Cats, rats and other predators produce a chemical signal that terrifies mice, according to new research.
Scientists in the US discovered that when mice detect specific proteins found in cat saliva and rat urine they react with fear.
These proteins, called Mups, act on cells in a special sensory organ in the mouse, called the vomeronasal organ.
The team describe in the journal Cell how the proteins trigger a fearful reaction in the mice.
This shows that mice, and possibly other mammals, have evolved receptors that are able to pick up chemical signals from other species.
The vomeronasal organ contains neurons that detect these chemical signals. The organ is connected to areas of the brain that are involved in memory, emotion and hormonal release.
In many mammals, it is already known to detect pheromones, chemical messengers that carry information between individuals of the same species. These pheremones can have a direct effect on animals' behaviour.
But in this study, the researchers discovered that, in mice, the neurons in the vomeronasal organ were also stimulated by chemical signals from their predators. These proteins caused the mice to display signs of fear - freezing or keeping close to the ground as they carefully sniffed and investigated their surroundings.
The study was led by Professor Lisa Stowers of the Scripps Research Institute in La Jolla, California.
She explained that the discovery made "perfect sense" - once animals had evolved a receptor for one type of Mup protein, the underlying genes could enable them to evolve new receptors capable of detecting the proteins produced by other kinds of animals.
And evolving a receptor that detected signals from their predators would help them avoid being eaten.
What surprised Professor Stowers, though, were the results of experiments in which she and her team disabled the vomeronasal organs of mice and then allowed them to interact with an anesthetised, but very much alive, rat.
Unable to detect the Mups, those mice with no prior experience with rats showed absolutely no evidence of fear at all, even though they could still see the rat right in front of them.
"One test mouse curled up and went to sleep next the rat," Professor Stowers said. "We think it was cold."