When the male muroid rodent is in a state of reproductive readiness, i.e. during the reproductive season and in possession of a breeding territory, his social behaviors are organized into a complex, appropriate, interrelated pattern, truly a social strategy. He "stakes out" a territory that he patrols and scent-marks with pheromones that advertise his potency, attract other conspecifics including potential mates, and stimulate the female's reproductive physiology. When he encounters a conspecific male, he attacks with motor patterns mediated by the offense motivational system. When he encounters a female he is inhibited from attacking her and makes appropriate sexual advances instead.
The male's reproductive readiness state is activated by androgen. Androgen activates the motivational system of exploration/marking (brain site 2) that he uses to maintain the territory. It activates the tuned olfactory filters (brain sites 7 and 8) that facilitate offense against conspecific males, but suppress offense against females. And it activates the approach to females (brain site 8) and the male sex motivational system (brain site 4) that are responsible for his sexual behavior towards receptive females. Androgen also changes the quality of the male's pheromones so that they became specific attractants for females (brain site 9), advertising his sexual potency; in addition, as an unfortunate byproduct, they also advertise his sexual potency to potential male competitors for whom they serve as motivating stimuli for offense (site 7).
There are limits upon the fighting of males in a state of reproductive readiness that are reflected in the operation of the submission motivational system and the ritualization of offensive attack. The male in a state of reproductive readiness not only maintains a territory, but also he is surrounded by other territorial males with whom he is acquainted and with whom he maintains a kind of "loyal opposition." Territorial males are each dominant upon their own territories, but submissive upon the territories of adjacent individuals. When such a social organization is stable, it can greatly reduce the amount of overt fighting. Although at times the argument is couched in terms of kin selection (Hamilton, 1971) or group selection (Wynne-Edwards, 1962), I don't think such an appeal is necessary. The "loyal opposition" arrangement can confer an advantage upon each participating individual. If a territorial male were to kill or seriously injure its neighbor, it would upset the stability of the territorial arrangement. A new animal would take over the territory of the former neighbor, and the two animals would have to start at the beginning of a new relationship, testing their strength in serious combat. If this event were to occur many times over, as would happen if a territorial male regularly killed its neighbors, one would find that this territorial male was at a disadvantage compared to others. He would expend greater energy in fighting, with greater risk of injury both from opponents and from predators, to accomplish a result which his other conspecific competitors would be accomplishing more easily by means of ritualized territorial relations. The metabolic cost of fighting, estimated to be 20% above that of other routine behaviors (Davis and Golley, 1963), be an important factor by itself. Finally, one can imagine that females may have developed a preference for males who spend minimal time in fighting; the genes from such males might give her male offspring an advantage compared to offspring of more aggressive males.