One of the main goals in environmental endocrinology during the last two decades has been to understand the interactions between aggression and sex steroids within males, as modeled by the Challenge Hypothesis. This hypothesis proposes that male-male territorial aggression results in an increase in testosterone levels, which then feeds back to support the aggressive behavior. However, less attention has been focused on the role of sex steroids in male-female social interactions. One area where sex steroids could have a profound effect on male-female interactions is in fine-tuning the timing of breeding within socially monogamous pairs. In tropical birds, breeding seasons are generally long and there is significant variation within the population as to when breeding occurs. This presents an ideal opportunity to elucidate the importance of male-female interactions in the timing of breeding. Previous findings in tropical male rufous-collared sparrows, Zonotrichia capensis, have suggested that male-male interactions do not affect testosterone levels, implying that testosterone is not functioning within the Challenge Hypothesis model. In addition, preliminary data indicate that within monogamous pairs, gonadal state is similar, and independent of the rest of the population. This presents the possibility that male-female interactions may be more important than male-male interactions in determining gonadal activity and testosterone levels. The proposed research will investigate the role of hormonally mediated social cues between males and females in timing of breeding. The following specific questions will be addressed: 1) Within monogamous pairs, is there a correlation between male and female gonadal stage and plasma sex steroid levels? 2) Are hormonally mediated social cues used to fine tune and coordinate reproductive timing within breeding pairs? The results of these studies will clarify the differences documented between high latitude and tropical birds in terms of hormone-behavior interactions. This is important as two-thirds of the world's vertebrates are tropical and thus the typical high-latitude mechanistic relationships may not be adequate to describe them.
