Some Implications of Relative Biornechanical Neck Length in Hominid Femora

Demonstration of a size-based influence on relative biomechanical neck length of the femur predicts relatively longer necks for smaller femurs. Fossil hominids through the middle Pleistocene appear to have relatively longer femur necks than expected from this relation, excepting the two small australopithecine females. I t is suggested that this variation results from smaller crania a t birth in the fossils, and the possibility is raised that australopithecine populations were characterized by marked brain size differences a t birth. Biomechanical length of the femoral neck was defined to be the best, easily reproducible, osteological measure of the distance between the weight bearing point on the femur head and the abductor attachment on the greater trochanter (Lovejoy e t al., '73: p. 762). Its length, relative to femur length, is one of the features which most consistently separates australopithecine and living human femora (Lovejoy and Heiple, '72; Lovejoy, '75; Walker, '73; Wolpoff, '76a). I t has been suggested that this difference corresponds to the smaller birth canal (or pelvic inlet) in australopithecines relative to total pelvis size (Lovejoy et al., '73). Yet, the fact that most fragmentary australopithecine femora and all of the complete or reconstructable ones are very small, compared to sample means for living humans represented in most comparative collections, suggests that systematic sizebased variation might have to be taken into account for valid comparisons between the australopithecines and living human samples. The question of whether such size-based variation in biomechanical neck length exists was examined using a sample of femora from the Northeast Ohio Libben Amerind site that had been sexed by pelvic indicators. In a mixed cantly different from 1.0, and the linear correlation of 0.683 is significant a t the 0.001 level. Moreover, the best fitting power curve predicts virtually no difference in relative biomechanical neck length (100 x biomechanical neck length/morphological femur length) over a range of femur lengths from 250 mm (15.5) to 500 mm (15.3). However, in living humans the relative size of the pelvic inlet differs between males and females; holding total pelvis size constant, the inlet is broader in females (Lovejoy, '75) since i t functions as a birth canal. The relatively narrower male pelvic inlet increases the horizontal distance between the abductors and the acetabulum on the innominate, and there should be a corresponding increase in this distance on the femur for the same mechanical relations of the hip musculature to be maintained. Thus, the positive correlation and the magnitude of the linear slope in the Libben sample may be a function of more than just the size difference between males and females. The longer biomechanical necks in the male femora may also respond to the relatively narrow inlet. The effect of both of these ' Research underlying this study wau supported by NSF Grants GS33035 and BNS 75-21756. sex sample of 148-individu&, biomechanical neck length was found to be positively and linearly related to morphological (i.e., "in POsition??) bone length; the least squares fitted ' Lovejoy, Heiple, and Burstein define the measure as "the right angle distance from the lateral-most point of the greater trochanter to Its tangential point of intersection with the must cephaladpoint of the femur h e a d (p. 762). The measure is easily taken by l a y l n ~ the proximal portion of a femur an its lateral side with the shaft horizontal, and finding the point of intersection between the head and a vertical. power function exponent of 0.98 is not signifiAM. J. PHYS. ANTHROP. (1978) 48: 143-148. IY positioned ruler.