Abstract
Energy deficiency can suppress reproductive functions in vertebrates. As the orchestrator of reproductive function, endocrine activity of the hypothalamo-pituitary-gonadal (HPG) axis is potentially an important mechanism mediating such effects. Previous experiments in wild-caught birds found inconsistent relationships between energy deficiency and seasonal reproductive function, but these experiments focused on baseline HPG axis activity and none has investigated the responsiveness of this axis to endocrine stimulation. Here, we present data from an experiment in Abert's Towhees, Melozone aberti, using gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) challenges to investigate whether energy deficiency modulates the plasma testosterone (T) responsiveness of the HPG axis. Wild-caught birds were either ad libitum-fed or energetically constrained via chronic food restriction during photoinduced reproductive development. Energy deficiency did not significantly affect the development of reproductive morphology, the baseline endocrine activity of the HPG axis, or the plasma T response to GnRH challenge. Energy deficiency did, however, decrease the plasma T responsiveness to LH challenge. Collectively, these observations suggest that energy deficiency has direct gonadal effects consisting in decreased responsiveness to LH stimulation. Our study, therefore, reveals a mechanism by which energy deficiency modulates reproductive function in wild birds in the absence of detectable effects on baseline HPG axis activity.
Highlights
During prolonged periods of energy deficiency, animals prioritize investment of energy in the requirements for survival over investment in reproductive processes (Lack, 1954; Gittleman and Thompson, 1988; Hill et al, 2008)
Summary statement Energy deficiency in an adult male songbird had no detectable affect on the development of reproductive morphology, but constrained the endocrine responsiveness of the testes
Body mass of ad libitum-fed birds was similar throughout the experiment (Tukey HSD, P > 0.05), whereas the body mass of food-restricted birds decreased after two weeks (Tukey HSD, P < 0.05), and remained at the reduced mass for the duration of the study
Summary
During prolonged periods of energy deficiency (i.e., when energy expenditure exceeds energy intake), animals prioritize investment of energy in the requirements for survival over investment in reproductive processes (Lack, 1954; Gittleman and Thompson, 1988; Hill et al, 2008). Reproduction is regulated by the activity of the hypothalamo-pituitary-gonadal (HPG) axis, beginning with the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus (Sharp, 2005; Zohar et al, 2010; Clarke, 2011). This neuropeptide stimulates the secretion of luteinizing hormone (LH) from the anterior pituitary gland (Kuenzel, 2000; Zohar et al, 2010; Clarke, 2011). Testosterone regulates a variety of male reproductive characteristics, such as the development of primary (Kempenaers et al, 2008) and secondary sexual characteristics (Buchanan et al, 2003), courtship, and aggression towards conspecific males (Hegner and Wingfield, 1987; Landys et al, 2010)
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