Durophagous crabs successfully hunt hard-shelled prey by subjecting them to extremely strong biting forces using their claws. Here I show that, for a given body mass, six species of Cancer crabs (Cancer antennarius, Cancer branneri, Cancer gracilis, Cancer magister, Cancer oregonensis and Cancer productus) were able to exert mean maximum biting forces greater than the forces exerted in any other activity by most other animals. These strong biting forces were in part a result of the high stresses (740-1350 kN m(-2)) generated by the claw closer muscle. Furthermore, the maximum muscle stress increased with increasing mean resting sarcomere length (10-18 microm) for the closer muscle of the claws of these six Cancer species. A more extensive analysis incorporating published data on muscle stresses in other animal groups revealed that stress scales isometrically with the resting sarcomere length among species, as predicted by the sliding filament model of muscle contraction. Therefore, muscle or filament traits other than a very long mean sarcomere length need not be invoked in explaining the high stresses generated by crustacean claws.

Relative brain size and the relative size of six brain regions (main olfactory bulbs, accessory olfactory bulbs, telencephalon, optic tectum, cerebellum and brain stem) in ten species of anurans from five habitats were examined to determine whether there was any evidence of adaptation in brain structure. A previously published data set was also reanalysed. Arboreal frogs have larger body-size corrected brains than frogs from other habitats. Arboreal ranid (Platymantis vitiensis) and hylid (Hyla versicolor) possess slightly larger cerebella than the ranids and hylids from other habitats. Platymantis vitiensis lacks an accessory olfactory bulb. The fully-aquatic Xenopus laevis (Pipidae) has a smaller optic tectum and cerebellum than the non-fossorial hylids and ranids. Adaptation to life underground appears to explain the modified brains of two fossorial frogs, Hemisus guineensis (Ranidae) and Rhinophrynus dorsalis (Rhinophrynidae). Both species of fossorial frogs have reduced optic tecta, larger main olfactory and smaller accessory olfactory bulbs, and larger torus semicircularis than non-fossorial species. Our data showed a strong negative correlation between the size of the optic tectum and the size of the main olfactory bulbs. We conclude that, although anuran brains are very similar across taxa in qualitative and general structure, there are some interesting, apparent adaptations, to fossorial and arboreal life.

Performance data for the claws of six sympatric species of Cancer crabs confirmed a puzzling pattern reported previously for two other decapod crustaceans (stone crabs, Menippe mercenaria, and lobsters, Homarus americanus): Although biting forces increased, maximum muscle stresses (force per unit area) declined with increasing claw size. The negative allometry of muscle stress and the stress at a given claw size were fairly consistent within and among Cancer species despite significant differences in adult body size and relative claw size, but were not consistent among decapod genera. Therefore, claw height can be used as a reliable predictor of maximum biting force for the genus Cancer, but must be used with caution as a predictor of maximum biting force in wider evolutionary and biogeographical comparisons of decapods. The decline in maximum muscle stress with increasing claw size in Cancer crabs contrasts with the pattern in several other claw traits. Significantly, three traits that affect maximal biting force increased intraspecifically with increasing claw size: relative claw size, mechanical advantage, and sarcomere length of the closer muscle. Closer apodeme area and angle of pinnation of the closer muscle fibers varied isometrically with claw size. The concordant behavior of these traits suggests selection for higher biting forces in larger crabs. The contrast between the size dependence of muscle stress (negative allometry) and the remaining claw traits (isometry or positive allometry) strongly suggests that an as yet unidentified constraint impairs muscle performance in larger claws. The negative allometry of muscle stress in two distantly related taxa (stone crabs and lobsters) further suggests this constraint may be widespread in decapod crustaceans. The implications of this performance constraint for the evolution of claw size and the "arms-race" between decapod predators and their hard-shelled prey is discussed.

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.