The evolution and maintenance of sexual size dimorphism in horseshoe crabs: an evaluation of six functional hypotheses

• We examine the evolution of sexual size dimorphism in horseshoe crabs.
• Larger females laid more eggs than smaller females during peak spawning.
• The costs for males maturing earlier at smaller sizes appear to be minimal.
• Fecundity selection and higher lifetime reproductive success favours large females.
• A demographic advantage of reproducing earlier favours small males.

Ultimate hypotheses to explain sexual size dimorphism (SSD) generally focus on differences in reproductive roles between males and females. For example, intrasexual competition for mates can favour larger males in some taxa, and fecundity selection can favour larger females in other taxa. However, SSD is not simply due to one selection pressure on males or females, but is the outcome of numerous conflicting pressures on both sexes. Despite this, few studies have tested multiple hypotheses to explain the factors underlying SSD. Horseshoe crabs, Limulus polyphemus, have female-biased SSD in all populations, and are an interesting species for the study of SSD because of their unusual evolutionary history and life-history strategy. We used the hypothetico-deductive method to evaluate multiple predictions from six hypotheses to understand the evolution and maintenance of SSD in horseshoe crabs. We examined these hypotheses using field studies and data from the literature. We found little support for four hypotheses. However, predictions for the fecundity advantage hypothesis were supported (e.g. larger females lay more eggs than smaller females). Our results predict that, for every centimetre increase in size, females will lay 1672 ± 285 more eggs per spawning bout. Thus, selection is likely to favour the future compounded gain in fecundity from one extra year of growth over the gain from one additional mating opportunity if females mature earlier. Predictions for the protandry hypothesis were also supported. The costs for males maturing at smaller sizes (i.e. a large-size advantage for competing males, and increased ejaculate volume) appear to be minimal. Therefore, males that mature and reproduce earlier will likely be selected for, given the demographic advantage of their offspring hatching and reproducing earlier. Our study illustrates the importance of investigating multiple hypotheses for both males and females in the study of SSD.