Size variation and sexual dimorphism in and living hominoids

Abstract

New fossils recovered from the Pliocene of Ethiopia and attributed to Australopithecus afarensis (White et al., 1993; Kimbel et al., 1994) provide an opportunity to reassess size variation and sexual dimorphism in jaws and limb bones of these early hominids. In this study two related questions are addressed using exact randomization methods. First, is it possible to observe the fossil size range within samples of modern hominoids? Second, and moving beyond minimum/maximum possibilities to probabilities , what is the likelihood of sampling such differences in extant hominoids? Fossil minimum/maximum ranges in size and shape are defined by linear measurements from mandibular specimens A.L. 444-2b vs. A.L. 207-13, humeri MAK-VP-1/3 vs. A.L. 288-lm, and the proximal femora A.L. 333-3 vs. A.L. 288-lap. Modern reference samples include gorillas, orang-utans, common chimpanzees and several human groups (all of known sex). All possible pairwise contrasts (in size ratios and average Euclidean distances) are computed within each sample [e.g. within a sample of 50 gorillas (25 males, 25 females) 1225 total contrasts are possible of which 625 are opposite sex contrasts], and the probability of finding a difference as great as that seen in the fossil pair is noted. The maximum observed size (sex) difference in the fossil mandibles, humeri and proximal femora is either rare or not observed in human and chimpanzee samples, is rare to uncommon in orang-utans, and is unusual even in gorillas. Specimens recovered in the future will undoubtedly increase the fossil ranges, and thereby further reduce the probabilities of matching such differences in modern taxa. These results could imply that multiple taxa exist within the fossil assemblage. If the fossils from Hadar and Maka (and Laetoli) are assumed instead to be from one sexually dimorphic species, then the degree of sexual dimorphism of A. afarensis would have been at least as extreme as that of the most dimorphic living apes, the gorilla and orang-utan. It follows that a strictly monogamous social structure would have been highly unlikely.