Paedophile mice are kept at bay by the tears of their potential victims, research has shown.
The tears contain a pheromone signalling chemical that puts off the predatory males, scientists discovered.
Two-week-old pre-pubescent mice produce 50 times more of the pheromone, called ESP22, than adults.
When scientists blocked the ability of adult males to sense the chemical, they witnessed a dramatic worsening of behaviour.
Sexual advances towards pre-pubescent females increased "strikingly" with numerous mating attempts.
A similar response was seen when young mice were prevented from producing ESP22 in their tears.
Painting the chemical onto the juveniles protected them again.
Adult mice detect the chemical through their vomeronasal organ, a smell sensor in the nasal cavity designed to pick up pheromones.
An international team of scientists led by Dr Stephen Liberles, from Harvard Medical School, wrote in the journal Nature: "We describe a pheromone of sexually immature mice that controls an innate social behaviour, a response pathway through the accessory olfactory system and a new role for vomeronasal organ signalling in inhibiting sexual behaviour towards young.
"These findings provide a molecular framework for understanding how a sensory system can regulate behaviour."
Experts are divided about whether a vomeronasal organ exists in adult humans.
There is no evidence of a human nasal organ that responds to pheromones in the same way as it does in many animals.