Sexually dimorphic weaponry in monogamous snapping shrimp: investigating the roles of seasonal variation, female aggression, and mate choice
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Heuring, Whitney Lynn
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Abstract
Sexual dimorphisms in weaponry often result from intrasexual selection in polygynous mating systems: high levels of competition between males can favor larger weaponry. Snapping shrimp have sexually dimorphic weaponry but are monogamous; males have larger chelae than females, but females are more aggressive. Here, I determined the relationship between competitive ability and both body and chela size, and used field data to determine if weapon size and sexual dimorphism vary seasonally. Body size but not chela size affected competitive ability in both sexes. However, sexual dimorphism increases during the reproductive season due to increases in male chela size and female body size, and decreases in female relative chela size. These data suggest intrasexual selection is at best a partial explanation for sexually dimorphic weaponry in snapping shrimp. Seasonal changes in relative weapon size in females suggested a role for female aggression in mating system dynamics: highly aggressive females were predicted to increase female spatial distribution and make it difficult for males to access multiple females. I found that female aggression influences spacing, but does not appear to influence the pair stability, suggesting mechanisms other than female aggression maintain high pair stability in the system. In monogamous systems, size-assortative pairing is often due to mutual mate choice. I found that male snapping shrimp prefer larger females; however, females have no preference for male shrimp of different body or chela sizes, but prefer their pair mate male. Larger females are more fecund and are better competitors; male mate choice for larger females combined with intrasexual competition among males and female pair choice may lead to stable size-assortative pairing in snapping shrimp. Finally, I determined that individual variation in coloration was not a result of multiple cryptic species; I showed that multiple <i>Alpheus angulosus</i> clades in the <i>Alpheus armillatus</i> species complex were present in South Carolina.