No abstract

The surprising incidence of nonlethal skeletal fractures implies that many organisms operate near their upper performance limits, yet we know little about the loads at which biological structures break or about the material properties of those structures. In addition, biologically realistic estimates of how closely normal maximal loads approach breaking strengths (i.e., safety factors) remain elusive. We measured cuticular breaking strength (a material property) and safety factors (breaking force/maximum biting force) for intact claws of six species of predatory Cancer crabs (Crustacea, Brachyura). Cuticular breaking stresses in Cancer claws (40-120 MN m-2) exceeded those reported for the carapace of shore crabs (Carcinus) and swimming crabs (Scylla), but were similar to published values for the claws of stone crabs (Menippe). Cuticular breaking strength increased towards the tip of the pollex (fixed finger), correlated with visible changes in the claw cuticle, but decreased with increasing claw size. Safety factors of the pollex varied within and among Cancer species and ranged from 2 to 7. Safety factors increased with increasing claw size ({alpha}manus length0.6), due to proportionally thicker cuticle ({alpha}manus length1.31+/-0.078) and proportionally lower maximum biting forces ({alpha}manus length1.49+/-0.082). Why larger crabs have proportionally lower biting forces remains an important unsolved problem. The higher safety factors of larger claws appear adaptive, however, since costs of failure and unpredictability of cuticle strength increase with increasing size. Patterns of intraspecific size-dependence offer an attractive test of whether safety factors vary adaptively. A brief review of the literature suggests that positive size-dependence often signals adaptive variation in safety factors, whereas negative size-dependence may signal the action of constraints on growth or form.

To better understand how safety factors of biological structures evolve, we examined the frequency of claw failure, and the intra-and interspecific 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 Pacific: 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, suggesting a coupling that maintains a more stable safety factor over the moult cycle. In some species, size-adjusted maximum biting forces were higher for males than females, but this paralleled differences in breaking strength, so safety factors did not differ between the sexes. Among the six Cancer species, one exhibited an unusually high safety factor (C. oregonensis, 7.4) and another an unusually low one (C. magister, 2.6). The remaining four species were similar to each other and exhibited an intermediate safety factor (3.6). From a phylogenetic perspective, the species with more extreme safety factors appeared to be derived from a common ancestor with an intermediate safety factor. From an ecological perspective, species more closely associated with rocky substrata, and presumably a higher incidence of hard-shelled prey, exhibited higher safety factors. But safety factors were also correlated with relative claw size, and sexual dimorphism in claw size. Although we cannot say whether habitat, diet or sexual selection are primarily responsible for the differences in safety factors observed among species, the cost of producing a relatively larger claw seems an unlikely explanation because safety factors did not differ between males and females in any of the sexually dimorphic species.