Arginine vasotocin promotes calling behavior and call changes in male túngara frogs.

In the túngara frog, Physalaemus pustulosus, males alter calling behavior with changes in their social environment, adding 'chucks' to their advertisement calls in response to the calls of conspecific males. Other studies demonstrate that adding chucks increases the attractiveness of calls to females but also increases the risk of bat predation. In the current study, subcutaneous injections of the neuropeptide hormone arginine vasotocin (AVT) significantly increased chuck production in male túngara frogs. The effects of AVT on chuck production did not depend on the presence of playback stimuli, suggesting that AVT increased either the males' general motivation to produce chucks or their responsiveness to the calls of distant males. Injections of AVT also increased the probability that males would call and decreased the latency to call after injection, supporting the hypothesis that AVT influences motivation to call. Finally, AVT inhibited a drop in call rate after the termination of a playback stimulus and increased call rate at a lower dose of AVT. The effects of AVT on chucks and call rate appear to be independent of each other, as there was no correlation between change in chuck production and change in call rate in individual males. We conclude that AVT may play an important role in socially-mediated call changes that result from competition for mates. The behavioral changes induced by AVT might increase a male's attractiveness to females, and also may be consistent with an aggressive response to another túngara frog male.

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.