AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 86521-536 (1991) Mechanical Implications of Chimpanzee Positional Behavior KEVIN D. HUNT Department of Anthropology, Indiana University, Bloomington, Indiana 47405 KEY WORDS Arm-hanging, Vertical climbing, Brachiation, Hu- meral abduction, Suspensory behavior ABSTRACT Mechanical hypotheses concerning the function of chimpan- zee anatomical specializations are examined in light of recent positional behavior data. Arm-hanging was the only common chimpanzee positional behavior that required full abduction of the humerus, and vertical climbing was the only distinctive chimpanzee positional behavior that required forceful retraction of the humerus and flexion of the elbow. Some elements of the chimpanzee anatomy, including an abductible humerus, a broad thorax, a cone-shaped torso, and a long, narrow scapula, are hypothesized to be a coadapted functional complex that reduces muscle action and structural fatigue during arm-hanging. Large muscles that retract the humerus (latissi- mus dorsi and probably sternocostal pectoralis major and posterior deltoid) and flex the elbow (biceps brachii, probably brachialis and brachioradialis) are argued to be adaptations to vertical climbing alone. A large ulnar excursion of the manus and long, curved metacarpals and phalanges are interpreted as adaptations to gripping vertical weight-bearing structures during vertical climbing and arm-hanging.A short torso, an iliac origin of the latissimus dorsi, and large muscles for arm-raising (caudal serratus, teres minor, cranial trapezius, and probably anterior deltoid and clavicular pectoralis major) are interpreted as adaptations to both climbing and unimanual suspension. Apes share a distinctive anatomy, most motion (Keith, 1891,1899,1923;Avis, 1962). notably long forelimbs (fingers included), Muscles particularly large or distinctively mobile shoulders, distinctively wide, shal- shaped in apes were reasoned to be espe- low, short torsos, and no tail (Keith, 1891, cially important as brachiating propulsors 1899,1903,1923; Schultz, 1930,1936,1953; (Keith, 1891; Miller, 1932; Campbell, 1937; Washburn, 1950; Erikson, 1963). Myologi- Inman et al., 1944; Ashton and Oxnard, cally they are distinguished by large muscles l963,1964a, b; Erikson, 1963; Oxnard, 1963; that flex the elbow, retract the humerus, and Corruccini and Ciochon, 1976). The wide raise the arm (Ashton and Oxnard, 1963, thorax and the accompanying relocation of 1964a; Napier, 1963; Oxnard, 1963, 1967; the scapula to a more dorsal position on the Ashton et al., 1965; Tuttle, 1969b). Keith thorax hypothetically oriented the scapulo- (1891, 1899, 1903) proposed that such ana- humeral joint laterally, increasing the ex- tomical specializations in gibbons were ad- cursion of the humerus by removing the aptations to brachiation; others extended chest wall as a barrier (Miller, 1932; Avis, the hypothesis to all apes (Gregory, 1916, 1962).A long, narrow scapula’ was hypothe- 1928,1934; Morton, 1922,1926; Frey, 1923; Miller, 1932; Midlo, 1934; reviewed in Tut- tle, 1974; Andrews and Groves, 1976). ‘Mediolaterally reduced and axially elongated The distance The short lumbar region (Schultz, 1936) between the glenoid fossa and the angle of the superior vertebral and high intermembral index of apes were border is small compared to the distance from either the glenoid interpreted as aspects of an overall reduction fossa or the angle of the vertebral border to the inferior angle of the scapula. of body parts not active (or disadvantageous: - Hildebrand, 1974) during suspensory loco- Received October 8,1990; accepted June 4,1991. @ 1991 WILEY-LISS,INC. 522 K.D. HUNT sized as serving to increase the mechanical Quadrumanous climbing was offered as a advantage of trapezius and serratus anterior more universal hominoid behavior than bra- during the scapular rotation necessary for chiation (Washburn, 1968,1973;Conroy and arm-raising (Ashton and Oxnard, 1963, Fleagle, 1972; Fleagle, 1976; Cartmill and 1964a; Oxnard, 1963,1967).Reduced articu- Milton, 1977; Fleagle et al., 1981). Such a lation between the ulna and the carpus and a reassessment was more a nomenclatural neomorphic ball and socket-like joint be- clarification than an advance in understand- tween the radius and the ulna (Midlo, 1934; ing since brachiation (sensu lato) had come Lewis, 1965 et seq.) compared to Old World to encompass essentially the same behaviors monkeys (Benton, 1967; Jones, 1967; Jen- proposed for quadrumanous climbing; i.e., kins, 1973; O’Connor, 1975, 1976) were in- suspensory locomotion, suspensory posture, terpreted as adaptations to wrist rotation vertical climbing, and walking on inclined or during brachiation. small-diameter weight-bearing structure(s) Although widely accepted, many of these (WBS) (Washburn, 1968,1973; Cant, 1986). hypotheses were simplistic or incorrect. Bra- Two developments refined the climbing chiation (sensu stricto)2 was rarely observed hypothesis, changing its focus and giving it a in naturalistic studies of great apes; instead, powerful theoretical orientation. The first terrestrial knuckle-walking dominated Afri- was the functional linking of high intermem- can ape behavior (reviewed in Tuttle, 1986; bra1 indices and vertical climbing. Long Yerkes and Yerkes, 1929). The carpus and arms were hypothesized as functioning to metacarpus of chimpanzees and gorillas re- increase friction between the tree bole and flect this adaptation in being less flexible the pes, allowing apes to ascend larger WBS and more reinforced compared to orangu- than monkeys (Kortlandt, 1968, 1974; Cart- tans and gibbons (Tuttle,l965,1969a,c; Jen- mill, 1974; Jungers, 1976; Mendel, 1976; kins and Fleagle, 1975). Knuckle-walking, Stern et al., 1977; Fleagle et al., 1981; however, cannot explain most ape synapo- Jungers and Stern, 1980, 1981, 1984; morphies since it differs from the terrestrial Jungers and Susman, 1984; Sarmiento, quadrupedalism of other primates princi- 1987). The second was electromyography pally in the orientation of the wrist and (EMG) research, demonstrating that many manus, suggesting that specializations muscles particularly large in apes were more should be limited to those structures, since active during vertical climbing than during quadrupedalism is no more common among knuckle-walking or brachiation (Miller, apes than most other primates (Feldesman, 1932; Inman et al., 1944; Ashton and Ox- 1982; Tuttle, 1986; Hunt, 1991a). nard, 1963, 1964a,b; Oxnard, 1963; Fleagle, Whereas Lewis (1965 et seq.) maintained 1974, 1977; Corruccini and Ciochon, 1976; that distal displacement of the pisiform and Tuttle et al., 1972; Tuttle and Basmajian, a reduced ulnar-triquetral articulation were 1974a,b,c, 1977, 1978a,b; Stern et al., 1977; evolved to allow extensive wrist rotation Susman and Stern, 1979; Jungers and Stern, during brachiation (sensu stricto), the gib- 1980; Swindler and Wood, 1982; Larson and bon, the preeminent brachiator, was shown Stern, 1986,1987). It appeared that the more to have the least mobile wrist of all apes restricted mode, vertical climbing, as op- (Conroy and Fleagle, 1972). Furthermore, posed to the more general quadrumanous “brachiating” characters were observed in climbing, could be the behavior for which ape lorises (Cartmill and Milton, 1977), indicat- synapomorphies were evolved, since it (hy- ing that a flexible wrist may be an adapta- pothetically) necessitated shoulder mobility tion to slow (= quadrumanous) climbing (in reaching up for a new handhold), elon- rather than brachiation. Jenkins (1981) gated forelimbs (for climbing large trunks), showed that in fact the articulation between and ape muscular specializations, all “bra- the styloid process of the ulna and the trique- chiating” hallmarks (Stern et al., 1977, tral and pisiform had little to do with wrist 1980a,b; Fleagle et al., 1981). rotation, which occurs mostly in the midcar- Although it was not clearly reconcilable pal joint. with this interpretation, it remained obvious to these researchers and others that sus- pensory posture also was an important as- ‘The term brachiation (sensu stricto) means hand-over-hand pect of the ape positional behavior and must suspensory locomotion, with or without a period of free flight, as have accompanying anatomical adaptations opposed to a more liberal usage (sensu lato). Iticochetal brachia- tion is reserved for gibbon-like brachiation with a period of free (Ellefson, 1968,1974; Chivers, 1972; Andrews flight. and Groves, 1976; Fleagle, 1976, 1988; Git- CHIMPANZEE BIOMECHANICS 523 tins, 1983; Srikosamatara, 1984; Sabater Pi, tween the torso and the arm bear the body 1979; Susman et al., 1980; Sugardjito, 1982; weight they create bending forces on the ribs Hollihn, 1984; Kano and Mulavwa, 1984; and tensile stress on vertebrocostal liga- Susman, 1984; Fleagle and Kay, 1985; Sug- ments, ultimately resulting in fatigue (Bas- ardjito and van Hooff, 1986; Cant, 1987a,b; majian, 1965; MacConnaill and Basmajian, Fleagle, 1988; Hunt 1989a,b,l990,1991a,b). 1969). EMG research demonstrated that the digital Here, mechanical hypotheses explaining flexors were virtually the only active mus- chimpanzee anatomical specializations (many cles during a~-m-hanging.~This implies sig- of which are shared by all apes) are exam- nificant skeletal and ligamentous (but not ined in the context of chimpanzee positional muscular) adaptations (ibid.) to arm-hang- behavior data (Hunt, 1991b). An attempt is ing to assure that body weight is borne by made to follow the