Australopithecus robustus societies – one-male or multimale?

Katarzyna A Kaszycka

doi:10.17159/sajs.2016/20150165

Katarzyna A Kaszycka

Open Access

https://doi.org/10.17159/sajs.2016/20150165

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Abstract

Determining the sex of individual specimens is important in estimating the degree of sexual dimorphism. Sexual dimorphism, in turn, provides clues for reconstructing the social organisation and mating systems of extinct species. In an article published in Science, Lockwood et al. (Lockwood CA, Menter CG, Moggi-Cecchi J, Keyser AW. Extended male growth in a fossil hominin species. Science. 2007;318:1443–1446.) suggested an uneven sex ratio (in favour of males) for the known individuals of the South African Pleistocene hominid, Australopithecus robustus, and claimed evidence of an extended period of growth (delayed maturity) for the males of this species. They concluded that this finding, combined with estimates of sexual size dimorphism, suggests a polygynous reproductive strategy, and a social system similar to that of silverback gorillas (i.e. one-male harems). On re-examination of these claims, and based on further analysis, I agree with Lockwood et al. that morphologically A. robustus exhibits an increased (almost gorilla-like) level of facial dimorphism, but propose using an alternate (clustering) technique for grouping the specimens of highly dimorphic species into sexes, and argue that their pronouncements regarding a polygynous social structure of these early hominids are inconclusive. I contend instead that the habitat occupied by this species suggests rather that a one-male harem social structure would have been counterproductive.

Highlights

Sexual dimorphism is a common phenomenon among both extant and extinct primates.[1]
If the set of ecological variables of hominids changed from those of their African ancestors, it can be expected that their social organisation changed too to meet the environmental changes
I have re-examined the evidence presented by Lockwood et al.[18] for a polygynous, one-male harem social structure in the South African Pleistocene hominid, Australopithecus robustus, and have shown that the conclusions reached in the Lockwood et al paper[18] can be questioned

Summary

Introduction

Sexual dimorphism (genetically determined differences between the sexes) is a common phenomenon among both extant and extinct primates.[1] The dimorphism in secondary sexual characteristics (i.e. those not directly related to reproduction) can be found in a variety of morphological features. In primates, these characteristics are primarily differences in overall body size and dimensions of the canine teeth and are among the skeletal features which can be observed in fossils. Female gorillas start breeding at the age of 10, while male gorillas only 5 years later (at about 11 years of age, males start becoming larger than females).[9]

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