Female ornaments signal own and offspring quality in a sex‐role‐reversed fish with extreme male parental care

Although female ornaments have been described in many taxa, the full spectrum of information conveyed by such traits together with the potential male fitness benefits are far from fully understood. Here, we used a sex‐role‐reversed species, the black‐striped pipefish, Syngnathus abaster, where females are the ornamented sex and intensively compete for mates who present an extreme form of paternal care (male pregnancy). We investigated what information is conveyed by female traits and if males are using it during mate choice. We further assessed which traits would reflect offspring quality at birth. We found that although body length generally portrays information on female reproductive potential (gonadosomatic index and oocyte diameter), it does so indirectly. Different aspects of the female traits, such as stripe width and trunk broadness, were found to be better direct indicators. When size is kept constant, males prefer females with wider stripes. Moreover, stripe coloration was found to reflect offspring quality as darker‐striped females produced larger newborns. Our observations suggest that in a species with exclusive paternal care, independently from the male's direct investment in reproduction, female contribution decisively impacts male fitness. Thus, at least in sex‐role‐reversed species such as the black striped pipefish, female ornaments can be selected in an analogous way to those of males in species with conventional sex roles (i.e. by mate choice).