A reanalysis of strontium isotope ratios as indicators of dispersal in South African hominins

Dispersal patterns in primates have major implications for behavior and sociality but are difficult to reconstruct for fossil species. This study applies novel strontium isotope methodologies that have reliably predicted philopatry and dispersal patterns in chimpanzees and other modern primates to previously published strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of two South African hominins, Australopithecus africanus and Australopithecus robustus. In this study, the difference or ‘offset’ was calculated between the ⁸⁷Sr/⁸⁶Sr of each fossil tooth compared to local bioavailable ⁸⁷Sr/⁸⁶Sr as defined by cluster analysis of modern plant isotope ratios. Large teeth (presumably belonging to males) have low offsets from local ⁸⁷Sr/⁸⁶Sr proxies, while small teeth (presumably from females) have greater offsets from local ⁸⁷Sr/⁸⁶Sr proxies. This supports previous conclusions of male philopatry and female dispersal in both A. africanus and A. robustus. Furthermore, A. robustus shows more extreme differences between presumed males and females compared to A. africanus. This is analogous to differences seen in modern olive baboons compared to chimpanzees and suggests that A. africanus may have had a larger home range than A. robustus. Neither hominin species has ⁸⁷Sr/⁸⁶Sr consistent with riparian habitat preferences despite the demonstrated presence of riparian habitats in South Africa at the time.