MOORE, I.T.*; WINGFIELD, J.C.: Latitudinal gradient in sensitivity of the hypothalamic-pituitary-gonadal axis to gonadotropin-releasing hormone.
Field investigations have revealed complex patterns of testosterone levels in vertebrates. Even in closely related taxa and within populations (especially birds) patterns of testosterone secretion may be dramatically different. The mechanisms underlying this variation in patterns remain unclear. At the secretion level, differences in plasma testosterone levels could result from differences in activity or sensitivity of the hypothalamic-pituitary-gonadal (HPG) axis. To address this we challenged birds from different latitudes with similar doses of gonadotropin-releasing hormone (GnRH) and measured resulting plasma levels of testosterone. If bird populations respond similarly to the GnRH challenge then observed differences in plasma testosterone may be the result of differences in activity of the HPG axis. If bird populations respond differently to the GnRH challenge then observed differences in plasma testosterone may be the result of differences in sensitivity of the HPG axis. We performed the experiment in two populations of white crown sparrows: the high latitude breeding Gamble’s white crown sparrow (Zonotrichia leucophrys gambelii) in Toolik, Alaska and the mid latitude breeding Puget Sound white crown sparrow (Z. l. pugetensis ) from Washington. Birds were captured and received a 500ng of GnRH and a blood sample obtained either 10 or 30 minutes later. GnRH treated birds from both populations had significantly higher plasma levels of testosterone after 30 minutes that was not evident at 10 minutes post injection. There was no difference in the response between the mid and high latitude populations of birds. We are currently extending this study to a low latitude population of rufous-collared sparrow (Z. capensis) in central Ecuador.