Does the tie fit the female? Melanin-based colouration, aggressive personality and reproductive investment in female great tits (Parus major)

Abstract

The evolution and function of female trait exaggeration in species with traditional sex roles are poorly understood. Competition for social and ecological resources, rather than mating opportunities, may be a key selection pressure in females of socially monogamous species. Moreover, such pre-existing resource competition-selected traits (armaments) may become the object of mating preferences (ornaments); however, the benefits that males may gain in choosing more ornamented females remain unclear. Here, using observational data on free-living female great tits (Parus major), we explore whether tie size, a well-known melanin-pigmented plumage trait, covaries with intrasexual aggressiveness during the breeding season and the investment in current reproduction (fledgling number and mass). We found consistent individual differences (i.e. personality variation) in most of the aggressiveness parameters studied. Moreover, we found support for a signalling role of tie size with respects to how close females approached intruders. Interestingly, this relationship was only evident in older females, not first-year breeding females, suggesting age-related changes in the information content of tie size in the context of same-sex aggressiveness. Additionally, although first-year breeding females, on average, produced fledglings with lower body mass compared to older females, tie size was positively associated with fledgling mass, suggesting that males could potentially gain benefits by choosing females with larger tie size. Experimental studies are now necessary to confirm whether females with larger tie size have a selective advantage in female competition (i.e. armament), as well as whether males base their mate choice on female tie size (i.e. ornament). Why females of many species with traditional sex roles often have exaggerated traits very similar to those found in males remains poorly understood. We explored the functional significance of a well-known melanin-based plumage trait (tie size) in females of a socially monogamous passerine bird, the great tit. Female tie size positively covaried with consistent individual differences in aggressiveness during female-female interactions, but only so in older females and not first-year breeding females. In addition, females with larger tie size produced fledglings with higher body mass, suggesting that it could potentially be used by males in mate choice. Our study emphasizes that greater attention should be paid to female trait exaggeration in socially monogamous species in the context of female-female competition and male mate choice, as well as to potential age-related changes in the information they convey.