Sexually selected nest‐building – Pomatoschistus minutus males build smaller nest‐openings in the presence of sneaker males

Both natural selection and sexual selection may act on nest‐building. We tested experimentally how different regimes of egg‐predation and male–male competition influence nest‐building before mating, using the marine fish sand goby, Pomatoschistus minutus . Males with sneaker males present built the smallest nest‐openings, smaller than males held alone or with Pomatoschistus microps males (which may predate eggs and compete over nest‐sites but not compete over fertilizations). Males with visual access to other nest‐building males tended also to build smaller openings than males held alone or with P. microps . Males with egg‐predators present built nests with openings not differing significantly from any other treatment. Our results indicate that the small nest‐openings found in the sneaker male treatment are sexually selected through protection against sneaking or by female choice. Across treatments, time span before a male started to build his nest also explained variation in nest‐opening width; males starting late built larger nest‐openings.