Researchers have found the chemicals that make mice scared stiff if they smell a predator, such as a cat, rat or snake.
Publishing in the Journal Cell, a team from California wanted to know what it was about these predators that caused stress hormone levels in mice to rise and why they'd flatten themselves against the floor - even if the predator wasn't visible.
Lisa Stowers and colleagues discovered that the trigger was a group of proteins, found in urine, known as MUPs (major urinary proteins). These are secreted by just about every vertebrate on the planet but they are very species-specific. And one section of the mouse's nose is very sensitive to these proteins.
The researchers already knew that the mouse vomeronasal organ could pick up pheromones from other mice but the idea that they were sensitive to those of other mammals is new. In their tests they placed mice which had an inactive vomeronasal organ close to an anaesthetised rat (so it wasn't going to eat them!). And because the mice couldn't smell any of these MUPs they showed no signs of fear. One even curled up and went to sleep next to the rat. So it shows that the visual spectacle of a rat on its own doesn't play a part in predator recognition.
The fear of cat, rat and snake smell must therefore be hardwired, since these mice have been bred in labs for nearly eighty years and very few would have met Mr Tibbles in that time.
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