Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus

Abstract

Social behaviour of fossil hominoid species is notoriously difficult to predict owing to difficulties in estimating body size dimorphism from fragmentary remains and, in hominins, low canine size dimorphism. Recent studies have shown that the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids. Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species, whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species. The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny. Although speculative owing to small sample sizes, these results suggest that digit ratios represent a supplementary approach for elucidating the social systems of fossil hominins.

Figure 1.

(a) Histogram of the frequencies of 2PP : 4PP in the modern human sample (white arrow; Qafzeh 9) with Neanderthal fossils superimposed as black arrows: 1, Spy; 2, Le Regourdou; 3, Kebara; 4, La Ferrassie; 5, Shanidar. (b) Contemporary humans (10 000 resampled means) with Neanderthal mean 2PP : 4PP (black arrow). Monte Carlo analysis gives a 0.19 per cent probability of drawing a sample of five modern humans with a mean 2PP : 4PP ratio as low as the observed Neanderthal mean.

Figure 2.

Histogram of variation in extant hominoid 2PP : 4PP with Miocene and Pliocene fossils transposed. Miocene apes and Ar. ramidus are classified as non-pair-bonded (NPB), while Australopithecus is classified as having a social system that is pair-bonded (PB) monogamous or intermediate (i.e. between pair-bonded and non-pair-bonded). Frequencies of individuals within species are scaled to the same dimensions; for sample sizes, see table 1).