, by implanting a simulated heart in a dead rat that replicated the size, rate and stroke volume of a rodent heart. They then tested how the constriction effort varied as snakes constricted rats with: a simulated heartbeat throughout constriction; a simulated heartbeat for the first half of constriction and then shut off, and no heartbeat. The results suggest tightness and duration of a constricting snake’s coils are timed to perfection, matching the heartbeat and weakening state of the snake’s prey.

Snakes constricting dead prey with a simulated heart beat constricted for a much longer time than in previous studies (averages of 12 vs 23 minutes). The authors propose that longer constriction times may have been required prior to the evolution of endotherms (birds and mammals) because ectotherms have slower metabolisms and can survive for longer periods of time with reduced amounts of oxygen.

S. M. Boback, A. E. Hall, K. J. McCann, A. W. Hayes, J. S. Forrester and C. F. Zwemer. 2012. Snake modulates constriction in response to prey's heartbeat. Biology Letters doi: 10.1098/rsbl.2011.1105.

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