title: The ecological and physiological bases of variation in the phenology of gonad growth in an urban and desert songbird.
abstract: Birds often adjust to urban areas by advancing the timing (phenology) of vernal gonad growth. However, the ecological and physiological bases of this adjustment are unclear. We tested whether the habitat-related disparity in gonad growth phenology of male Abert's towhees, Melozone aberti, is due to greater food availability in urban areas of Phoenix, Arizona USA or, alternatively, a habitat-related difference in the phenology of key food types. To better understand the physiological mechanism underlying variation in gonad growth phenology, we compared the activity of the reproductive system at all levels of hypothalamo-pituitary-gonadal (HPG) axis. We found no habitat-associated difference in food availability (ground arthropod biomass), but, in contrast to the seasonal growth of leaves on desert trees, the leaf foliage of urban trees was already developed at the beginning of our study. Multiple estimates of energetic status did not significantly differ between the non-urban and urban towhees during three years that differed in the habitat-related disparity in gonad growth and winter precipitation levels. Thus, our results provide no support for the hypothesis that greater food abundance in urban areas of Phoenix drives the habitat-related disparity in gonad growth phenology in Abert's towhees. By contrast, they suggest that differences in the predictability and magnitude of change in food availability between urban and desert areas of Phoenix contribute to the observed habitat-related disparity in gonad growth. Endocrine responsiveness of the gonads may contribute to this phenomenon as desert - but not urban - towhees had a marked plasma testosterone response to GnRH challenge.
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA. Electronic address: 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: Samuel
authors.initials: S
authors.lastname: Lane
authors.affiliation: The Roslin Institute & Royal (Dick) School of Veterinary Studies, University of Edinburgh, Roslin, Midlothian EH25 9PS, UK.
authors.forename: Simone L
authors.initials: SL
authors.lastname: Meddle
authors.affiliation: Laboratory of Integrative Brain Sciences, Department of Biology and Center for Medical Life Science, Waseda University, Tokyo 162-8480, Japan.
authors.forename: Kazuyoshi
authors.initials: K
authors.lastname: Tsutsui
authors.affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Pierre
authors.initials: P
authors.lastname: Deviche
mesh_terms: D000818:Animals; D001130:Arizona; D001835:Body Weight; D002947:Cities; D003889:Desert Climate; D017753:Ecosystem; D005502:Food; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology; D006066:Gonads / Q000254:growth & development*; D008297:Male; D012098:Reproduction / Q000502:physiology; D012621:Seasons; D020308:Songbirds / Q000033:anatomy & histology / Q000097:blood / Q000254:growth & development* / Q000502:physiology*; D013739:Testosterone / Q000097:blood; D014197:Trees / Q000254:growth & development
chemical_list: D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone
keywords: Food abundance; Gonad cycles; Phenotypic plasticity; Reproductive neuroendocrinology; Urbanization