From Nature:
A whiff of a single type of protein from urine is enough to make a male mouse pick a fight, researchers have found. Pheromone scents that elicit aggressive behaviour have long been predicted, but have proven elusive until now.
Male mice will attack other mice they see as a threat, such as males that invade their territory, but will generally welcome females and leave juveniles or castrated males alone. When they do attack it can be quite aggressive. “The resident will chase the intruder, bite, kick and wrestle with him,” says Lisa Stowers, a biologist at the Scripps Research Institution in La Jolla, California who along with her colleagues has identified a protein that provokes this aggression.
Stowers and her colleagues filtered mouse urine by fractionation to sort the molecules by size. They then tested to see which samples — when dabbed on a castrated male — elicited an aggressive response from resident males. The researchers narrowed the search down to a group of molecules called the major urinary proteins (MUPs), whose role in chemical communication has been only suspected until now.
More here.