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.

Figure 1.

Back-transformed means (±95% CI) of post-capture testosterone levels did not differ between control and simulated territorial intrusion (STI) males of the low- and the high-altitude populations. However, testosterone levels increased after a GnRH injection within the same individual. Sample sizes are given above bars. Light grey bars, post-capture; dark grey bars, GnRH induced.

Figure 2.

Back-transformed means (±95% CI) of post-capture testosterone levels did not differ between control males and males who were challenged with a simulated intruder or a real intruder. Sample sizes are given above bars.

Figure 3.

Back-transformed means (±95% CI) of post-capture testosterone levels were similar during the early and the parental breeding phase, and between the low- and the high-altitude population. However, the physiological potential to mount an androgen response to GnRH was significantly higher during the early breeding than during the parental phase, and higher in the high than in the low-altitude population. Sample sizes are given above the bars. Light grey bars, post-capture; dark grey bars, GnRH induced.

Figure 4.

Post-capture testosterone levels were highly correlated with GnRH-induced levels (r = 0.61, p < 0.0001). Data are shown for both populations taken together for the early (black circles) and the parental breeding phase (white circles).

Figure 5.

Post-capture testosterone levels plotted against the duration of the simulated territorial intrusion (STI). In males that attacked the decoy (white circles, n = 12), testosterone levels stayed high, while in males that did not attack the decoy, testosterone levels declined with STI duration (black circles, n = 31).