Investigating the influence of food on reproductive physiology: urbanization as a natural experiment

In many organisms, food availability is a proximate cue that synchronizes seasonal development of the reproductive system (recrudescence) with optimal environmental conditions. Reproductive recrudescence is orchestrated by the hypothalamo-pituitary-gonadal (HPG) axis. However, the physiological mechanisms by which food availability modulates activity of the HPG axis are poorly understood. It is thought that the timing of recrudescence is modulated by an individual’s energetic status. We tested this hypothesis by examining whether food availability modulates the activity of the HPG axis. Specifically, we food-restricted captive adult male Abert’s Towhees, Melozone aberti, for two or four weeks during photoinduced reproductive recrudescence. A third group (control) received ad libitum food throughout. We measured multiple aspects of the reproductive system that play a crucial role in recrudescence, including: hypothalamic gonadotropin-releasing hormone (GnRH), plasma testosterone (T), and reproductive morphology. Furthermore, because gonadotropin-inhibitory hormone (GnIH) and neuropeptide Y (NPY) potentially integrate information on food availability into seasonal reproductive recrudescence, we also measured the brain levels of these peptides.