Endocranial Volume Estimates for Sts 25 (Australopithecus cf. africanus)

ABSTRACT: Sterkfontein specimen Sts 25 is filled with calcified sediment and still partly encased in matrix. The only published endocranial volume estimate for this specimen (350–375 cm³) falls outside the range of variation for Australopithecus africanus adults. The purpose of this study was to estimate Sts 25’s endocranial volume and to explore the usefulness of parietal regressions for estimating brain size in other fragmentary hominin specimens. We used single-variable and multivariate polynomial regressions and combined chimpanzee/early hominin comparative samples to predict endocranial volumes for Sts 25 and 10 fragmentary hominin specimens from six chord and arc variables. Point estimates for Sts 25 ranged between 412–501 cm³, with random-effects means and 95% prediction intervals of 453 cm³ (393–512 cm³) from single-variable regressions and 446 cm³ (377–514 cm³) from multivariate regressions. New endocranial volume estimates ~450 cm³ for Sts 25 are consistent with values for other A. africanus specimens with similar dimensions of the vault and basicranium. Volume estimates for Sts 58 (468–559 cm³) and MLD 1 (509–595 cm³) are larger than previous estimates for these specimens and help refine the A. africanus range. Endocranial volume estimates for other crania are largely consistent with existing predictions, establishing the value of these polynomial regression equations for estimating brain size in early hominins.