Sexual Size Dimorphism in Australopithecus: Postcranial Dimorphism Differs Significantly Among Australopithecus...

Area of Science:

• Paleoanthropology
• Primatology
• Evolutionary Biology

Background:

• Sexual dimorphism in fossil hominins provides insights into social structure and mating systems.
• Previous studies on postcranial dimorphism in Australopithecus afarensis have yielded conflicting results.
• Direct statistical comparisons of dimorphism between early hominin species are lacking.

Purpose of the Study:

• To resolve discrepancies in previous dimorphism estimates for Australopithecus afarensis.
• To statistically compare postcranial size dimorphism between Australopithecus afarensis, Australopithecus africanus, and extant hominids.
• To investigate the evolutionary pressures driving dimorphism in early hominins.

Main Methods:

• Analysis of eight postcranial variables for size in gorillas, humans, chimpanzees, Australopithecus afarensis, and Australopithecus africanus.
• Application of a modified geometric mean method for significance testing of sexual size dimorphism.
• Direct statistical comparisons of dimorphism between fossil hominin samples, accounting for missing data patterns.

Main Results:

• Both Australopithecus afarensis and Australopithecus africanus exhibit significantly higher sexual dimorphism than chimpanzees and modern humans.
• Australopithecus afarensis displays significantly greater dimorphism compared to Australopithecus africanus.
• Effect sizes for dimorphism differences were large enough to achieve statistical significance even with low statistical power.

Conclusions:

• High dimorphism in Australopithecus species suggests intense sexual selection.
• Differences in the degree of dimorphism across Australopithecus species indicate varied species-specific selective pressures.
• The findings contribute to understanding the social and mating systems of early hominins.