Comparing population level sexual selection in a species with alternative reproductive tactics

The description of a species’ mating patterns is often based on observations from a single exemplar population; however, environmental variation can lead to variation in mating patterns and to differences in the strength of sexual selection among populations. In this study, we explored how resource distribution across a species’ range affects competition and the strength of sexual selection in a northern and southern population of plainfin midshipman (Porichthys notatus), a species with 2 male reproductive tactics. Male plainfin midshipman can be guarders that compete for nest sites and court females, or sneakers that attempt to steal fertilizations from the guarder males during spawning. Males from the north population grow larger, suggesting that there might be more competition among males in the north. However, we found that the variance in body size and in nest availability were similar between populations, suggesting instead a similar degree of male-male competition. We found no significant population differences in reproductive success (north: 517±50 eggs/nest ± SE; south: 412±68 eggs/nest ± SE), paternity (north: 52%; south: 58% for the guarding male), or tactic frequencies (north: 88% guarders; south: 91% guarders). There was a marginally steeper Bateman gradient in the south population but no difference at 8 other measures of the strength of sexual selection between the 2 populations. Thus, despite a wide geographic distance, our results show remarkable conservation of mating patterns between the north and south populations of this benthic toadfish.