Gape and Bite Force in the Rodents Onychomys leucogaster and Peromyscus maniculatus: Does Jaw-Muscle Anatomy Predict Performance? Susan H. Williams, 1 * Erika Peiffer 2 and Sonya Ford 2 1 Department of Biomedical Sciences, Ohio University College of Osteopathic Medicine, Athens, Ohio 45701 2 Department of Biological Sciences, Ohio University, Athens, Ohio 45701 ABSTRACT Compared with the deer mouse, Peromy- scus maniculatus, the grasshopper mouse, Onychomys leucogaster , exhibits modifications in its jaw-muscle architecture that promote wide gapes and large bite forces at wide gapes to prey upon large vertebrate prey. In this study, we determine whether jaw-muscle anat- omy predicts gape and biting performance in O. leu- cogaster , and we also assess the influence of gape on bite force in the two species. Although O. leucogaster has an absolutely longer jaw, which facilitates larger gapes, maximum passive gape is similar in both species, aver- aging 12.5 mm. Thus, when scaled to jaw length, O. leucogaster has a smaller maximum passive gape. These results suggest that predatory behaviors of O. leu- cogaster may not require remarkably large gapes. On the other hand, both absolute and relative bite forces exerted by O. leucogaster are significantly larger than those of P. maniculatus. The largest bite forces in both species occur at 5.0 mm of gape at the incisors, or 40% of maximum gape. Although bite force in both species decreases at larger gapes, O. leucogaster does maintain a larger percentage of maximum bite force at gapes larger than 40% of maximum passive gape. Therefore, although structural modifications in the masticatory ap- paratus of O. leucogaster may constrain gape, they may help to maintain bite force at large gapes. These results suggest that increases in gape differentially influence the length-tension properties of the jaw muscles in the two species. Finally, these results highlight the impor- tance of considering the effect of muscle stretch on force production in comparative studies of bite force. As a first approximation, it appears that gapes of 40–50% of maxi- mum gape in rodents optimizes bite force production at the incisors. J. Morphol. 000:000–000, 2009. Ó 2009 Wiley-Liss, Inc. KEY WORDS: bite force; gape; Onychomys leucogaster; Peromyscus maniculatus; rodent; jaw muscles INTRODUCTION Maximum bite force and gape are measures of performance associated with an animal’s feeding ecology, diet and foraging behavior that facilitate ecological separation within or among species. Whereas maximum bite force has been correlated with differences in food mechanical properties and dietary preferences (e.g., Kiltie, 1982; Binder and Van Valkenburgh, 2000; Herrel et al., 2001a, 2001b; Aguirre et al. 2002; Vinyard et al., 2004), maximum gape may limit the size of food that can be ingested or may facilitate certain feeding behav- iors and dietary specializations (Herring and Her- ring 1974; Herring, 1972, 1975; e.g., Emerson and Radinsky, 1980; Smith, 1984; Wheelwright, 1985; Shine, 1991; Rodriguez-Robles et al., 1999; Herrel et al., 2002; Vinyard et al., 2003; Herrel and O’Reilly, 2006; Satoh and Iwaku, 2008). For exam- ple, Verwaijen et al. (2002) demonstrate a positive correlation between bite force and prey hardness in two species of lacertid lizards, and argue that these differences contribute to separation in prey selection because of the effect that it has on prey handling efficiency. With respect to gape, work on callitrichid primates has shown that common mar- mosets routinely use gapes that are almost as large as their maximum passive gape to gouge and score trees to elicit exudate flow. Moreover, maximum passive gapes in marmosets are also larger relative to jaw length than in nongouging callitrichids, suggesting that large gapes are fundamental to their unique gouging behavior (Taylor and Vinyard, 2004). Both bite force and gape are influenced by cra- nial morphology and jaw-muscle anatomy, includ- ing muscle architecture, orientation and position. For example, bite force may be increased by increased packing of fibers resulting from greater fiber pinnation, larger physiological cross-sectional areas of the jaw muscles and/or increased me- chanical advantage of the jaw muscles (Smith Contract grant sponsor: NSF; Contract Grant number: IOB- 0520855; Contract grant sponsor: Ohio University Provost’s Under- graduate Research Award. *Correspondence to: Susan H. Williams, Department of Biomedi- cal Sciences, Ohio University College of Osteopathic Medicine, 228 Irvine Hall, Athens, Ohio 45701. E-mail: willias7@ohio.edu Received 26 November 2008; Revised 18 March 2009; Accepted 5 April 2009 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10761 JOURNAL OF MORPHOLOGY 000:000–000 (2009) Ó 2009 WILEY-LISS, INC.