To date, however, very few reports have described the foot muscle morphology of great apes (Tuttle, 1970, 1972 Thorpe et al., 1999 Oishi et al., 2009b), although there are more complete datasets of these species for the muscle morphology of the hindlimb, but excluding intrinsic foot muscles (Carlson, 2006 Payne et al., 2006a Myatt et al., 2011) and the forelimb (Ogihara et al., 2005 Carlson, 2006 Oishi et al., 2008, 2009a Myatt et al., 2012). The existence of such differences in the foot skeletal anatomy between orangutans and chimpanzees suggests that there may also exist substantial differences in foot muscle dimensions between the two species. Moreover, the intertarsal ligaments of chimpanzees are more developed than those of orangutans the plantar calaneonavicular (spring) ligament is strong to support the talar head against plantar motion during inversion (Gomberg, 1981, 1985 Gebo, 1992), although the foot of the apes, unlike human, dorsiflexes at the calcaneocuboid joint as the heel leaves the ground (Susman, 1983 Aiello & Dean, 1990). Associated with the relatively long and wide heels, the larger plantar flexors of chimpanzees provide an important advantage as a propulsive mechanism during plantigrade quadrupedalism and vertical climbing (Tuttle, 1970, 1972 Gomberg, 1981 Payne et al., 2006a).
The feet of chimpanzees have a higher ‘power arm (from the calcaneal tuberosity to the center of the talar trochlea)’ to ‘load arm (from the center of the talar trochlea to the head of metatarsal III)’ ratio (Schultz, 1963). Furthermore, the subtalar, transverse tarsal, and metatarsophalangeal joints, and the ligaments of the corresponding joints in orangutans are specialized for the inversion-eversion movements to adjust the foot to the substrate in arboreal environments (Lewis, 1980a, b Gomberg, 1981, 1985 Langdon, 1984 Rose, 1988, Kanamoto et al., 2011), resulting in a highly inverted foot position on the ground compared to the flattened foot of African apes (Tuttle, 1970 Tuttle & Beck, 1972 Susman, 1983 D'Août et al., 2002 Griffin et al., 2010), although the foot pressures of both orangutans and bonobos are similar in bearing weight predominately on the lateral aspect of the foot (Vereecke et al., 2003 Crompton et al., 2008, 2012). These features allow the flexed toe tips to be locked into the plantar aspect of the metatarsophalangeal joints and then the locked digits to be rolled into the sole (the double-locking mechanism) when orangutans grip the small-diameter supports (Schultz, 1963 Gomberg, 1981). As exhibited by analogous bones of the hand, the foot of orangutans possesses relatively elongated and curved metatarsal and phalangeal bones and a flexion set of the toe joints, resulting in the hook-like appearance of the foot (Schultz, 1963 Tuttle, 1970, 1972 Gomberg, 1981 Rose, 1988 Griffin & Richmond, 2010 Marchi, 2010). The morphology of the bones and joints enable the orangutan foot to function as a suspensory supporting organ, whereas chimpanzees have a plantigrade foot. These differences in foot muscle dimensions of the two species suggest that the orangutan is more specialized for hook-like digital gripping without involvement of the rudimentary hallux, while the chimpanzee is adapted to hallux-assisted power gripping in arboreal locomotion. The mass and PCSA ratios of the hallucal muscles were larger in chimpanzees.
Moreover, the medial components of the intrinsic muscles exhibited relatively larger mass and PCSA ratios in orangutans. The results indicate that the pedal interosseous and the intrinsic pedal digital extensor muscles in the orangutans probably have higher capacity for force production due to their relatively larger PCSAs than in chimpanzees. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire foot muscles for normalization. The hindlimbs of two orangutans and four chimpanzees were dissected, and muscle parameters (mass, fascicle length, and physiological cross-sectional area: PCSA) were determined to explore possible interspecies variation in muscle dimensions.