View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Lirias J. Anat. (2005) 206, pp453–476 FunctionalBlackwell Publishing, Ltd. analysis of the foot and ankle myology of gibbons and bonobos Evie E. Vereecke,1 Kristiaan D’Août,1 Rachel Payne2 and Peter Aerts1 1Laboratory for Functional Morphology, Department of Biology, University of Antwerp, Belgium 2Royal Veterinary College, University of London, UK Abstract This study investigates the foot and ankle myology of gibbons and bonobos, and compares it with the human foot. Gibbons and bonobos are both highly arboreal species, yet they have a different locomotor behaviour. Gibbon locomotion is almost exclusively arboreal and is characterized by speed and mobility, whereas bonobo locomotion entails some terrestrial knuckle-walking and both mobility and stability are important. We examine if these differ- ences in locomotion are reflected in their foot myology. Therefore, we have executed detailed dissections of the lower hind limb of two bonobo and three gibbon cadavers. We took several measurements on the isolated muscles (mass, length, physiological cross sectional area, etc.) and calculated the relative muscle masses and belly lengths of the major muscle groups to make interspecific comparisons. An extensive description of all foot and ankle muscles is given and differences between gibbons, bonobos and humans are discussed. No major differences were found between the foot and ankle musculature of both apes; however, marked differences were found between the ape and human foot. The human foot is specialized for solely one type of locomotion, whereas ape feet are extremely adaptable to a wide variety of locomotor modes. Apart from providing interesting anatomical data, this study can also be helpful for the interpretation of fossil (pre)hominids. Key words ankle; bonobo; foot myology; gibbon; locomotion. The human foot is paradigmatic in reflecting the Introduction species’ locomotor adaptations, because of its striking In this study we set out to investigate to what extent specializations for habitual bipedalism (Morton, 1935). the specific locomotor adaptations of apes are reflected However, the form–function relationship of the foot in their functional morphology. In primates both hands of non-human primates is undoubtedly as significant in and feet interact with the environment and are there- an evolutionary context. Arboreal primates are known fore most likely to reflect the locomotor behaviour and to have a flexible foot, with powerful grasping muscles habitat of the species (Sigmon & Farslow, 1986). However, and an opposable hallux (Morton, 1924; Tuttle, 1970, we chose to focus on the foot and ankle complex of 1972). Terrestrial primates, by contrast, and ultimately apes because the hand morphology might show some humans, possess a more robust and compact foot with locomotion-manipulation compromises (Tuttle, 1972) and lever and shock-absorbing capabilities (Jacob, 2001). because we were particularly interested in hind-limb- Establishing viable form–function relationships in the dominated locomotor modes, such as bipedalism. foot and ankle complex of extant primates is not only crucial for thorough investigation of primate locomo- tion but can also be helpful in the reconstruction of the locomotor behaviour of extinct hominoids. Correspondence Comparisons of the linear proportions of the various Dr Evie Vereecke, Laboratory for Functional Morphology, Department foot segments have repeatedly been used to investigate of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium. T.: +32 3820 22 60; F: +32 3820 22 71; the adaptation of the primate foot (Morton, 1924; E: [email protected] Schultz, 1963, 1973; Lessertisseur & Jouffroy, 1973). Accepted for publication 4 March 2005 Clearly, this is of considerable functional relevance, but © Anatomical Society of Great Britain and Ireland 2005 454 Foot myology of gibbons and bonobos, E. E. Vereecke et al. the importance of the muscles, as actuators of these foot et al. 2000). Beside this, their locomotor repertoire segments, should not be underestimated. In addition, also contains climbing (4–20%), leaping (6–20%) and bone is a dynamic structure, sensitive to mechanical bipedal walking on large branches (4–11%), all executed loading, and the observed structure might therefore at high speeds (Carpenter, 1964; Ellefson, 1967; Tuttle, rather be a reflection of activity patterns than of actual 1972; Andrew & Groves, 1976; Fleagle, 1976; Gittins, adaptations. Gross anatomical features of the musculat- 1983; Sati & Alfred, 2002). They live in the middle to ure, such as the distribution, origin and insertion, and upper levels of the forest canopy and rarely come to the the presence or absence of muscles, are more con- ground (Carpenter, 1964; Tuttle, 1972). Observations servative than bony structure and might thus better of terrestrially walking gibbons are infrequent in the reflect the evolutionary pathway and adaptations of wild and occur predominantly when crossing gaps and the species (Gibbs et al. 2002). roads in fragmented forested regions (Sati & Alfred, 2002; Unfortunately, previous papers investigating the foot B. Rawson and G. Thampy, personal communication). and ankle myology of non-human apes are very scarce. Thus, the gibbon is characterized by a fast arboreal Bisschoff (1870) and Kohlbrügge (1890/91) provide a locomotion. This combination of swift movements gross anatomical description of the gibbon, and Wilder and a complex three-dimensional environment requires (1863), Miller (1952), Sokoloff (1972) and Swindler & highly mobile and flexed limbs (Schmitt, 1999). When a Wood (1973) give information on the gross anatomical gibbon swings at high speed through the forest, it must musculature of bonobos and chimpanzees. A more have the ability to grasp a branch in almost every orienta- detailed description of the hip and thigh musculature tion and it must also be capable of quickly changing is given by Sigmon (1974, 1975), and Tuttle (1970, 1972) direction and speed. Obviously, arm-swinging is a provides a functional analysis of the hand and foot forelimb-dominated locomotion type and the hind morphology of non-human apes. Other researchers limbs are mostly kept flexed at hip and knee (Jungers have used EMG to investigate the recruitment of the & Stern, 1976). Nevertheless, mobility of the hind limb hind limb muscles during gait (e.g. Tuttle et al. 1978; and foot are also crucial, as these swinging phases Stern & Susman, 1981; Shapiro & Jungers, 1988, 1994). are alternated with short and fast bipedal bouts on More recently, Thorpe et al. (1999) and Payne (2001) large branches, with jumps, and with quadrumanous have made detailed studies that provide quantitative climbing (Tuttle, 1972). In view of their important data on the fore- and hind limb musculature of all ape prehensile function, flexibility of both hands and feet species. These are all very valuable studies but to date is essential in gibbon locomotion. the only information on the foot and ankle myology of In bonobo locomotion, by contrast, different features primates is provided by Langdon (1990). In this work he can be premised. Although bonobos are larger and heavier combines observations from original dissections (n = 67) than gibbons, they are also gracile and arboreal apes. and from the literature to investigate the variation in They most commonly travel using arboreal quadrupe- cruropedal musculature throughout different primate dalism, quadrumanous climbing and scrambling taxa (14 families), including the apes. Although this is (Susman et al. 1985; Doran, 1993), often performed at an extensive and very comprehensible work, a detailed a slow deliberate pace. Faster locomotion types, such as functional description of the hominoid foot and ankle diving, leaping and arm-swinging, are observed in myology is still warranted. agitated or fleeing animals but fast ricochetal brachia- We chose to study gibbons (Hylobates sp.), bonobos tion as seen in gibbons is absent (Susman et al. 1985; (Pan paniscus) and compare them with modern humans Doran, 1993). In contrast with the fully arboreal gibbons, because their locomotor anatomy and behaviour are bonobos regularly come to the ground and travel strikingly different. In addition, gibbons, bonobos and terrestrially (Susman et al. 1985; Doran, 1993; Doran & humans all belong to the same superfamily Hominoidea Hunt, 1994). They most often do so using quadrupedal (Goodman et al. 1994; Gibbs et al. 2002) and all three knuckle-walking, supporting 40% of their body weight species can and do walk bipedally despite their markedly on the knuckles of the forelimbs and 60% on the hind different morphology (Carpenter, 1964; Susman et al. limbs (Reynolds, 1985; Susman et al. 1985; Doran, 1993). 1980). Beside this, bipedalism and tripedalism are also occasion- Gibbons are lightly built apes, specialized for very ally used during terrestrial travel (Susman et al. 1985; fast ricochetal arm-swinging or brachiation (Chang Kano, 1992; Doran, 1993). A robust and compact foot © Anatomical Society of Great Britain and Ireland 2005 Foot myology of gibbons and bonobos, E. E. Vereecke et al. 455 is most suitable for terrestrial walking, in order to sup- which is the same individual as in the study of Payne port high compressive stresses and to generate large (2001), and a female. The male died of a heart attack, propulsive forces (Morton, 1935). However, high foot the female from