Social competition and selection for sexual monomorphism

The evolution of ornamental traits is frequently explained as a consequence of the competition for mating and reproductive opportunities. However, other mechanisms not directly related to reproduction may also play a role. For example, competition for non-sexual resources such as food can exert selection pressure on traits that are associated with competitive success. Moreover, such competition is likely to involve both males and females and would therefore lead to sex-independent expression of these traits. A difficulty in testing for the role of non-sexual social competition lies in separating sexual from non-sexual competitive contexts. The present project solves this by making use of a cichlid fish, in which females clearly do not compete for reproductive opportunities or parental care. Rather, both males and females compete for individual feeding territories. Both sexes display a conspicuous color pattern with a wide yellow bar on a black body. I hypothesize that (1) selection on female color pattern arises from competition over feeding territories but not from mate preferences, and (2) that sexual monomorphism in color pattern is promoted because there are no differences between males and females in (a) the strength of non-sexual social selection on color pattern nor in (b) the strength of the association between color pattern and fitness. As the study species evolved from ancestors with non-ornamented non-territorial females, the project will inform about mechanisms driving the evolution of mutual ornamentation. The effect of color pattern variation on the outcome of territorial contests will be tested. Additionally, intruder experiments, in which focus fish can intrude on territories held by 3D-printed dummy fish, will determine whether the color pattern painted onto the dummy influences the behavior of the intruder. The effects of color pattern on both male and female mate choice will be examined. The association between color pattern variation and variation in fitness gains (here: differential acquisition of food) will be ascertained in a foraging competition experiment. As the adult color pattern is determined by the distribution of pigment cells on the body, it may not be able to respond to conditions experienced as adult, but rather reflect conditions experienced as juvenile. The effect of developmental stress on adult color pattern will be tested by raising fish under good and under poor feeding conditions. A thorough characterization of the color pattern (development, pigment chemistry, correlation with age and body size) will complement the study. An asset of the project is the combination of two understudied forces of trait evolution, i.e. female competition and social competition for non-sexual resources. Significant advances in our understanding of the role of non-sexual social selection in trait evolution are expected.

In many animals, males and females differ in body size, color and behavior. Often, differences in size and color can be linked to sex-specific behavior, such as when territorial males rely on body size, impressive coloration, weapons or ornaments in order to secure success in contest competition. In contrast, females will not develop such costly phenotypes if they do not contribute to their fitness. In some species, however, males and females are equally large and colorful. The present project addressed the question whether females, which adopt male-like social behavior (such as territoriality), will concomitantly evolve male-like body traits in order to compete successfully against males. We studied this question in a cichlid fish species, in which both males and females show territorial behavior, with each individual defending its own territory (except for a short period while mated). Both sexes display a conspicuous color pattern with a wide yellow bar on a black body. The yellow color is produced by carotenoids. We tested for links between the color pattern (specifically, the width of the yellow bar) and success in territorial competition, and investigated the mechanisms behind associations between color pattern, social interactions and fitness components. We detected a correlation between the color pattern trait and performance in contest competition, but only in females. This result supports the hypothesis that competition for resources selects for female coloration. It is also consistent with an evolutionary scenario, in which the acquisition of male-like social roles triggered the evolution of male-like phenotypes in females. The link between color pattern and contest performance suggests that the color pattern is correlated with the physiological condition of its bearer and can therefore inform about its quality and competitive ability. Our experiments revealed an intimidating effect of bar width on opponents, but only in case of males. Furthermore, fish with wider bars were quicker in exploring a novel environment. Both of these correlated traits can prove advantageous in contest situations. Regarding the information content of the color pattern, we also tested whether the bar width of adult fish responded to either their current condition or to conditions experienced as young juveniles. We found no evidence for an effect of benign or adverse environmental conditions on the width of the yellow bar. Rather, our breeding experiment allowed us to demonstrate a parent-offspring correlation in bar width, suggesting a genetic component for variation among individuals. Together, these results suggest that bar width is at most a poor indicator of environmentally induced variation in body condition. Additionally, we detected high concentrations of carotenoids in the black body areas flanking the yellow bar, which means that variation in bar width does not translate in substantial variation in the demand for precious carotenoids.