The large chelipeds of male Orconectes rusticus have an adaptive function related to inter-male competition for mates. Large clawed males dominate over smaller clawed, but otherwise physically similar, competitors. Additionally, males with large chela are better able to secure, and more quickly orient, females into the copulatory position. Large clawed males also copulate for longer periods than smaller clawed competitors. The significance of copulation duration is unclear, but is likely related to mate guarding. Sexual selection for increased cheliped size in 0. rusticus has been more intense on males than females. Sperm of the second male to inseminate a dually mated female takes precedence over that of the previous male. The paternity attributable to the second male was determined as approximately 92 % .