The question above is one Andy Rooney never asked, but biologists are a curious lot. Boas (yes, their Latin name is Boa constrictor) squeeze their prey, killing them by suffocation, after first biting them to get a grip.
I’ve put below a video of a boa constrictor killing a large rat this way, but here’s a WARNING: don’t watch it if you’re squeamish! Predation is never pretty, but of course natural selection doesn’t make predators kind.
Squeezing prey in this way is costly for the snake. It takes a lot of energy for a cold-blooded (ectothermic) reptile to squeeze that hard (its metabolism goes up sevenfold when squeezing), and during the act the snake itself could be subject to predation.You don’t want to squeeze longer than you have to, but how do you know your prey is dead?
Many of you might have said, “When the breathing stops,” but that’s apparently not correct.
The answer, which might seem obvious but is still cool, is in a new paper in Biology Letters by Scott Boback et al. (access is free), is that the snake monitor’s the prey’s heartbeat, and stops squeezing when the heart stops beating.
Well, the experiment is gruesome, and personally I wouldn’t kill rats to answer this question, but Boback et al. did the carnage. They used 16 wild-caught boas, and offered them killed but still-warm rats, kept warm by a sort of heating pad. The rats were also implanted with water-filled bulbs to measure the pressure of the snake’s squeezing, and then—the key—an artificial heart, formed by another water-filled bulb that was set to beat in three treatments: 1. no heartbeat, 2. heartbeat throughout a 20-minute period (the average time a snake squeezes a rat) and 3. heartbeat stops after 10 minutes.
- snakes periodically tightened their coils in the constant-heartbeat treatment, but didn’t do that when there was no heartbeat from the outset
- rats with the constant heartbeat were squeezed nearly twice as long as those with no heartbeat, and the pressure for the former was about 20% higher.
- in the treatment for which artificial heartbeat was discontinued after 10 minutes, the snakes stopped their periodic increase in pressure at about 10 minutes, and pressures and squeezing times were intermediate between the full-heartbeat and no-heartbeat treatment. :’
Is this learned or innate? Experiments with naive boas, who had never squeezed a live prey, suggest there’s at least a strong innate component, i.e., an evolved, genetically-based ability to stop squeezing when you detect a stopped heart. But snakes also apparently learn to squeeze less when the heart stops, too. Regardless, this is the first demonstration that snakes can detect and monitor the heartbeat of prey that they’re squeezing.
The authors raise one caveat: warm-blooded (“endothermic”) prey like birds and mammals are killed much more quickly than 20 minutes, and snakes could use the cessation of movement as a cue, saving them lots of squeezing time. Why the extra time and heartbeat monitoring? The authors theorize that the heartbeat monitoring originally evolved when the snakes preyed on cold-blooded (“ectothermic”) animals like lizards, which can live a lot longer than a few minutes without respiration and movement, and so cessation of heartbeat was a more reliable cue to death.
This explanation is interesting, but if boas’ current prey are mostly endothermic, there should have been selection to eliminate the heartbeat cue. I have no idea what kinds of prey predominate in a modern boa’s diet.
Boback, S. M., A. E. Hall, K. J. McCann, A. W. Hayes, J. S. Forrester and C. F. Zwemer. 2012. Snake modulates construction in response to prey’s heartbeat. Biology Letters online, doi: 10.1098/rsbl.2011.1105