Canine sexual dimorphism in Ardipithecus ramidus was nearly human-like

Body and canine size dimorphism in fossils inform sociobehavioral hypotheses on human evolution and have been of interest since Darwin’s famous reflections on the subject. Here, we assemble a large dataset of fossil canines of the human clade, including all available Ardipithecus ramidus fossils recovered from the Middle Awash and Gona research areas in Ethiopia, and systematically examine canine dimorphism through evolutionary time. In particular, we apply a Bayesian probabilistic method that reduces bias when estimating weak and moderate levels of dimorphism. Our results show that Ar. ramidus canine dimorphism was significantly weaker than in the bonobo, the least dimorphic and behaviorally least aggressive among extant great apes. Average male-to-female size ratios of the canine in Ar. ramidus are estimated as 1.06 and 1.13 in the upper and lower canines, respectively, within modern human population ranges of variation. The slightly greater magnitude of canine size dimorphism in the lower than in the upper canines of Ar. ramidus appears to be shared with early Australopithecus, suggesting that male canine reduction was initially more advanced in the behaviorally important upper canine. The available fossil evidence suggests a drastic size reduction of the male canine prior to Ar. ramidus and the earliest known members of the human clade, with little change in canine dimorphism levels thereafter. This evolutionary pattern indicates a profound behavioral shift associated with comparatively weak levels of male aggression early in human evolution, a pattern that was subsequently shared by Australopithecus and Homo.

Sex Predictability by Using Mandibular Canine Index

Background: Canines are known for their higher resistance to trauma and pathological alterations when compared to other teeth. Among all the teeth, canines show highest sexual dimorphism.Methods: This descriptive cross-sectional study was commenced from January 2019 to April 2019 after the ethical approval from Institutional Review Committee. The maximum mesio-distal widths of right and left mandibular canines and mandibular inter-canine arch width were measured on the cast with the help of a divider and digital vernier callipers. Mandibular canine index was calculated by dividing the mesio-distal width of each mandibular canine with inter-canine arch width. Data was entered and analyzed using Statistical Package for the Social Sciences version 21. Results: Sex predictability by using mandibular canine index in the present study showed poor sex predictability (57.5% - 62.5%).Conclusions: Sex determination should be done by other methods and mandibular canine index should be used cautiously in Nepalese population.Keywords: Canine dimorphism; forensic anthropology; forensic dentistry; forensic identification, sex determination.