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Forelimb Proportions and Kinematics: How Are Small Primates Different from Other Small Mammals?

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Forelimb Proportions and Kinematics: How Are Small Primates Different from Other Small Mammals? 3775 The Journal of Experimental Biology 211, 3775-3789 Published by The Company of Biologists 2008 doi:10.1242/jeb.019802 Forelimb proportions and kinematics: how are small primates different from other small mammals? Manuela Schmidt Institut für Spezielle Zoologie und Evolutionsbiologie, Friedrich Schiller Universität Jena, Erbertstrasse 1, D-07743 Jena, Germany e-mail: [email protected] Accepted 25 September 2008 SUMMARY The crouched limb posture of small mammals enables them to react to unexpected irregularities in the support. Small arboreal primates would benefit from these kinematics in their arboreal habitat but it has been demonstrated that primates display certain differences in forelimb kinematics to other mammals. The objective of this paper is to find out whether these changes in forelimb kinematics are related to changes in body size and limb proportions. As primates descended from small ancestors, a comparison between living small primates and other small mammals makes it possible to determine the polarity of character transformations for kinematic and morphometric features proposed to be unique to primates. Walking kinematics of mouse lemurs, brown lemurs, cotton-top tamarins and squirrel monkeys was investigated using cineradiography. Morphometry was conducted on a sample of 110 mammals comprising of primates, marsupials, rodents and carnivores. It has been shown that forelimb kinematics change with increasing body size in such a way that limb protraction increases but retraction decreases. Total forelimb excursion, therefore, is almost independent of body size. Kinematic changes are linked to changes in forelimb proportions towards greater asymmetry between scapula and radius. Due to the spatial restriction inherent in the diagonal footfall sequence of primates, forelimb excursion is influenced by the excursion of the elongated hind limb. Hindlimb geometry, however, is highly conserved, as has been previously shown. The initial changes in forelimb kinematics might, therefore, be explained as solutions to a constraint rather than as adaptations to the particular demands of arboreal locomotion. Key words: joint kinematics, angular excursion, intralimb proportions, limb length scaling, Microcebus murinus, Eulemur fulvus, Saguinus oedipus, Saimiri sciureus. INTRODUCTION Tomita, 1967; Shapiro and Raichlen, 2005; Wallace and Demes, A support that has a small diameter relative to the size of an animal 2007), it is superior to the lateral footfall pattern in terms of the places particular challenges on the locomotor performance and dynamic stability of locomotion (control and transfer of moments morphology of the musculoskeletal system. A small-diameter imposed on the body axes). As the diagonally paired fore- and support is inherently unstable; twigs and branches may swing, yield hindlimb make contact with the support concurrently, a dynamic or even break. An animal travelling on such a support has two major weight shift from side to side (=balance) is possible at any moment concerns – balance and compliance. Balance prevents the animal of a stride cycle. At the same time, the other fore- and hindlimbs from falling down. Compliance reduces the branch oscillations, swing forward synchronously, thus counterbalancing the momentum which would otherwise disturb cyclic locomotor performance and on the transverse body axis. Compliance is basically provided by a increase the energy costs of motion enormously. The distinctive crouched limb posture which extant arboreal primates certainly characteristics of primate locomotion – powerful pedal grasping, inherit from their non-primate ancestors. hind limb dominance and diagonal sequence of footfalls (Martin, Along with the locomotion-related primate features listed by 1968; Martin, 1986) – have been interpreted as adaptive solutions Martin (Martin, 1968; Martin, 1986), various relative characters have to locomotion on terminal branches smaller in diameter than the been proposed to be unique to primates: larger limb excursion, animal (Cartmill, 1972; Rose, 1973; Cartmill, 1974; Sussman, 1991; greater step length, lower step frequency and longer limbs Cartmill et al., 2002). Powerful prehensile feet enable primates to (Alexander et al., 1979; Alexander and Maloiy, 1984; Reynolds, influence their substrate reaction forces via simultaneously 1987; Larson et al., 2001). The adaptive advantage of these features transferred substrate reaction moments (Preuschoft, 2002; Witte et for locomotion on narrow branches is discussed in numerous recent al., 2002). The counter-transfer of moments onto the trunk permits publications. For example, lower step frequency means longer a dynamic weight shift from the forelimbs to the hindlimbs similar contact time for the limbs, which significantly reduces the peak to the mechanism proposed by Reynolds (Reynolds, 1985). forces the limbs are subjected to by gravity and, thus, further Combined with a diagonal footfall pattern – hindlimb contact prior enhances the compliance of primate walking (Demes et al., 1990; to contralateral forelimb contact (Hildebrand, 1967) – this enables Schmitt, 1999). Although assessment of the polarity of these relative the hindlimbs to carry most of the body weight at the moment of characters greatly depends on sample composition, phylogenetic forelimb touchdown (Reynolds, 1985; Cartmill et al., 2002). hypotheses have often played a minor role in selecting species for Although the diagonal footfall pattern is less advantageous in terms comparison. Rather, comparative studies between ‘typical’ primates of the static stability of locomotion relating the support polygon of belonging to Cebidae, Cercopithecidae, and even Hominoidea and the limbs to the location of the centre of body mass (Gray, 1944; ‘typical’ members of the artificial taxon ‘non-primates’ (e.g. cats, THE JOURNAL OF EXPERIMENTAL BIOLOGY 3776 M. Schmidt dogs, horses) form the majority of literature in this field of research. in four species of small arboreal quadruped primates (mouse lemur, Furthermore, small sample size often weakens some of the most brown lemur, cotton-top tamarin and squirrel monkey) with regard frequently cited references. For example, the notion that primates to the kinematic principles displayed by other small mammals: the have longer limb bones and, thus, longer limbs than other mammals predominance of scapula excursion in limb protraction and (Alexander et al., 1979) is based on data from six primate species. retraction, the parallel motion of scapula and forearm and the Reynolds’ assumption (Reynolds, 1987) that primates display function of the intrinsic limb joints in providing limb compliance. greater hindlimb angular excursion is based on a sample of four As the three-segmented fore- and hindlimbs of quadruped primates (chimpanzee, gibbon, spider monkey and brown lemur). mammals are constrained to display the same pivot height and Larson (Larson, 1998) and Larson et al. (Larson et al., 2000; Larson angular excursion, intralimb proportions and the length ratio between et al., 2001) went to great lengths to test the hypothesis proposed fore- and hindlimbs play a crucial role in adjusting limb kinematics by Reynolds on the basis of a much larger sample (53 primates and to certain biomechanical demands such as postural stability and 49 ‘non-primates’ of several phylogenetic groups). Although this stress reduction. A crucial factor in the primate-specific diagonal outstanding sample could potentially have allowed the ancestral sequence gait is the relationship between limb length and body size pattern for each phylogenetic lineage to be derived, the authors because long limbs increase the risk of interference between compared the mean values of each group, making it impossible to ipsilateral fore- and hindlimbs. It can be hypothesized that the estimate character polarity. The comparative evidence relating to relationship between limb length and body size and the ratio between whether limb lengths, angular excursion and step length in primates fore- and hindlimb length act as constraints on limb geometry. are uniquely large thus needs to be surveyed critically with regard Therefore, the second part of this study examines the scaling pattern to character polarity. In an earlier study, Schmidt (Schmidt, 2005a) of forelimb length, the length ratio between forelimbs and hind limbs compared the hindlimb kinematics of small arboreal quadruped and the intralimb proportions of the forelimb. Fischer and Blickhan primates with those of other non-cursorial mammals and suggested demonstrated that the crouched forelimb posture of small mammals that the differences that occur with increasing body size result from is combined with skeletal intralimb scapula, humerus and radius the decreasing angular excursion in cursorial mammals, with larger proportions of approximately 1:1:1 (Fischer and Blickhan, 2006). primates merely retaining the primitive condition of large hindlimb A more extended limb posture requires asymmetrical proportions excursion seen in the smaller primates, tree-shrews, rodents and for self-stability (Seyfarth et al., 2001). In this morphometric part marsupials. of the paper, a broader sample of quadrupeds is considered in an Fischer and his team (Fischer et al., 2002) proposed kinematic attempt to test whether primates in general differ from other principles for the locomotion of small mammals, which is suggested mammals or whether previously suggested differences in limb bone as being adaptive to postural stability in
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