Temperature affects operational sex ratio and intensity of male-male competition: Experimental study of sand gobies, Pomatoschistus minutus

The operational sex ratio is intimately related to the intensity of sexual selection, but factors governing variation in the operational sex ratio and their effects on mating competition are still poorly understood. In this study, temperature was found to affect both the operational sex ratio and the intensity of male-male competitive interactions in the sand goby [Pomatoschistus minutus (Pallas)]. In an experiment with two different temperature treatments, the operational sex ratio became male biased in the warm treatment (15 degrees C) and males in that treatment interacted more frequently than in the cold treatment (8.5 degrees C). These results were as predicted since the potential reproductive rate of males increases faster with temperature than does the potential reproductive rate of females. Thus, an environmental factor, water temperature, affects not only the reproductive rates of the sexes, but also the operational sex ratio and mating competition, and thereby the intensity of sexual selection. Operational sex ratio was not found to be correlated with male behavior. This may suggest a direct effect of temperature or potential reproductive rates on mate competition. The mechanism behind the evolution of such a direct relationship would, however probably be the impact of potential reproductive rates on operational sex ratio, which in turn directly affects sexual selection.