Boa constrictor snakes sense prey's fading heartbeat
Boa constrictors have a drawn-out method of subduing their prey - wrapping their muscular bodies around it and slowly squeezing it to death.
Scientists have now discovered that snakes sense exactly how long they need to squeeze; the predators can sense their victim's heartbeat, and let go only when it stops.
The findings are reported in the Royal Society journal Biology Letters.
They reveal how snakes evolved to take physical "cues" from their prey.
End Quote Scott Boback Dickinson College
A snake actually 'feels' the heartbeat of their prey”
This allows the animals to sense when they have successfully incapacitated their quarry.
It is a crucial trick for a predator that has to balance its need for food with the energy it spends constricting its muscles tightly enough - and for long enough - to suffocate an animal.
The research team, led by Dr Scott Boback from Dickinson College in the US state of Pennsylvania, wanted to find out if the snakes were able to "measure" the amount of time they needed to keep squeezing.
The scientists used rats to entice their captive Boa constrictor to attack.
But the team did not subject the rodents to death by constriction, instead implanting dead rats with "simulated hearts" - water-filled bulbs connected to a pump, which replicated the blood-pumping action of a live rat's heart.
- Boa constrictors can grow up to 4m (13ft) in length and have a lifespan of up to 30 years
- The snakes have no venom, instead using their small, hook-shaped teeth to grip prey while wrapping their muscular bodies around it
- Boas tackle a wide variety of prey, including rodents, monkeys and wild pigs. As with most snakes, their specialised, "flexibly attached" lower jaw enables them to swallow prey whole
When the snake struck out at one of the already-dead prey, the scientists were able to control this "fake heart" remotely.
They also used pressure sensors on the rat's bodies to measure how the snake adjusted its squeeze.
"I couldn't believe my eyes the first time we tested a snake with a rat with a simulated heart," Dr Boback told BBC Nature.
"It was writhing and squeezing the rat in an apparent effort to kill it."
When the scientists kept the simulated heart pumping, they found that "the boas constricted rats... longer than any previous observation of a snake constricting a prey item - live or dead".
The snakes' reactions to rats without a simulated heartbeat were completely different. The boa constrictors "would strike, form their coils, constrict the rat, then gradually ease off".
Dr Boback explained: "There was such a clear difference I knew we were discovering something interesting."
In a summary of the study, the scientists wrote: "During constriction a snake actually 'feels' the heartbeat of their prey.
"Many of us think of snakes as audacious killers, incapable of the complex functions we typically reserve for 'higher' vertebrates.
"We found otherwise."
The acuity of snakes' sense of touch, the scientists say, might mean that the animals are "capable of things that we did not realise before".
"For instance," said Dr Boback, "snakes may utilise this acute tactile sense to coordinate complex movements associated with limbless locomotion."