Mate competition and resource competition are inter-related in sexual selection

Sexual selection can be affected by the competition for limited breeding resources and/or the competition for limited mates. Although there is ample evidence for each type of competition by itself, little is known about their relative importance and interaction. To address these questions, we established 48 experimental breeding populations of the two-spotted goby (Gobiusculus flavescens), a substrate-breeding fish with paternal care. In three experimental treatments, males were limited in the access to either nest sites or mates or were provided with both nests and mates in excess. We quantified male competition behaviour (agonistic and courtship), the opportunity for selection and selection on male body size. Limited access to nests and mates produced similar opportunities for selection, but only limited access to mates increased male competitive behaviours and caused positive selection on male body size. Selection on body size in the mate-limited treatment was due both to larger male...