How and why the winner effect forms: influences of contest environment and species differences

Winning aggressive social encounters can enhance the probability of future victories. This so-called winner effect occurs in diverse species and is thought to be an intrinsic phenomenon mediated by postencounter hormone release. Yet, recent evidence suggests the possibility that certain extrinsic factors also influence the winner effect’s formation within an individual, possibly by affecting the expression of hormone titers that follow a fight. We first investigated in the monogamous and territorial male California mouse (Peromyscus californicus) whether the effect of residency, an extrinsic factor, influences the winner effect’s formation and the endogenous secretion of testosterone and/or progesterone after a dispute. We found that California mice that acquire winning experience in unfamiliar physical locations do not form a full winner effect. Furthermore, this species does not experience a testosterone pulse after a fight in an unfamiliar cage. These findings indicate that environmental context can mediate the winner effect’s formation, possibly by affecting the expression of postencounter testosterone pulses. Second, we compared the winner effect of the California mouse to that of the white-footed mouse (Peromyscus leucopus), a close relative that is promiscuous and less-territorial. We found that, compared with California mice, white-footed mice exhibit neither a full winner effect, despite similar past winning experiences, nor a postencounter surge in testosterone. This finding suggests that these behavioral and physiological phenomena vary among even closely related species and are possibly linked to aspects of social biology, including the degree to which individuals of each species are territorial.