Dietary Correlates of Temporomandibular Joint Morphology in the Great Apes Claire E. Terhune* Department of Community and Family Medicine, Duke University Medical Center, Box 104780, Durham, NC KEY WORDS primate feeding behavior; masticatory morphology; geometric morphometrics ABSTRACT Behavioral observations of great apes have consistently identified differences in feeding behav- ior among species, and these differences have been linked to variation in masticatory form. As the point at which the mandible and cranium articulate, the temporoman- dibular joint (TMJ) is an important component of the masticatory apparatus. Forces are transmitted between the mandible and cranium via the TMJ, and this joint helps govern mandibular range of motion. This study examined the extent to which TMJ form covaries with feeding behavior in the great apes by testing a series of biomechanical hypotheses relating to specific components of joint shape using linear measurements extracted from three-dimensional coordinate data. Results of these anal- yses found that taxa differ significantly in TMJ shape, particularly in the mandibular fossa. Chimpanzees have relatively more anteroposteriorly elongated joint surfa- ces, whereas gorillas tend to have relatively anteroposter- iorly compressed joints. Orangutans were most commonly intermediate in form between Pan and Gorilla, perhaps reflecting a trade-off between jaw gape and load resist- ance capabilities. Importantly, much of the observed vari- ation among taxa reflects differences in morphologies that facilitate gape over force production. These data therefore continue to emphasize the unclear relationship between mandibular loading and bony morphology, but highlight the need for further data regarding food material properties, jaw gape, and ingestive/food process- ing behaviors. Am J Phys Anthropol 150:260–272, 2013. V V C 2012 Wiley Periodicals, Inc. Observations of African ape feeding behavior suggest a gradient in dietary resistance 1 across taxa. Chimpanzees consume less resistant foods compared to mountain goril- las (Watts, 1984; Williamson et al., 1990), while bonobos and lowland gorillas fall between these two extremes (Badrian and Malenky, 1984; Tutin and Fernandez, 1993; Malenky and Wrangham, 1994). A similar gradient has been documented within and between orangutan species (Galdikas, 1988; Knott, 1998; Fox et al., 2004; Wich et al., 2006), and comparisons of African and Asian apes suggest orangutans consume food items, such as seeds, that are at least as—if not considerably more—resistant than foods consumed by gorillas (Elgart-Berry, 2004; Vogel et al., 2008). Food processing behaviors also vary across ape taxa, with orangutans and chimpanzees relying more con- sistently on their anterior teeth for processing than goril- las (Lucas et al., 1994; Ungar, 1994; Lambert, 1999). A number of researchers have attempted to link these feed- ing behaviors to great ape craniofacial variation with varying success (e.g., Krogman, 1931a,b; Schultz, 1969; Daegling, 1989, 2007; Taylor, 2002, 2005, 2006a,b, 2009; Constantino, 2007; McCollum, 2007; Taylor et al., 2008). Though this prior work suggests differences in mastica- tory form among the great apes in general (e.g., Taylor, 2002, 2003, 2006a,b), and the TMJ in particular (Taylor, 2005), this work did not incorporate all functionally im- portant aspects of this joint, including the cranial compo- nent of the TMJ and condylar curvature. The study pre- sented here addresses this gap by linking known differen- ces in diet and feeding behavior to variation in both the cranial and mandibular components of the TMJ. The great apes are an important clade in which to evaluate craniofacial and TMJ variation for several rea- sons. Temporal bone morphology, including the mandibu- lar (i.e., glenoid) fossa, has been used to distinguish among extant great ape taxa, for quantifying expected levels of variation in fossil taxa, and for recovering phy- logenetic relationships among great apes (Harvati, 2003; Lockwood et al., 2002, 2004; Terhune et al., 2007). Simi- larly, temporal bone form, again, including the mandibu- lar fossa, has been shown to reflect genetic variation among modern humans (Smith et al., 2007; von Cramon- Taubadel, 2009). Thus, further evaluating great ape TMJ shape from a functional perspective, especially the relatively less studied cranial component of this joint, may help to elucidate the extent to which phylogenetic variation in this clade reflects functional differences in the masticatory system. Finally, examining the func- tional correlates of TMJ shape in the great apes will add to a growing body of data describing TMJ form across primates (e.g., Smith et al., 1983; Bouvier, 1986a,b; Wall, 1995, 1999; Vinyard, 1999; Vinyard et al., 2003; Taylor, 2005; Terhune, 2010, 2011a,b) and may assist in recon- structions of feeding behavior in fossil taxa. 1 I use the term ‘‘resistant’’ here to collectively refer to foods that are fracture resistant (tough) and/or stress-limited (stiff) (Lucas, 2004; Williams et al., 2005). Additional Supporting Information may be found in the online version of this article. Grant sponsor: National Science Foundation, the Leakey Founda- tion, Arizona State University; Grant number: NSF DDIG 0752661. *Correspondence to: Department of Community and Family Medi- cine, Duke University Medical Center, Box 104780, Durham, NC 27710, USA. E-mail: claire.terhune@duke.edu Received 18 January 2012; accepted 9 November 2012 DOI 10.1002/ajpa.22204 Published online 7 December 2012 in Wiley Online Library (wileyonlinelibrary.com). V V C 2012 WILEY PERIODICALS, INC. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 150:260–272 (2013)