Meeting Abstract
Why traits that strongly affect fitness vary is a standing question in evolutionary biology. Many such traits may vary across a species’ range, including body size, life-span, development rate, song structure, and mating strategy. Variation in these traits have often been attributed to factors such as season length and temperature, localized ecological factors, and population specific sexual selection. While studies explaining changes in these types of traits across populations are common, few studies have investigated how gametes differ across a species’ range. In particular, little is known about the extent to which sperm traits vary across populations and, if variation exists, the factors or selective pressures that lead to patterns of variation. Sperm traits are strongly linked to fitness, especially in systems with high levels of sperm competition, and thus an understanding of the factors that affect sperm trait evolution is essential. In this study, we examine differences in sperm traits across populations of the American horseshoe crab, Limulus polyphemus. We find that populations differ in average ejaculate size, sperm concentration, and sperm velocity. Population variation in the operational sex ratio and body size seem to influence most strongly the differences in ejaculate size and sperm concentration. These results suggest that populations may adapt to the local level of sperm competition risk.
