Meeting Abstract
Opportunistic breeding is a strategy used to maximize reproductive success in unpredictable environments. Unlike species that breed seasonally, opportunistic breeders are thought to maintain partial activation of the reproductive axis in order to rapidly initiate breeding when environmental conditions become suitable. The initiation of breeding in opportunistic species is likely to be fine-tuned with respect to an array of environmental variables, but the physiological mechanisms that influence reproductive readiness across heterogeneous environments remain relatively unexplored. Zebra finches (Taeniopygia guttata) are found across the majority of the Australian continent from the arid center to the monsoonal belt in the northern tropics. Zebra finches initiate breeding in response to unpredictable precipitation and are an ideal species to explore the physiological underpinnings of opportunistic breeding in a free-living system. We examined trade-offs between reproductive readiness and stress responsiveness in wild zebra finches at five field sites with varying degrees of rainfall across a latitudinal gradient in Australia. Using adrenocorticotropic hormone (ACTH) and gonadotropin-releasing hormone challenges, we assessed maximum output of corticosterone (CORT) and testosterone. Across all sites, zebra finches released similar amounts of CORT in response to ACTH challenges. However, zebra finches in the northern most (predictable) sites responded to restraint stress with lower levels of CORT output compared to zebra finches in the southern (unpredictable) sites suggesting a downregulation of the HPA axis. Examining differences in HPG axis function between sites will allow us to interpret these results in terms of trade-offs between the HPA and HPG axes.
