The capacity for additional matings does not affect male mating competition in the sand goby

Abstract

Models of sexual selection predict that mating competition within a population will increase with a biased operational sex ratio (OSR). However, these models do not consider any influence of the individuals' capacity for additional matings. Using the sand goby, Pomatoschistus minutus, we tested whether intensity of mating competition (measured as the fraction of time spent displaying) is influenced by a male's imminent capacity for additional matings (nest capacity, i.e. additional space available for egg laying in the nest). We manipulated nest size and number of females (zero to three) allowed to spawn in each male's nest, and then used display rate to estimate the male's intensity of mating competition. The nests were never filled completely, but always had room for eggs from at least one more mating. We found no significant difference in display rate between males with large or small nests, whether nest size was measured as nest capacity or area already covered by eggs. In fact, males with many eggs in their nests displayed as much as males with empty nests. However, male display decreased significantly with time. As male display behaviour was not influenced by whether the nest was empty or was partially filled, the number of males and females ready to mate (i.e. the OSR) provides a good estimate of the intensity of male mating competition. Nevertheless, many other species may adjust mating competition to their capacity for additional matings, which may thus influence the accuracy of OSR-based estimates of mating competition and sexual selection.