Nest tending increases reproductive success, sometimes: environmental effects on paternal care and mate choice in flagfish

Parents should adjust parental care if the costs and benefits of that care vary. Traditionally, the benefit of care has been assumed to be increased offspring fitness, yet increasing evidence indicates that care can also increase mating success. We examined male behaviour and reproductive success across environments in flagfish, Jordanella floridae, a pupfish found across a range of salinities in Florida, U.S.A. Care may be more beneficial to offspring in freshwater habitats than in brackish ones. If so, female preferences for care-giving males should be stronger in fresh water. We quantified male behaviour in fresh and brackish water for four populations and examined whether male behaviour influenced the probability of spawning or the number of eggs spawned. A male's behaviour influenced his reproductive success, but did so differently in fresh and brackish water. In fresh water, the male's behaviour prior to spawning was a strong predictor of whether or not he would spawn, whereas in brackish water, postspawning behaviour of males predicted additional spawning success. These results suggest that the traits that females use to assess potential mates differ depending on the salinity of the environment. Despite the importance to a male's spawning success of different activities in different salinities, male behaviour did not differ consistently between salinities. We discuss possible benefits to females of a phenotypically plastic mate choice criterion and examine explanations for why male behaviour does not covary with the strength of sexual selection across environments.