Eight forelimbs of three orangutans and four chimpanzees were dissected and the muscle mass, fascicle length and physiological cross-sectional area (PCSA) of all forelimb muscles were systematically recorded to explore possible interspecies variation in muscle dimensions. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire forelimb muscles for normalization. The results indicate that the mass and PCSA ratios of the monoarticular elbow flexors (M. brachialis and M. brachioradialis) are significantly larger in orangutans. In contrast, the mass ratios of the biarticular muscles in the upper arm (the short head of M. biceps brachii and the long head of M. triceps brachii) are significantly larger in chimpanzees. For the rotator cuff muscles, the force-generating capacity of M. subscapularis is significantly larger in orangutans, whereas the opposite rotator cuff muscle, M. infraspinatus, is larger in chimpanzees. These differences in forelimb muscle dimensions of the two species may reflect functional specialization for their different positional and locomotor behaviors.

A left brachial plexus and axillary artery of bonobo (Pan paniscus) were examined, and the interrelation between the brachial plexus and the axillary artery was discussed. This is the first report of the brachial plexus and the axillary artery of bonobo. The bonobo brachial plexus formed very similar pattern to that of other ape species and human. On the other hand, the branches of the bonobo axillary artery had uncommon architecture in comparison with human case. The axillary artery did not penetrate the brachial plexus and passes through all way along anterior to the brachial plexus. Only 4.9% of human forelimbs have this pattern. Moreover, the brachial artery runs through superficially anterior to branches of the brachial plexus.

We dissected the left upper limb of a female orangutan and systematically recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA), in order to quantitatively clarify the unique muscle architecture of the upper limb of the orangutan. Comparisons of the musculature of the dissected orangutan with corresponding published chimpanzee data demonstrated that in the orangutan, the elbow flexors, notably M. brachioradialis, tend to exhibit greater PCSAs. Moreover, the digital II-V flexors in the forearm, such as M. flexor digitorum superficialis and M. flexor digitorum profundus, tend to have smaller PCSA as a result of their relatively longer fascicles. Thus, in the orangutan, the elbow flexors demonstrate a higher potential for force production, whereas the forearm muscles allow a greater range of wrist joint mobility. The differences in the force-generating capacity in the upper limb muscles of the two species might reflect functional specialization of muscle architecture in the upper limb of the orangutan for living in arboreal environments.

In the ''Methods'' section, the first sentence of the first paragraph should read: A left upper limb of a female Sumatran orangutan (Pongo abelii) was obtained for dissection from Higashiyama Zoo and Botanical Gardens, Aichi, Japan, through the Great Ape Information Network (GAIN).

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Comparative analysis of the foot muscle architecture among extant great apes is important for understanding the evolution of the human foot and, hence, human habitual bipedal walking. However, to our knowledge, there is no previous report of a quantitative comparison of hominoid intrinsic foot muscle dimensions. In the present study, we quantitatively compared muscle dimensions of the hominoid foot by means of multivariate analysis. The foot muscle mass and physiological cross-sectional area (PCSA) of five chimpanzees, one bonobo, two gorillas, and six orangutans were obtained by our own dissections, and those of humans were taken from published accounts. The muscle mass and PCSA were respectively divided by the total mass and total PCSA of the intrinsic muscles of the entire foot for normalization. Variations in muscle architecture among human and extant great apes were quantified based on principal component analysis. Our results demonstrated that the muscle architecture of the orangutan was the most distinctive, having a larger first dorsal interosseous muscle and smaller abductor hallucis brevis muscle. On the other hand, the gorilla was found to be unique in having a larger abductor digiti minimi muscle. Humans were distinguished from extant great apes by a larger quadratus plantae muscle. The chimpanzee and the bonobo appeared to have very similar muscle architecture, with an intermediate position between the human and the orangutan. These differences (or similarities) in architecture of the intrinsic foot muscles among humans and great apes correspond well to the differences in phylogeny, positional behavior, and locomotion.

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. Muscle mass and PCSA were divided by the total mass and total PCSA of the entire foot muscles for normalization. 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. Moreover, the medial components of the intrinsic muscles exhibited relatively larger mass and PCSA ratios in orangutans. The mass and PCSA ratios of the hallucal muscles were larger in chimpanzees. 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.

ABSTRACT. We dissected the hindlimb of a female western lowland gorilla and determined the muscle dimensions (mass, fascicle length, and physiological cross-sectional area: PCSA). Comparisons of the muscle parameters of the measured gorilla with corresponding reported human data demonstrated that the triceps surae muscles were larger and had more capacity to generate force than the other muscle groups in both species, but this tendency was more prominent in the human, probably as an adaptation to strong toe-off during bipedal walking. On the other hand, PCSAs of the extrinsic pedal digital flexors and digiti minimi muscles were larger in the western lowland gorilla, suggesting that the foot, particularly the fifth toe, has a relatively high grasping capability in the lowland gorilla.KEY WORDS: foot, gorilla, muscle architecture.J. Vet. Med. Sci. 71(6): 821-824, 2009 Gorillas are generally regarded as the most terrestrial of the extant hominoids. However, the degree of arboreality is known to vary among subspecies. Mountain gorillas (G. gorilla beringei) living in eastern central Africa seem to be the least arboreal, and the amount of time they spend above ground is only 7% in females and 2% in males [1]. On the other hand, western lowland gorillas (G. g. gorilla) in western central Africa are observed to be more arboreal and are frequently found in trees higher than 20 m [1,9]. Such differences in the degree of arboreality among the subspecies are correlated with foot anatomy. The foot of the western lowland gorilla has a relatively longer free portion of the first toe capable of opposing to the other four toes for grasping, whereas that of the mountain gorilla is relatively more humanlike and adapted for terrestrial locomotion [10,11].Therefore, understanding subspecies variations in the muscular characteristics of the gorilla foot is important for interpreting functional adaptations of the foot in hominoids. However, although Payne et al. [8] have reported the muscle architecture of the gorilla's hindlimb, no studies so far have provided complete quantitative data on all of the foot muscles, including the intrinsic muscles.In this study, we dissected the left hindlimb of a female western lowland gorilla to provide complete quantitative * CORRESPONDENCE TO: OISHI, M., First Department of Anatomy, School of Veterinary Medicine, Azabu University, 1-17-71, Fuchinobe, Sagamihara, Kanagawa 229-8501, Japan. e-mail: dv0502@azabu-u.ac.jp Tables 1 and 2.