Cuticle Strength and the Size-Dependence of Safety Factors in Cancer Crab Claws

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.