Two extinct hominins, Australopithecus afarensis and Australopithecus africanus, were significantly more dimorphic than chimpanzees and modern humans; Australopithecus afarensis were also significantly more dimorphic than Australopithecus africanus, according to Dr. Adam Gordon, a paleoanthropologist at the University at Albany and Durham University.
Forensic facial reconstruction of Australopithecus afarensis. Image credit: Cicero Moraes / CC BY-SA 3.0.
Sexual size dimorphism isn’t just a physical trait — it reveals something deeper about behavior and evolutionary strategy.
Consistent with sexual selection theory, high sexual size dimorphism in living primates typically correlates with strong male-male competition and social structures allowing for polygynous mating systems, where one or a few large males monopolize reproductive access to multiple females.
In contrast, low sexual size dimorphism can be found in any species, but tends to be found in those with pair-bonded social structures and low competition for mating opportunities.
Modern human populations exhibit low to moderate sexual size dimorphism, where males tend to be slightly larger than females on average but with substantial overlap in size between the sexes.
Fossil data are often fragmentary, and determining the sex of ancient individuals is nearly impossible.
To work around this, Dr. Gordon used a geometric mean method that allows for size estimation from multiple skeletal elements — including the humerus, femur, tibia and others.
He then applied resampling techniques to simulate thousands of comparisons between fossil hominins and modern primates, ensuring that the statistical models mirrored the incomplete and uneven nature of real fossil samples.
Data from modern gorillas, chimpanzees and humans with known sex and complete skeletons were used to build a comparative framework.
Unlike past studies, which sometimes interpreted weak or inconclusive statistical results as evidence of similarity, Dr. Gordon’s methods revealed clear and significant differences even when using relatively small fossil samples.
To rule out the possibility that body size changes in Australopithecus afarensis reflected evolutionary trends rather than sex differences, Dr. Gordon also tested for chronological trends across a 300,000-year span of fossils from the Hadar Formation in Ethiopia.
His analysis found no significant size increase or decrease over time, indicating that the observed variation is best explained by differences between males and females — not by evolutionary drift or long-term increases in average size.
“These weren’t modest differences,” Dr. Gordon said.
“In the case of Australopithecus afarensis, males were dramatically larger than females — possibly more so than in any living great ape.”
“And although both of these extinct hominin species exhibited greater sex-specific size differences than modern humans do, they were also more different from each other in this respect than living ape species are, suggesting a greater diversity of evolutionary pressures acting on these closely-related species than we had previously appreciated.”
Australopithecus africanus. Image credit: J.M Salas / CC BY-SA 3.0.
Dr. Gordon’s previous research suggests that high sexual size dimorphism in living primates can also be associated with intense resource stress — when food is scarce, small healthy females can get enough food to meet their own metabolic needs and store energy for reproduction faster than larger females can, leading to more offspring with smaller mothers in the next generation and a resulting greater difference in male and female size.
The high sexual size dimorphism identified in both Australopithecus species suggests a high degree of competition among males, similar to that of chimpanzees or even gorillas, while the difference between the two fossil species may be due to a difference in the intensity of those forces of sexual selection and/or a difference in the intensity of resource stress in their environments (e.g., a difference in the length of dry seasons with low fruit availability) and its impact on female body size.
In any event, the high sexual size dimorphism in these fossil hominins contrasts sharply with the more balanced size seen in modern humans and offers a glimpse into a different model of early hominin life — one where large size may have been a key factor in male reproductive success for competitive reasons, and small size may have been a key factor for females for energetic reasons.
The implications of the findings are wide-ranging. Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago, is widely regarded as either a direct ancestor of modern humans or a species very closely-related to a direct ancestor.
Yet, its high degree of sexual dimorphism suggests that early hominins may have lived in social systems that were far more hierarchical and competitive than once thought.
Meanwhile, the less dimorphic Australopithecus africanus — which overlapped in time with Australopithecus afarensis but first shows up and last appears in the fossil record slightly later, between roughly 3.3 and 2.1 million years ago — may represent a different evolutionary branch on the hominin tree, or perhaps a transitional stage in the development of more human-like social behavior.
“We typically place these early hominins together in a single group called the gracile australopiths, a group of species that are thought to have interacted with their physical and social environments in very similar ways,” Dr. Gordon said.
“And while that’s true to a certain extent — the evidence suggests that both these species may have had social organizations more like gorillas than modern people — the significant difference in the amount of dimorphism in these two extinct species suggests that these closely-related hominin species were subject to selection pressures more distinct than the selection pressures applied to any pair of similarly closely-related living ape species, highlighting the diversity of ways that our extinct ancestors and close relatives interacted with the world.”
The findings appear in the American Journal of Biological Anthropology.
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Adam D. Gordon. 2025. Sexual Size Dimorphism in Australopithecus: Postcranial Dimorphism Differs Significantly Among Australopithecus afarensis, A. africanus, and Modern Humans Despite Low-Power Resampling Analyses. American Journal of Biological Anthropology 187 (3): e70093; doi: 10.1002/ajpa.70093