Temperature affects male and female potential reproductive rates differently in the sex-role reversed pipefish, Syngnathus typhle

Abstract

The differences in potential reproductive rate between the sexes can be used to predict the operational sex ratio and the patterns and intensity of mating competition and hence sexual selection in a population. This article describes how one environmental component, temperature, affects potential reproductive rates of the two sexes in the paternally brooding, sex-role reversed pipefish (Syngnathus typhle). Males brooded embryos much longer (on average 58 days) in cold water (about 10°C) than in warmer water (35 days at about 15°C). As a consequence, the potential reproductive rate (number of eggs brooded per day) of males was significantly higher in warm water. In females, however, potential reproductive rate, i.e., number of eggs produced per day given an unlimited access to mates, was not significantly different between temperatures. In both sexes, potential reproductive rate was positively related to body size. At both temperatures, females had the potential to reproduce faster than males. As a result, the operational sex ratio will become female biased and sex-roles reversed, as is the case in this species. Since temperature differently influenced the potential reproductive rates of males and females, with the sexual difference larger at lower temperatures, more intense female-female competition is predicted at low temperatures.