Palmer A R, Taylor G M, Barton A
Biol Bull. 1999 Jun;196(3):281-294. doi: 10.2307/1542953.
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.
非致命性骨骼骨折的惊人发生率表明，许多生物体的运作接近其性能上限，但我们对生物结构断裂时的负荷以及这些结构的材料特性知之甚少。此外，对于正常最大负荷与断裂强度接近程度（即安全系数）的生物学现实估计仍然难以捉摸。我们测量了六种捕食性黄道蟹（甲壳纲，短尾亚目）完整爪子的表皮断裂强度（一种材料特性）和安全系数（断裂力/最大咬合力）。黄道蟹爪子的表皮断裂应力（40 - 120 兆牛顿/平方米）超过了报道的滨蟹（Carcinus）和游泳蟹（Scylla）甲壳的断裂应力，但与石蟹（Menippe）爪子的已发表值相似。表皮断裂强度朝着螯指（固定指）尖端增加，与爪子表皮的可见变化相关，但随着爪子尺寸增大而降低。螯指的安全系数在黄道蟹物种内部和之间有所不同，范围从 2 到 7。安全系数随着爪子尺寸增大而增加（α掌节长度^0.6），这是由于表皮成比例变厚（α掌节长度^1.31 ± 0.078）和成比例较低的最大咬合力（α掌节长度^1.49 ± 0.082）。为什么较大的螃蟹咬合力成比例较低仍然是一个重要的未解决问题。然而，较大爪子的较高安全系数似乎具有适应性，因为随着尺寸增加，角质层强度的失效成本和不可预测性也会增加。种内尺寸依赖性模式为安全系数是否适应性变化提供了一个有吸引力的测试。对文献的简要回顾表明，正尺寸依赖性通常表明安全系数存在适应性变化，而负尺寸依赖性可能表明对生长或形态的限制作用。
