- 1According to the ‘territorial behaviour’ hypothesis, red grouse population cycles are caused by delayed density-dependent changes in male aggressiveness influencing recruitment. These lagged changes in aggressiveness might be caused by changes in the kin structuring of male populations and differential aggressive behaviour between kin and non-kin (‘kinship’ hypothesis).
- 2A population-level manipulation of male aggressiveness in autumn affected the kin structure of male populations, their subsequent aggressiveness, and recruitment of both sexes. On two moors, we implanted the old territorial cocks in autumn with testosterone on an experimental area (T-areas) and with sham implants on a control area (C-areas).
- 3Increased aggressiveness in autumn t reduced recruitment in autumns t and t+ 1, and breeding density of both sexes in springs t+ 1 and t+ 2, confirming previous studies elsewhere. A new observation was that cocks on T-areas had bigger combs (an ornament whose size is testosterone-dependent) than those on C-areas for at least 1·5 years after treatment, evidence that they remained more aggressive.
- 4Increased aggressiveness reduced not only subsequent density but also kin structuring among territorial cocks. This is consistent with the ‘kinship’ hypothesis that changes in the kin structure of male populations mediate year-to-year changes in male aggressiveness.
- 5Increased aggressiveness did not increase intensity of infection by the dominant intestinal nematode Trichostrongylus tenuis, which might have affected recruitment through reduced breeding success. Moreover, breeding success after treatment was no lower on the T- than on the C-areas.
- 6The results show for the first time that increased aggressiveness affects both kin structure and subsequent recruitment, supporting a key assumption of the kinship hypothesis for red grouse population cycles.