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Function, Design, Scaling, and Sexual Differences of Dimorphic Chelae in the Land Crab, Cardisoma Carnifex

Abstract

Crab chelae are a model system for studying the relationship between the biomechanics of an organism’s structure and its ecolgical role. This study investigated how chelae dimorphism may correlate with specialization in function in the land crab Cardisoma carnifx (Herbst 1791). This was achieved by comparing field observations of preferential claw usage during diurnal activities to a mechanical model drived from anatomical claw measurements, claw closing effort of captured specimens, and calculations of expected closing force. Behavior, mechanical scaling, and effort were also compared between males and females. Foraging, eating, and lead claw entering burrow showed significant differences in claw use frequency. It was also found that the major and minor claws scaled differently with respect to carapace length, with the major claw displaying positively allometric scaling and the minor claw displaying near-isometric scaling. Measurement of claw closing effort with respect to claw length showed high correlation in male minor and female major claws. In males, expected force measurements showed a greater rate of growth in the minor claw than in the major claw with respect to claw lengh, but in females, expected force in the major claw exhibited a greater rate of growth. A possible explanation for the differences in design between sexes may be that there are functional differences between male nd female chelae, such as the primary use of male minor claw and female major claw in gripping objects when stressed.

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