Body Size and Scaling of Long Bone Geometry, Bone Strength, and Positional Behavior in Cercopithecoid Primates

Abstract

Allometry in the strictest biometrical sense—size-correlated differences in shape — explains nothing. It is also not a biological “principle” (Smith, 1980; Jungers, 1984; Jungers et al., 1995; contra Gould, 1975; contra Martin, 1993). Rather, allometry is merely a quantitative description or signal that may or may not serve to test an explicit hypothesis. Without explicit hypotheses of how and why things should change as a function of body size (i.e., similarity criteria), allometry cannot be diagnosed except with respect to the statistical, dimensional null hypothesis of “isometry” or geometric similarity. In special circumstances, isometry can itself be a hypothetical criterion of biological similarity (Alexander et al., 1979; Biewener, 1990; Prothero, 1992). If such criteria cannot be specified and justified a priori, it also follows that even when allometry is discovered, it cannot be assumed that the observed size-correlated differences are evidence of size-required changes sufficient to insure “functional equivalence” (Smith, 1980). Empirical lines used to describe allometric patterns of interspecific scaling can rarely, if ever, be rationalized into meaningful, adaptive “criteria of subtraction” for the subsequent analysis of residuals (Smith, 1984; Jungers et al., 1995). The scaling of mammalian long-bone dimensions makes these points clearly and unequivocally: although long bone robusticity is expected to increase with body size according to most biomechanical theories, positively allometric distortions in the shape of the long bones of larger vertebrates do not produce functional equivalence in any mechanical or behavioral sense. To the contrary, further behavioral and structural modifications are still required to maintain adequate safety factors at larger body sizes (Biewener, 1982, 1990; Rubin and Lanyon, 1984; Selker and Carter, 1989; Bertram and Biewener, 1990; Demes and Jungers, 1993; Jungers and Burr, 1994).