Abstract
Sexual dimorphism in body size is often used as a correlate of social and reproductive behavior in Australopithecus afarensis. In addition to a number of isolated specimens, the sample for this species includes two small associated skeletons (A.L. 288-1 or “Lucy” and A.L. 128/129) and a geologically contemporaneous death assemblage of several larger individuals (A.L. 333). These have driven both perceptions and quantitative analyses concluding that Au. afarensis was markedly dimorphic. The Template Method enables simultaneous evaluation of multiple skeletal sites, thereby greatly expanding sample size, and reveals that A. afarensis dimorphism was similar to that of modern humans. A new very large partial skeleton (KSD-VP-1/1 or “Kadanuumuu”) can now also be used, like Lucy, as a template specimen. In addition, the recently developed Geometric Mean Method has been used to argue that Au. afarensis was equally or even more dimorphic than gorillas. However, in its previous application Lucy and A.L. 128/129 accounted for 10 of 11 estimates of female size. Here we directly compare the two methods and demonstrate that including multiple measurements from the same partial skeleton that falls at the margin of the species size range dramatically inflates dimorphism estimates. Prevention of the dominance of a single specimen’s contribution to calculations of multiple dimorphism estimates confirms that Au. afarensis was only moderately dimorphic.
Highlights
Sexual dimorphism varies substantially in primates whether in response to sexual or ecological selection or a combination of both (Plavcan, 2012b)
The smaller non-333 sample shows a similar pattern (Fig. 4B), it just fails to reach statistical significance at p = 0.05 level for both chimpanzee and gorillas (Table 2). These data confirm previous analyses using the Template Method (TM) that indicate that Au. afarensis skeletal dimorphism was significantly below that of gorillas and unlikely to be as low as chimpanzees
For the largest Combined Afar (CA) sample using Lucy as a template, Au. afarensis is significantly different from both chimpanzees and gorillas and is indistinguishable from human levels of dimorphism (Table 2)
Summary
Sexual dimorphism varies substantially in primates whether in response to sexual or ecological selection or a combination of both (Plavcan, 2012b). Gorillas and orangutans are highly dimorphic in canine size, body mass, and skeletal size, largely reflecting intense single male competition in both genera. Chimpanzees are moderately dimorphic with respect to mass and canine size and are essentially monomorphic in skeletal size. The latter principally reflects multimale group composition with patrilineal territorial defense and sperm competition (Morin, 1993; Reno et al, 2003). Evaluating body mass dimorphism in humans is complicated by sex differences in body composition related to muscle and adipose mass (Plavcan, 2012b). Most striking is the absence of human canine dimorphism brought about by dramatic feminization of the male tooth (Holloway, 1967; Washburn & Ciochon, 1974; Lovejoy, 2009; Suwa et al, 2009)
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