Agression pheromones have been identified in mice that encourage fights between rival males.
Scientists from Scripps Research Institute in California found that mice excrete at least two chemicals in their urine which make them brawl.
Nerves in the noses of male mice are able to detect these proteins in the urine of rivals.
This is enough to stimulate aggressive behaviour in these highly territorial rodents, the journal Nature reports.
Scientists already knew that something in the urine of male mice promoted aggression in other males.
Previous experiments had shown that castrated males did not produce the aggression pheromone and did not elicit belligerent behaviour from rivals.
But when scientists swabbed urine from uncastrated males on the backs of castrated mice, other males displayed hostile behaviour towards them once again.
Lisa Stowers from Scripps and her colleagues have now analysed the responses of sensory neurons in the rodents' noses to various protein components in mouse urine, and assessed the behaviour of the males.
They eventually identified two molecules that were sufficient to promote male-on-male aggression. The authors then characterised the cells and circuits that these molecules acted on.
Male mice display innate territorial aggression, but the logic underlying this behaviour has been difficult to unravel.
Dr Stowers and colleagues hypothesis that pheromones may transmit information about the characteristics of other mice such as gender, age, or status, but this remains unproven. ( BBC )