FLIES avoid being swatted in just the same way Keanu Reeves dodges flying bullets in the movie The Matrix - by watching time pass slowly.

To the insect, that rolled up newspaper moving at lightning speed might as well be inching through thick treacle.

Like Reeves standing back and side-stepping slo-mo bullets, the fly has ample time to escape.

And it is not alone in its ability to perceive time differently from us. New research suggests that across a wide range of species, time perception is directly related to size.

Generally the smaller an animal is, and the faster its metabolic rate, the slower time passes.

The evidence comes from research into the ability of animals to detect separate flashes of fast-flickering light.

"Critical flicker fusion frequency" - the point at which the flashes seem to merge together, so that a light source appears constant - provides an indication of time perception.

"A lot of researchers have looked at this in different animals by measuring their perception of flickering light," said Dr Andrew Jackson, from Trinity College Dublin.

"Some can perceive quite a fast flicker and others much slower, so that a flickering light looks like a blur.

"Interestingly, there's a large difference between big and small species. Animals smaller than us see the world in slo-mo. It seems to be almost a fact of life. Our focus was on vertebrates, but if you look at flies, they can perceive light flickering up to four times faster than we can."

The animals studied covered more than 30 species, including rodents, eels, lizards, chickens, pigeons, dogs, cats and leatherback turtles.

Smaller, more agile creatures had the most refined ability to perceive information in a unit of time, said the researchers writing in the journal Animal Behaviour. In other words, they were able to see more flickers of light per second.

Time perception is just another aspect of evolution and survival, the scientists believe.

"Our results lend support to the importance of time perception in animals where the ability to perceive time on very small scales may be the difference between life and death for fast-moving organisms," said co-author Kevin Healy, a Phd student at the School of Natural Sciences at Trinity College Dublin.