The progesterone challenge: steroid hormone changes following a simulated territorial intrusion in female Peromyscus californicus.

There is a growing body of evidence that the rapid but transient increase in male androgens, particularly testosterone (T), following a single social encounter such as a territorial intrusion occurs in a wide array of vertebrate taxa. Yet, this phenomenon, often called the Challenge Hypothesis, has rarely been investigated in females. Moreover, when studying male challenge effects, researchers have rarely investigated other hormones that can be important to the expression of aggression, such as progesterone (P4) and estradiol (E2). We conducted 10-min aggression trials using the resident-intruder paradigm in cycling female California mice, Peromyscus californicus, a species in which both sexes show territorial behavior. By comparing the hormone levels of test females to control females, we found a decrease in P(4) and the P4/T ratio, but no change in T, E2, corticosterone, E2/P4, or E2/T. Interestingly, these hormone changes were observed even when the resident was not aggressive toward the intruder, suggesting that the stimulus cueing the hormone changes was the mere presence of the intruder and not the amount of aggression displayed by the resident. Generally, T has a positive relationship with aggression, whereas P4 inhibits male and nonmaternal female aggression. Thus, decreasing the P4/T ratio following an encounter may serve to increase future aggression in females. These results suggest that females may use different hormonal mechanisms than do males to mediate aggression in a challenge situation.