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 adaptations. It has been suggested, for instance, that the cross-sectional strength of 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 carnivorans and 59 rodents. Primates differ from carnivorans and rodents in having 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 other mammals. When cross-sectional dimensions are regressed against bone length, primates appear to have more gracile bones than other mammals. These two seemingly contradictory findings can be reconciled by recognizing that most limb bones experience bending as a predominant loading regime. After regressing cross-sectional strength against the product of body mass and bone length, a product which should be proportional to the bending moments applied to the limb, primates are found to overlap considerably with carnivorans and rodents. Consequently, primate humeri and femora are similar to those of nonprimates in their resistance to bending. Comparisons between arboreal and terrestrial species within the orders show that the bones of arboreal carnivorans have greater cross-sectional properties than those of terrestrial carnivorans, thus supporting Kimura's general notion. However, no differences 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 species do not show consistent differences in long bone reinforcement, and Kimura's conclusions must be modified to take into account the interaction of bone length and cross-sectional geometry.

Skeletal allometry and interlimb scaling patterns in mustelid carnivorans.

To address the effects of an evolutionary increase in body size on long bone skeletal allometry, scaling patterns relating body mass, bone length, limb length, midshaft diameters, and cross-sectional properties of the humerus and femur were analyzed for four species of scansorial mustelids. Humeral and, to a lesser extent, femoral allometry is consistent with expectations of elastic similarity: bone and limb length scale with negative allometry on body mass while bone robusticity (cross-sectional parameters against bone length) scales with strong positive allometry. Differences between fore- and hindlimb scaling patterns, however, are observed, with size-dependent increases in forelimb length and humeral strength and robusticity exceeding those of the hindlimb and femur. It is hypothesized that this greater fore-than hindlimb lengthening results in postural modifications that serve to straighten the hindlimb of larger bodied scansorial mustelids relative to smaller mustelids. Straightening of hindlimb joints would more precisely align the long axis of the femur with peak (vertical) ground reaction forces, thereby accounting for the reduction in relative bending stresses acting on the femur compared to the humerus.

Hominid radius from the middle Pliocene of Lake Turkana, Kenya.

A nearly complete left radius, KNM-ER 20419, was recovered from middle Pliocene sediments east of Lake Turkana, Kenya in 1988. Ape-like characteristics of the fossil include an eccentrically positioned articular fovea, relatively long radial neck, wide distal metaphysis, and large brachioradialis crest. The robustness of the radial neck in proportion to the radial head, and the semilunar shape of the distal diaphysis, however, clearly distinguish KNM-ER 20419 as hominid. The distal articular surface possesses a larger area for radius-lunate articulation than for radius and scaphoid, a radiocarpal arrangement that is associated with increased wrist adduction among quadrumanous climbers. Since this morphology is also found in hylobatids, Pongo, and other early australopithecines, it is argued to be plesiomorphic for hominoids. This further supports the argument that vertical climbing was an important locomotor behavior among both early hominoids and our more immediate prebipedal ancestors.