Physiological Cross-Sectional Area of the Masticatory Muscles in the Giraffe (Giraffa camelopardalis)

Abstract

Numerous studies have investigated how the skeletal morphology is related to feeding behavior (Christiansen and Adolfssen 2005; Wroe et al. 2005; Christiansen and Wroe 2007; Ellis et al. 2009; Koyabu and Endo 2009; Koyabu et al. 2009; Koyabu and Endo 2010), although remarkably few studies have focused on the architecture of masticatory muscles from which bite forces are produced. In this regard, the quantification of physiological cross-sectional areas (PCSA) of muscle is critical for estimation of bite forces. PCSA is generally proportional to and regulates maximum force-generating capacity (Santana et al. 2010). To date, PCSA of masticatory muscles are reported for bats (Burke et al. 1974; Santana et al. 2010), primates (Weijs and Hillen 1985; Anton 1999; Anapol et al. 2008), sloths (Naples 1985), rodents (Weijs and Dantuma 1975; Weijs 1980; Druzinsky 2010), rabbits (Weijs and Dantuma 1980), and pigs (Herring 1980, 1985). Since quantification of muscle PCSA is critical to estimate bite forces in animals, and since yet little is understood about the relationship between the variation of masticatory muscle PCSA and diversity in feeding behavior among mammals, further accumulation of knowledge on masticatory muscle PCSA is highly needed. Here, we report the first quantification of masticatory muscle PCSA in the giraffe. Among ruminants, giraffes have been reported that they can be clearly classified as browsers (Leuthold and Leuthold 1972; Dutoit 1990; Codron et al. 2007). Therefore, investigation on this species should provide useful information on the dichotomy of masticatory biomechanics between grazers and browsers. Their browsing behaviors are supported by some traits such as longer tongue, narrower muzzle, and narrower incisor row. Pellew (1984) have reported that Acacia shoots were most selected by Giraffa camelopardalis tippelskirchi to obtain high protein in the Serengeti National Park, Tanzania. Acacia flowers are also reported to be an important food resource for G. camelopardalis in a Southern African savanna (Dutoit 1990). To date, there have been very few studies on giraffe masticatory anatomy. Sasaki et al. (2001) has previously described the detailed macroscopic anatomy of the masseter muscles in the giraffe. However, the PCSA of masticatory muscles were not quantified, and other masticatory muscles were not incorporated in their study. Thus, in the present study, we macroscopically dissected three carcasses of giraffes and examined the muscle weight and PCSA of masticatory muscles, the medial and lateral pterygoid muscles, masseter muscle, temporal muscle, for the purpose of better understanding of their feeding strategy.