title: Negative energy balance in a male songbird, the Abert's Towhee, constrains the testicular endocrine response to luteinizing hormone stimulation.
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.
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA Present address: Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, USA Scott.Davies@asu.edu.
authors.forename: Scott
authors.initials: S
authors.lastname: Davies
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Sisi
authors.initials: S
authors.lastname: Gao
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Shelley
authors.initials: S
authors.lastname: Valle
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Stephanie
authors.initials: S
authors.lastname: Bittner
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Pierce
authors.initials: P
authors.lastname: Hutton
authors.affiliation: The Roslin Institute, The Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Easter Bush, Midlothian EH25 9RG, Scotland, UK.
authors.forename: Simone L
authors.initials: SL
authors.lastname: Meddle
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Pierre
authors.initials: P
authors.lastname: Deviche