Department of Evolutionary Anthropology, Duke University
March 26, 2015 3:30, Grand Ballroom C
Years ago, Smith and Savage (1959) suggested that high mandibular condyles of herbivores are more mechanically efficient than are low-positioned condyles because the high condyle is linked to larger jaw-closing moments of the masseter and medial pterygoid muscles during chewing. Others, including Rak and Hylander (2008), have considered additional competing hypotheses regarding condylar position, with a particular emphasis on robust australopiths. My purpose here is not to review competing hypotheses for condylar position. Instead, I’ll focus on observations regarding maximum gape data for baboons and geladas (Hylander, 2013), and these observations are relevant for why robust australopiths have highly positioned mandibular condyles.
It is well known that baboons have low-positioned condyles, whereas geladas have high-positioned condyles. Following Smith and Savage, high condyles are presumably linked to larger muscle-moments and bite force, whereas low condyles are linked to smaller muscle-moments and bite force. Importantly, and all things considered equal, larger moment arms should be associated with less gape, whereas smaller ones should be associated with more gape.
Surprisingly, relative gape (maximum gape/projected jaw length) in baboons and geladas are near identical. Values for male and female baboons and geladas are as follows: Papio anubis 1.12 and 0.87, Papio hamadryas 1.03 and 0.86, and Theropithecus gelada 1.05 and 0.90, respectively. Contrary to expectations, relative gape values (for each sex separately) are more or less identical. Thus, these data do not support the hypothesis that high condyles in robust australopiths are necessarily linked to increased muscle-moments and bite force.