Chemical cues influence a range of behavioral responses in rodents. The involvement of protein odorants and odorant receptors in mediating reproductive behavior, foraging, and predator avoidance suggests that their genes may have been subject to adaptive evolution. We have estimated the consequences of selection on rodent pheromones, their receptors, and olfactory receptors. These families were chosen on the basis of multiple gene duplications since the common ancestor of rat and mouse. For e...