A comparison of primate, carnivoran and rodent limb bone cross-sectional properties: are primates really unique?

The cross-sectional properties of mammalian limb bones provide an important source of information about their loading history and locomotor adaptation s. It has been suggested, for instance, that the cross-sectional strength o f primate limb bones differs from that of other mammals as a consequence of living in a complex arboreal environment (Kimura, 1991, 1995). In order to test this hypothesis more rigorously, we have investigated cross-sectional properties in samples of humeri and femora of 71 primate species, 30 carni vorans and 59 rodents. Primates differ from carnivorans and rodents in havi ng limb bones with greater cross-sectional strength than mammals of similar mass. This might imply that primates have stronger bones than carnivorans and rodents. However, primates also have longer proximal limb bones than ot her mammals. When cross-sectional dimensions are regressed against bone len gth, primates appear to have more gracile bones than other mammals. These t wo seemingly contradictory findings can be reconciled by recognizing that m ost limb bones experience bending as a predominant loading regime. After re gressing cross-sectional strength against the product of body mass and bone length, a product which should be proportional to the bending moments appl ied to the limb, primates are found to overlap considerably with carnivoran s and rodents. Consequently, primate humeri and femora are similar to those of nonprimates in their resistance to bending. Comparisons between arborea l and terrestrial species within the orders show that the bones of arboreal carnivorans have greater cross-sectional properties than those of terrestr ial carnivorans, thus supporting Kimura's general notion. However, no diffe rences were found between arboreal and terrestrial rodents. Among primates, the only significant difference was in humeral bending rigidity, which is higher in the terrestrial species. In summary, arboreal and terrestrial spe cies do not show consistent differences in long bone reinforcement, and Kim ura's conclusions must be modified to take into account the interaction of bone length and cross-sectional geometry. (C) 2000 Academic Press.