Mechanisms behind size-assortative nest choice by sand goby males in the absence of intrasexual competition

► We tested why sand goby males prefer nest sites matching their own body size, even without male-male competition. ► Size-matched males did not enjoy fewer nest take-overs or fan with less effort than size-mismatched males. ► Females did not prefer males with size-matched nest sites over males with size-mismatched nest sites. ► More studies are needed to explain why males choose nest sites size-assortatively.