Comparison of Jaw Muscle Morphology in Two Sympatic Callosciurine Squirrels (Callosciurus ErythraeusandDremomys Rufigenis) in Vietnam

Laboratory of Anatomy, Department of Basic Veterinary Sciences, Obihiro University of Agriculture and Veterinary Medicine, Obihiro 080-8555, JapanIn this study, we focus on the jaw muscle morphol-ogy of two callosciurine squirrel species, Callosciuruserythraeus and Dremomys rufigenis. Callosciuruserythraeus (Callosciurini: Sciuridae: Callosciurinae) isan arboreal squirrel that inhabits Southeast Asia fromeastern India to the Malayan archipelago, Indochina,southern China, and Taiwan (Nowak 1999; Can et al.2008; Francis 2008; Smith and Xie 2008). D. rufigenis,another member of the tribe Callosciurini, is found sym-patrically with C. erythraeus throughout Southeast Asia(Nowak 1999; Can et al. 2008; Francis 2008; Smith andXie 2008). Koyabu et al. (2009) previously reported thatthe sympatric C. erythraeus and D. rufigenis displaynotable difference in craniodental mechanics. Both C.erythraeus and D. rufigenis are known to prefer fruitsand insects, as generally seen in other tree squirrels.However, C. erythraeus also feeds extensively on hardseeds and tough tree bark, which are rarely included inthe diet of D. rufigenis. For example, C. erythraeus isreported to frequently forage on the hard seeds ofPinus, Camellia, and Castanopsis tree (Chou et al.1985; Sonoda et al. 2001; Lu 2003; Men et al. 2007).Callosciurus erythraeus is also known to often strip toughbark to obtain sugar from sweet tree saps (Tamura andOhara 2005), whereas this dietary behavior seems to berare or not reported in D. rufigenis (Medway 1969;Nowak 1999; Smith and Xie 2008). Koyabu et al.(2009) demonstrated that C. erythraeus possesses agreater mechanical advantage of the masseter and tem-poralis muscles for chewing compared to D. rufigenis.Callosciurus erythraeus was also found to display greaterleverage of the masseter and temporalis for incision.Given these findings, it was suggested that C. erythraeusshould be capable of generating greater bite force for agiven muscle force at the cheek teeth and that the cranio-dental divergence between C. erythraeus and D. rufigenismay facilitate their trophic separation under sympatry.In their study, however, only the analysis of mechani-cal leverages was conducted, and comparisons of jawmuscle morphology were not incorporated. Althoughthe craniodental arrangements and mechanical advantageof the masticatory muscles are critical parameters thataffect the amount of force production, muscle mass isalso another essential contributor to the bite forces ananimal can generate (Raadsheer 1999). Since the biteforces an organism can generate should constrain therange of food items it can eat (Lucas 2004), variation inmasticatory muscle mass has undoubtedly been funda-mental for the dietary diversification of rodents. Giventhe great range of diet and phenomenal level of morpho-logical diversity found across rodents, studies on rodentmuscle mass variation should provide promising avenuesto study the ecomorphological patterns of mammals ingeneral. To date, reports on muscle mass are availablefor some muroids (e.g., Hiiemae and Houston 1971;Weijs and Dantuma 1975; Satoh 1997; Satoh and Iwaku2004) and New World squirrels (Ball and Roth 1995).Although Thorington and Darrow (1996) provideddetailed anatomical descriptions of jaw muscles for someOld World squirrel species, still very little is describedquantitatively for them. Here, to bridge the gaps in ourknowledge of Old World squirrel muscle morphology