The U.S. scientists  : Snakes 'capable of things we did not realise before' They might spend an age stalking their prey before slowly squeezing it to death. But scientists have found that boa constrictors make up for lost time by letting go as soon as their victim's heart stops.
This accurate calculation of death, while seemingly just cruel, allows the snake to expend the minimum amount of energy.
Scientists, from Dickinson College, Pennsylvania, discovered boas can actually 'feel' their prey's heartbeat after a series of experiments.
First they used dead rats with implanted 'simulated hearts' (water-filled bulbs connected to a pump) to lure the snakes.
When the boas struck out at the rats the scientists controlled their fake hearts remotely.
They also measured the pressure of the squeeze on the rat's bodies to see whether the snake adjusted according to heartbeat strength.
When they kept the hearts pumping, the scientists found the snakes clung onto the rats for 'longer than any previous observation of a snake constricting a prey item - live or dead.'
'I couldn't believe my eyes the first time we tested a snake with a rat with a simulated heart,' lead researcher, Dr Scott Boback, from Dickinson College, told BBC Nature.
'It was writhing and squeezing the rat in an apparent effort to kill it.'
The team then tried the same experiment with live rats.
They found the boas constricted the rats and then gradually eased off as their prey's heartbeat dwindled.
'There was such a clear difference I knew we were discovering something interesting,' Dr Boback said.
In a summary of the study, published in the Royal Society journal Biology Letters, the scientists wrote: 'Many of us think of snakes as audacious killers, incapable of the complex functions we typically reserve for "higher" vertebrates.
'We found otherwise.'
They added the snakes' sense of touch may mean the serpents 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.