In most animals, males play the active role in courtship and compete for mates, whereas females are discriminatory in their mate choice. As a result of this, males in many animals have evolved costly ornamental traits such as gaudy coloration and long tails. In some species, females are also ornamented, but the reasons for this are poorly known. So far, the focus of most studies have been on sexual selection acting on males, although, under some circumstances males are expected to be choosy and females competitive. Despite the large interest the field of sexual selection has received over the last decades there are still challenging areas which are not fully understood, for example, the function and evolution of secondary sexual ornaments, the evolution of mate preferences and the existence of sex-roles. The aim of this project is to test questions related to these areas. The project will not only focus on selection acting on males, but will also investigate sexual selection acting on females, i.e. male mate choice and female-female competition. The project combines field studies with laboratory experiments. The animals under study are some gobiid fishes with paternal care and conventional sex roles: the sand goby, Pomatoschistus minutus, the common goby, P. microps and the two-spotted goby, Gobiusculus flavescens. The project will, for example, investigate costs and benefits of mate choice, the function of both male and female sexual ornaments, and the plasticity of reproductive behaviour and sex-roles.