Temperature modulates competitive behaviour: why sand goby males fight more in warmer water

Abstract

An operational sex ratio is defined as the ratio of males to females ready to mate. Temperature has been shown to affect patterns of mating competition over the breeding season, as ambient water temperature differentially influences the potential reproductive rates of male and female sand gobies, Pomatoschistus minutus (Pallas), creating an unbiased operational sex ratio in colder water, and a male biased operational sex ratio in warmer water. Here, the direct effect of water temperature on frequencies of inter- and intrasexual interactions was studied in a short (24 hr) time perspective in aquaria with an unbiased adult sex ratio. Males were found to interact significantly more with other males, and also tended to interact more with females, in a warm (16 °C) water treatment, as compared with a cold (8 °C) treatment, even though there was no general increase in total number of interactions in the warm water treatment. Female-initiated interactions were not significantly affected by temperature. An altered operational sex ratio is unlikely to account for the observed difference in male-male interactions between treatments, for two reasons: (1) the operational sex ratio, which was estimated after the last behavioural recording in each replicate, was slightly, but equally, male biased in both treatments; (2) the operational sex ratio of each replicate was not correlated with the number of male-male interactions recorded from that replicate. Instead, these results suggest that temperature may influence the intensity of male mating competition directly, and I propose that individual fish use water temperature as a proximate cue for estimating and forecasting operational sex ratios.