Life-history and hormonal control of aggression in black redstarts: Blocking testosterone does not decrease territorial aggression, but changes the emphasis of vocal behaviours during simulated territorial intrusions

Abstract

Introduction: Many studies in behavioural endocrinology attempt to link territorial aggression with testosterone, but the exact relationship between testosterone and territorial behaviour is still unclear and may depend on the ecology of a species. The degree to which testosterone facilitates territorial behaviour is particularly little understood in species that defend territories during breeding and outside the breeding season, when plasma levels of testosterone are low. Here we suggest that species that defend territories in contexts other than reproduction may have lost the direct regulation of territorial behaviour by androgens even during the breeding season. In such species, only those components of breeding territoriality that function simultaneously as sexually selected signals may be under control of sex steroids.

Results: We investigated black redstarts (Phoenicurus ochruros), a species that shows periods of territoriality within and outside of the breeding season. We treated territorial males with an anti-androgen and an aromatase inhibitor during the breeding season to block both the direct and indirect effects of testosterone. Three and ten days after the treatment, implanted males were challenged with a simulated territorial intrusion. The treatment did not reduce the overall territorial response, but it changed the emphasis of territoriality: experimental males invested more in behaviours addressed directly towards the intruder, whereas placebo-treated males put most effort into their vocal response, a component of territoriality that may be primarily directed towards their mating partner rather than the male opponent.

Conclusions: In combination with previous findings, these data suggest that overall territoriality may be decoupled from testosterone in male black redstarts. However, high levels of testosterone during breeding may facilitate-context dependent changes in song.

Figure 1

Photograph of an adult male black redstart during breeding.

Figure 2

Non-vocal territorial behaviours (A, B) and number of songs (C). Behaviours were shown in response to simulated territorial intrusions on day 3 and day 10 after treatment with a placebo or flutamide and letrozole (Flut/Let). Points represent means and error bars represent 95% CI.

Figure 3

Spectrogram of one song of a black redstart (Avisoft-SASLab Pro, sample rate 22.05 kHz, FFT = 256 points, hamming-window, overlap: 50%). Song parts are indicated on top of the spectrogram. Measures analysed were durations of parts A, B, C, of the total song and the pause duration between A and B; the number of elements of part A and C; the frequency bandwidth and the maximum frequency of part A, B, C (see text and [26] for further details).

Figure 4

The duration of songs (A) and of pauses between part A and B (B). Behaviours were shown in response to simulated territorial intrusions on day 3 and day 10 after treatment with a placebo or flutamide and letrozole (Flut/Let). Placebo-implanted males sang significantly longer songs with a longer pause between part A and B on day 10 than on day 3. Bars represent means and 95% CI.

Figure 5

The frequency bandwidth (kHz) of part B plotted against the duration of part B of songs sang in response to the STI. Data are shown separately for placebo- (left) and Flut/Let-treated males (right;filled circles: STI on day 3; open circles: STI on day 10). Lines represent regression lines for both days taken together.

Figure 6

Frequency bandwidth (kHz) of song parts A (A) and B (B). The song was recorded after simulated territorial intrusions on day 3 and day 10 after treatment with a placebo or flutamide and letrozole (Flut/Let). Placebo-implanted males sang both parts with a significantly broader frequency bandwidth than Flut/Let-implanted males. For details on song measurements and statistics see text.

Figure 7

The number of elements in part A plotted against the duration of the part A of songs sang after the STI. Data are presented separately for placebo- (left) and Flut/Let-treated males (right; filled circles and solid lines: STI on day 3; open circles and dashed lines: STI on day 10). Lines represent regression lines.