Physiology of reproductive senescence in Florida scrub-jays: results from a long-term study and GnRH challenge

In most vertebrates, production of reproductive hormones wanes with age, co-occurring with a decline in reproductive output. Measurement of these hormones can serve as a key marker of the onset of reproductive senescence. Longitudinal studies of physiological parameters in populations of free-living animals are relatively uncommon; however, we have monitored baseline concentrations of hormones for nine years in a population of Florida scrub-jays (Aphelocoma coerulescens). We hypothesized that concentrations of circulating reproductive hormones change with age, and predicted declines in reproductive hormones in the oldest jays. We found that baseline levels of luteinizing hormone (LH) and testosterone (T) were relatively low in both young and old male breeders and reach their highest levels in birds aged 4-7years. Conversely, we found no age-related patterns in baseline levels of LH or estradiol in female jays. In males we determined which component of the hypothalamo-pituitary-gonadal (HPG) axis is responsible for observed age-based differences, by challenging males of different ages with gonadotropin-releasing hormone (GnRH); thereby allowing assessment of pituitary and gonadal responsiveness by measuring plasma concentrations of luteinizing hormone and testosterone, respectively. The magnitude of increase in levels of both LH and T in response to GnRH challenge decreased with age in male breeders. Combined with the baseline levels, the results from the GnRH challenge suggest that younger birds have the capability to produce higher levels of reproductive hormone, whereas the old birds may be constrained by senescence in their ability to produce these hormones.