Intrasexual competition and throat color evolution in female three-spined sticklebacks

In a stickleback population, in which females have male-like red throats, the redder females are not more aggressive or dominant. But overall, females of this population are more aggressive and dominant than in the ancestral marine form. The absence of a clear relationship between color and agonism contradicts many studies of male animals, and some of females, in which more conspicuously colored individuals are often more aggressive and dominant.Interest in the evolution of female ornaments has been growing but explaining their presence remains a challenge for many animal taxa. Although female ornaments may be sexually selected via male choice, they can also function in female-female competition. Here, we studied a stream-resident population of three-spined sticklebacks (Gasterosteus aculeatus), in which females possess male-typical red throat coloration, and a nearby anadromous population that lacks such coloration and is thought to possess ancestral character states for most traits in this species complex. We asked first whether there were population-level differences in agonism that might be associated with the evolution of female ornaments. We tested if the derived population possessing some red-throated females exhibited relatively higher levels and clearer patterns of within-population aggression and dominance in dyadic trials and more frequent behavioral dominance over individuals of the population lacking red females. Second, to address the relationship between the derived coloration and female-female competition, we tested whether red-throated stream females were more aggressive and dominant toward dull-throated stream females in both dyadic and socially complex contexts. We found that clear dominance was established more often in pairs of stream females, and stream females were more often dominant in stream-anadromous dyads. However, we found no clear evidence that the red throat coloration was directly linked to a female competitive advantage within the stream population, suggesting alternative untested signaling or perhaps nonadaptive functions. Our study represents the first evaluation of the potential relationship between a male-typical nuptial trait and intrasexual competition in female three-spined sticklebacks.