Primate Models for Australopithecine Sexual Dimorphism

Several different models of sexual dimorphism in the South African australopithecines are compared with sexual dimorphism in the living primates. Australopithecine dimorphism is placed in an evolutionary context, and contrasting trends in the hominid and pongid lineages are shown. Evidence suggesting that the australopithecines were an extremely polytypic taxon is presented, and a high level of both interand intra-population variation is indicated. The evolution of sexual dimorphism has received little attention in recent decades, although it seemed of great importance before a substantial fossil record had been discovered (Bonnet, '19). Yet the extent of the fossil record currently known, and the large available body of knowledge concerning the living non-human primates, suggest that an understanding of the evolution of this phenomenon is of particularly critical importance both for its own sake, and as a possible explanation for the considerable amount of individual variability that seems to characterize almost every known fossil hominid site with more than one individual (Brace, '73). Of course, many attempts have been made to determine the sex of individual fossils as they are discovered. Few, however, have sought to understand the general pattern of evolution of the phenomenon in the hominid lineage. Furthermore, several recent attempts to arrive at such an understanding have been confused by various misstatements of sexual dimorphism hypotheses concerning the Pliocene and Lower Pleistocene hominids. This early sample comprises the first clearly recognizable group of human ancestors. An unambiguous determination of their sexual dimorphism is crucial in both tracing the evolution of the phenomenon and in helping to partition the causes of variation within the early hominid sample itself. The purpose of this work is to discuss various models of early hominid sexual dimorphism; to present an unambiguous model for sex determination in the early hominids and determine the degree of sexual dimorphism in the sample; and to place the sexual dimorphism established in an evolutionary context. TESTING THE NON-HYPOTHESIS In a recent publication, the non-hypothesis that the South African gracile australopithecines are females, and the South African robust australopithecines are males, has been once again resuscitated (Greene, '73). This contention has been stated by numerous authors (Holloway, '70; Pilbeam, '72; Robinson and Steudel, '73; and an anonymous Nature editorial (228: p. 3151, and considerable effort has been expended in its "refutation." However, an examination of the sources usually quoted as originating the "hypothesis" (Brace, '69; Brace et al., '71; Wolpoff, '71) reveals that the contention exists in only the publications of the quoters. The quotees have found it necessary to reiterate their position on several different occasions (Brace, 1 Supported in part by NSF Grant GS-33035 AM. J. PHYS. ANTHROP., 45; 497-510. 497 498 MILFORD H. WOLPOFF ’73; Wolpoff, ’75) and this will hopefully be the last time it is necessary. Greene has chosen to test the “hypothesis” that “the differences between Paranthropus and Australopithecus are no greater than those found between male and female gorilla,” explicitly quoting Brace (’71) as its source although what Brace actually said was: “Within group variation in the Australopithecines is comparable to the variation in modem primates where sexual dimorphism is large.”