The intensity of sexual selection and the absence of assortative mating patterns in Prunella modularis (Aves: Passeriformes)

Grazieli da Silva Cerqueira, Eduardo S. A. Santos , Benedikt Holtmann, Carlos Esteban Lara, Shinichi Nakagawa , Renato Chaves Macedo-Rego

When females and males prefer large mating partners, a size-assortative mating pattern can occur, and larger individuals tend to obtain more partners, thereby increasing the intensity of sexual selection. Considering this, we studied a population of dunnocks (Prunella modularis), a species with a variable mating system. We tested whether (A) assortativity (based on body size and/or condition) occurs, (B) sexual selection is higher in males than in females, as is typically expected, and (C) the sex ratio influences the strength of selection. Despite assessing multiple breeding seasons (individually and combined), we found no evidence of assortative mating. Because we found little evidence that large (or higher-condition) individuals produce more offspring, there should be no strong preference for body size or condition, and assortativity should not occur, as observed. We also found that males faced stronger sexual selection than females, which conforms to long-standing predictions within sexual selection theory. However, this sex difference was of little to moderate magnitude, which is reasonable for a polygynandrous species. Finally, both sexes faced stronger selection when the sex ratio was more male-biased, indicating that (a) some females are better than others in obtaining more than one mate, even when males are abundantly available, and/or (b) males are more willing to accept co-breeding males as the sex ratio becomes more biased.