title: Ignoring the challenge? Male black redstarts (Phoenicurus ochruros) do not increase testosterone levels during territorial conflicts but they do so in response to gonadotropin-releasing hormone.
abstract: Competition elevates plasma testosterone in a wide variety of vertebrates, including humans. The 'challenge hypothesis' proposes that seasonal peaks in testosterone during breeding are caused by social challenges from other males. However, during experimentally induced male-male conflicts, testosterone increases only in a minority of songbird species tested so far. Why is this so? Comparative evidence suggests that species with a short breeding season may not elevate testosterone levels during territory defence. These species may even be limited in their physiological capability to increase testosterone levels, which can be tested by injecting birds with gonadotropin-releasing hormone (GnRH). We studied two populations of black redstarts that differ in breeding altitude, morphology and the length of their breeding season. Unexpectedly, males of neither population increased testosterone in response to a simulated territorial intrusion, but injections with GnRH resulted in a major elevation of testosterone. Thus, black redstarts would have been capable of mounting a testosterone response during the male-male challenge. Our data show, for the first time, that the absence of an androgen response to male-male challenges is not owing to physiological limitations to increase testosterone. Furthermore, in contrast to comparative evidence between species, populations of black redstarts with a long breeding season do not show the expected elevation in testosterone during male-male challenges.
authors.affiliation: Abteilung für Verhaltensneurobiologie, Max-Planck-Institut für Ornithologie, Eberhard-Gwinner-Straße 6a, 82319 Seewiesen, Germany. apfelbeck@orn.mpg.de
authors.forename: Beate
authors.initials: B
authors.lastname: Apfelbeck
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Goymann
mesh_terms: D000378:Agonistic Behavior / Q000502:physiology*; D000531:Altitude; D000818:Animals; D005858:Germany; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D007558:Italy; D016014:Linear Models; D008297:Male; D046109:Passeriformes / Q000097:blood / Q000502:physiology*; D012098:Reproduction / Q000502:physiology; D018709:Statistics, Nonparametric; D013731:Territoriality; D013739:Testosterone / Q000097:blood* / Q000378:metabolism
chemical_list: D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone