Linking Male Reproductive Success to Effort Within and Among Nests in a Co-Breeding Stream Fish

Abstract Nest construction is an energetically costly behavior displayed by males in many taxa. In some species, males construct nests and co-breed with other males and they may construct multiple nests in a breeding season. However, little is understood about how allocation of effort within and among nests affects male reproductive success. We characterized reproductive effort of male bluehead chub (Nocomis leptocephalus) on nests in an entire breeding season using PIT antennas deployed around nests and linked effort within and among nests to reproductive success, measured by number of offspring assigned genetically to each male, in a small stream in South Carolina, USA. We monitored time spent by a total of 34 males on each of 18 nests during the spawning season in 2017. A Bayesian hierarchical analysis showed that larger males spent more time constructing and maintaining a given nest, and consequently were more reproductively successful than smaller males on the same nest. Combined with aggressive behavior displayed by larger males toward smaller males, this finding suggested that reproductive effort including agonistic interactions within nests was a determinant of reproductive success. In contrast, more males together constructed larger nests, which led to higher reproductive success of members that constructed those nests. Number of nests that male constructed, a measure of effort across nests, was not a predictor of reproductive success, indicating that reproductive success varied among nests due to nest size. Our study showed that male reproductive success was determined by both aggressive and cooperative behaviors in a co-breeding species.