Variation in safety factors of claws within and among six species of Cancer crabs (Decapoda: Brachyura)

To better understand how safety factors of biological structures evolve, we examined the frequency of claw failure, and the intra- and interspeci®c patterns of variation in maximum biting force and breaking strength in the claws of six species of Cancer (Linnaeus) crabs that live in sympatry along the coast of the northeastern Paci®c: C. antennarius, C. branneri, C. gracilis, C. magister, C. oregonensis and C. productus. Although the breakage frequencies in natural populations were similar among species ( » 6%), they were higher than predicted based on failure probabilities calculated from laboratory measurements of biting force and breaking strength for healthy pristine claws. The incidence of claw damage was correlated with the degree of wear, suggesting that claws later in the intermolt interval were more likely to fail. Within species, safety factors increased from 3.1 to 4.6 with increasing instar number due primarily to a decline in muscle stress (force per unit area of apodeme). Surprisingly, the lower maximum muscle stress generated by later instars appeared to be due to behavioral restraint, since it was not accompanied by relatively lower muscle mass. In addition, among individuals of the same claw size, lower breaking forces were correlated with lower maximum biting force, and both were correlated with lighter cuticle and closer muscle mass, suggestin