Forelimb proportions and kinematics: how are small primates different from other small mammals?

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1 3775 The Journal of Experimental Biology 211, Published by The Company of Biologists 08 doi: /jeb 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 Jena, Germany Accepted 25 September 08 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 A support that has a small diameter relative to the size of an animal places particular challenges on the locomotor performance and morphology of the musculoskeletal system. A small-diameter support is inherently unstable; twigs and branches may swing, yield or even break. An animal travelling on such a support has two major concerns balance and compliance. Balance prevents the animal from falling down. Compliance reduces the branch oscillations, which would otherwise disturb cyclic locomotor performance and increase the energy costs of motion enormously. The distinctive characteristics of primate locomotion powerful pedal grasping, hind limb dominance and diagonal sequence of footfalls (Martin, 1968; Martin, 1986) have been interpreted as adaptive solutions to locomotion on terminal branches smaller in diameter than the animal (Cartmill, 1972; Rose, 1973; Cartmill, 1974; Sussman, 1991; Cartmill et al., 02). Powerful prehensile feet enable primates to influence their substrate reaction forces via simultaneously transferred substrate reaction moments (Preuschoft, 02; Witte et al., 02). The counter-transfer of moments onto the trunk permits a dynamic weight shift from the forelimbs to the hindlimbs similar to the mechanism proposed by Reynolds (Reynolds, 1985). Combined with a diagonal footfall pattern hindlimb contact prior to contralateral forelimb contact (Hildebrand, 1967) this enables the hindlimbs to carry most of the body weight at the moment of forelimb touchdown (Reynolds, 1985; Cartmill et al., 02). Although the diagonal footfall pattern is less advantageous in terms of the static stability of locomotion relating the support polygon of the limbs to the location of the centre of body mass (Gray, 1944; Tomita, 1967; Shapiro and Raichlen, 05; Wallace and Demes, 07), it is superior to the lateral footfall pattern in terms of the dynamic stability of locomotion (control and transfer of moments imposed on the body axes). As the diagonally paired fore- and hindlimb make contact with the support concurrently, a dynamic weight shift from side to side (=balance) is possible at any moment of a stride cycle. At the same time, the other fore- and hindlimbs swing forward synchronously, thus counterbalancing the momentum on the transverse body axis. Compliance is basically provided by a crouched limb posture which extant arboreal primates certainly inherit from their non-primate ancestors. Along with the locomotion-related primate features listed by Martin (Martin, 1968; Martin, 1986), various relative characters have been proposed to be unique to primates: larger limb excursion, greater step length, lower step frequency and longer limbs (Alexander et al., 1979; Alexander and Maloiy, 1984; Reynolds, 1987; Larson et al., 01). The adaptive advantage of these features for locomotion on narrow branches is discussed in numerous recent publications. For example, lower step frequency means longer contact time for the limbs, which significantly reduces the peak forces the limbs are subjected to by gravity and, thus, further enhances the compliance of primate walking (Demes et al., 1990; Schmitt, 1999). Although assessment of the polarity of these relative characters greatly depends on sample composition, phylogenetic hypotheses have often played a minor role in selecting species for comparison. Rather, comparative studies between typical primates belonging to Cebidae, Cercopithecidae, and even Hominoidea and typical members of the artificial taxon non-primates (e.g. cats,

2 3776 M. Schmidt dogs, horses) form the majority of literature in this field of research. Furthermore, small sample size often weakens some of the most frequently cited references. For example, the notion that primates have longer limb bones and, thus, longer limbs than other mammals (Alexander et al., 1979) is based on data from six primate species. Reynolds assumption (Reynolds, 1987) that primates display greater hindlimb angular excursion is based on a sample of four primates (chimpanzee, gibbon, spider monkey and brown lemur). Larson (Larson, 1998) and Larson et al. (Larson et al., 00; Larson et al., 01) went to great lengths to test the hypothesis proposed by Reynolds on the basis of a much larger sample (53 primates and 49 non-primates of several phylogenetic groups). Although this outstanding sample could potentially have allowed the ancestral pattern for each phylogenetic lineage to be derived, the authors compared the mean values of each group, making it impossible to estimate character polarity. The comparative evidence relating to whether limb lengths, angular excursion and step length in primates are uniquely large thus needs to be surveyed critically with regard to character polarity. In an earlier study, Schmidt (Schmidt, 05a) compared the hindlimb kinematics of small arboreal quadruped primates with those of other non-cursorial mammals and suggested that the differences that occur with increasing body size result from the decreasing angular excursion in cursorial mammals, with larger primates merely retaining the primitive condition of large hindlimb excursion seen in the smaller primates, tree-shrews, rodents and marsupials. Fischer and his team (Fischer et al., 02) proposed kinematic principles for the locomotion of small mammals, which is suggested as being adaptive to postural stability in unanticipated situations within a disordered spatial arrangement of surfaces. These principles include a permanent crouched limb posture in which the most proximal element is predominant in the protraction and retraction of the limb. Intrinsic limb joints (shoulder, elbow, knee and ankle) mainly serve to provide limb compliance. It has further been suggested that some of these principles increase the self-stability of the limb and, thus, minimize neural control effort (Fischer and Blickhan, 06). These are the so-called pantograph behaviour (parallel motion of scapula and forearm and femur and tarsometatarsus, respectively) and the placement of the forelimb right below the eye. These features characterize the locomotion of small mammals regardless of their phylogeny. Their adaptive advantage for locomotion on irregular and uncertain substrates is further evident in the re-acquisition of a crouched posture in small-sized mammals that descent from larger-sized ancestors such as the hyraxes (Hyracoidea), the mouse deer (Tragulidae) or the ferrets (Mustelidae) (Jenkins, 1971; Fischer et al., 02). It, therefore, seems reasonable to assume that small arboreal primates would benefit greatly from retaining these principles but it has been demonstrated that primates display a more extended and more protracted forelimb posture at the beginning of a step cycle than other mammals (Larson, 1998; Larson et al., 00). The ultimate objective of the present study is to find out whether these changes in forelimb posture are related to changes in body size and/or to changes in the skeletal intra- and interlimb proportions. Considering the forelimb geometry of other small mammals on the one hand and the overall uniformity of hindlimb geometry in small mammals including primates on the other hand, it will be hypothesized that, in primates, changes in forelimb geometry are caused by constraints rather than by their increased adaptive value for arboreal locomotion on narrow supports. The present paper attempts to find out what kind of constraints act on forelimb geometry. The first part of the study investigates forelimb kinematics in four species of small arboreal quadruped primates (mouse lemur, brown lemur, cotton-top tamarin and squirrel monkey) with regard to the kinematic principles displayed by other small mammals: the predominance of scapula excursion in limb protraction and retraction, the parallel motion of scapula and forearm and the function of the intrinsic limb joints in providing limb compliance. As the three-segmented fore- and hindlimbs of quadruped mammals are constrained to display the same pivot height and angular excursion, intralimb proportions and the length ratio between fore- and hindlimbs play a crucial role in adjusting limb kinematics to certain biomechanical demands such as postural stability and stress reduction. A crucial factor in the primate-specific diagonal sequence gait is the relationship between limb length and body size because long limbs increase the risk of interference between ipsilateral fore- and hindlimbs. It can be hypothesized that the relationship between limb length and body size and the ratio between fore- and hindlimb length act as constraints on limb geometry. Therefore, the second part of this study examines the scaling pattern of forelimb length, the length ratio between forelimbs and hind limbs and the intralimb proportions of the forelimb. Fischer and Blickhan demonstrated that the crouched forelimb posture of small mammals is combined with skeletal intralimb scapula, humerus and radius proportions of approximately 1:1:1 (Fischer and Blickhan, 06). A more extended limb posture requires asymmetrical proportions for self-stability (Seyfarth et al., 01). In this morphometric part of the paper, a broader sample of quadrupeds is considered in an attempt to test whether primates in general differ from other mammals or whether previously suggested differences in limb bone lengths characterize only larger primates that display more derived locomotor behaviours such as terrestrial quadrupedalism. Finally, the discussion section proposes a hypothesis about the hierarchical structure of dependencies in the character evolution of primate locomotion. This section explores the way in which concurrence between initial adaptations to walking on narrow supports (prehensile hindlimbs, diagonal footfall sequence and dynamic weight shift mechanism) and subsequent adaptations to other locomotor modes constrain the limb geometry in primates. MATERIALS AND METHODS Animals Forelimb kinematics were compared in four species of arboreal quadrupedal primates: the grey mouse lemur (Cheirogaleidae: Microcebus murinus J. F. Miller 1777), the brown lemur (Lemuridae: Eulemur fulvus E. Geoffroy St Hilaire 1796), the cotton-top tamarin (Callitrichidae: Saguinus oedipus Linnaeus 1758) and the squirrel monkey (Cebidae: Saimiri sciureus Linnaeus 1758). Motion analysis was conducted on two adult individuals of each species. Their body mass, sex and age are listed in Table1. All animals were kept in accordance with German animal welfare regulations, and Table 1. Body mass, sex and age of the animals used for the kinematic analysis Individuals Body mass (g) Sex Age (years) Microcebus murinus 90 Male 2 Microcebus murinus 110 Male 3 Eulemur fulvus Male > Eulemur fulvus Female 10 Saguinus oedipus 450 Male 10 Saguinus oedipus 5 Female 17 Saimiri sciureus Male 6 Saimiri sciureus 850 Male 3

3 Forelimb kinematics and proportions in primates 3777 experiments were registered with the Committee for Animal Research of the Freistaat Thüringen, Germany. Criteria for species selection were derived from the hypotheses placing the adaptive origin of primates in a small branch milieu (Napier, 1967; Cartmill, 1972; Rose, 1973; Sussman, 1991). Accordingly, the animals needed to be small in terms of body size but had to span a significant size range in order for the influence of size variation to be studied. Animals had to use arboreal quadrupedalism as their preferred locomotor mode. The four selected species fulfil these criteria. They prefer to run and walk on horizontal and oblique branches but are also capable of leaping. Grey mouse lemurs are the smallest primates in the world. Cottontop tamarins and squirrel monkeys are small quadrupedal New World monkeys. Motion analysis Animals were habituated to walk on a raised pole or on a horizontal motor-driven rope-mill an arboreal analogue of a treadmill. The diameter of the support was adapted to the preferred natural substrate of the species (mouse lemur, 10 mm; cotton-top tamarin, 25mm; squirrel monkey, mm; brown lemur, 50mm). Data on substrate preferences were obtained from several sources (Walker, 1979; Garber, 1980; Arms et al., 02). The speed of the rope-mill was not fixed but was adjusted to obtain the animal s preferred walking velocity. The walking velocity of each species varies moderately. Isolated very slow or very fast strides were excluded from the study. To compensate for differences in body mass across the sample, velocity was converted into Froude number using the Formula Fr=v 2 /gl (Alexander and Jayes, 1983), where v is raw speed, g is gravitational acceleration and l is a characteristic length of the animal. The cube root of body mass was used here as a characteristic length variable instead of hip height or hindlimb length because geometric similarity of hindlimb geometry is not present among the four primates. Uniplanar cineradiographs were collected in lateral view at 150 frames per second. The methods of collecting and processing kinematic variables from cineradiographs have been described in detail elsewhere (Schmidt, 05b) and will be summarized only briefly here. The X-ray equipment consists of an automatic Phillips unit with one X-ray source which applies pulsed X-ray shots (Institut für den Wissenschaftlichen Film, Göttingen). The X-ray images were recorded from the image amplifier either onto 35 mm film (Arritechno R camera, Arnold & Richter Cine Technik, München, Germany) or using a high-speed CCD camera (Mikromak Camsys; Mikromak Service K. Brinkmann, Berlin, Germany). X-ray films were then analyzed frame-by-frame to identify previously defined skeletal landmarks (software Unimark by R. Voss, Tübingen, Germany) (Fig. 1A). The software Unimark calculates angles and distances based on the x and y coordinates of the landmarks, correcting the distortions of the X-ray maps automatically with reference to the x and y coordinates of a recorded grid. The complete dataset obtained for individuals of the four primate species in this study includes approximately 13,000 X-ray frames, with at least 25 steps analyzed for each species. The following kinematic variables were measured or calculated: (1) segment angles calculated relative to the horizontal plane (the term protraction is used for the cranial displacement of the distal end of each segment, retraction describes its caudal displacement) (Fig. 1B). (2) Limb joint angles defined anatomically and measured at the flexor side of each joint (Fig.1B). (3) Maximum amplitudes of joint excursions during the support phase difference between maximum extension angle and maximum flexion angle. (4) Total A B Shoulder blade Upper arm Forearm Hand Shoulder joint Elbow joint Wrist joint C Lift-off Retraction angle Total angular excursion Touchdown Protraction angle Fig. 1. Motion analysis: (A) skeletal landmarks of the forelimb exemplified on the brown lemur, (B) calculated joint and segment angles, (C) calculated excursion angles of the forelimb. angular excursions of the forelimb measured as the angle between the lines connecting the point of contact with the ground and the proximal pivot at touchdown and lift-off (Fig. 1C). The proximal pivot of the forelimb is the instantaneous centre of scapular rotation, held and guided by muscles. The pivot corresponds to the point of zero velocity and is usually marked by the intersection of the two overlaying scapular spines near the vertebral border. The forelimb pivot can thus generally be estimated to be the proximal end of the scapular spine. (5) Protraction angle and retraction angle of the forelimb total angular excursion was divided into an anterior and a posterior angle by drawing a vertical line through the point of ground contact (Fig. 1C). (6) The relationship between anatomical limb length and the shortest functional limb length distance between the proximal pivot and the point of ground contact at mid-support, which, here, is used as a kinematic key point, namely the vertical alignment of ground contact point and the proximal pivot of the limb. The term mid-support is normally defined as the instant of the peak vertical substrate reaction force, which nearly coincides with the instant at which the shoulder joint passes the wrist joint. Morphometry Skeletal specimens (N=222) of 110 mammalian species were examined at the Phylogenetisches Museum Jena, Germany, at the Museum für Naturkunde Berlin, Germany and at the Naturhistorisches Museum Bern, Switzerland. Over 50% of the sample was composed of specimens collected in the wild (N=113), nine specimens were captured wild and then kept in a zoo. The remaining specimens died in a zoo and were probably born in captivity. The adult status of the specimens was judged on the basis of the fusion of the epiphyses of the long bones. In those species for which more than one specimen was available, the largest specimen in terms of total fore- and hindlimb length was chosen. It was decided not to calculate mean values for each species because the intraspecific and interspecific allometry of limb bones can be different (e.g. Steudel, 1982). While static intraspecific allometry

4 3778 M. Schmidt Table 2. Morphometry: species (number of specimens), body mass and limb segment lengths Maximum articular Body length (mm) Specimen mass (g) Scapula Humerus Radius Primates Cheirogaleidae Cheirogaleus major Microcebus murinus (4) Microcebus myoxinus Microcebus rufus (3) 70* Lemuridae Eulemur coronatus (2) Eulemur fulvus fulvus (4) Eulemur fulvus collaris (2) Eulemur fulvus albifrons Eulemur macaco (3) * Eulemur mongoz (2) Hapalemur griseus (2) Lemur catta (3) 2680* Varecia variegata (4) Galagonidae Galago alleni (2) Galago senegalensis 193* Otolemur crassicaudatus (4) Otolemur garnetti Loridae Arctocebus aureus (2) 210* Loris tardigradus (3) Perodicticus potto (6) 10* Nycticebus coucang (2) 610* Daubentoniidae Daubentonia madagasc. (2) 2500* Callitrichidae Callimico goeldii (2) 500* Callithrix argentata (3) Callithrix geoffroyi 250* Callithrix jacchus (4) Cebuella pygmaea (2) 1* Leontopithecus rosalia (2) 550* Saguinus fuscicollis Saguinus imperator (2) Saguinus labiatus Saguinus midas (2) Saguinus oedipus (5) Cebidae Aotus nigriceps (2) Aotus trivirgatus 800* Cacajao calvus (2) Cacajao melanocephalus (3) 00* Callicebus moloch (3) 800* Cebus albifrons Cebus apella (4) Cebus capucinus 10* Chiropotes satanas 00* Pithecia irrorata (4) 2500* Pithecia monachus 1500* Pithecia pithecia 1000* Saimiri sciureus (3) 800* Primates Cercopithecidae Cercopithecus cephus 2900* Cercopithecus diana (2) 5000* Cercopithecus hamlyni 3680* Cercopithecus mona Chlorocebus aethiops (4) Erythrocebus patas (3) Lophocebus albigena (2) 7000* Macaca fascicularis Macaca mulatta (3) 9000* Maximum articular Body length (mm) Specimen mass (g) Scapula Humerus Radius Primates Cercopithecidae Macaca nemestrina 14500* Macaca nigra 4500* Macaca sylvanus (3) Miopithecus talapoin 8* Papio hamadryas (5) Theropithecus gelada (2) Colobus badius (2) Colobus guereza Colobus pennantii 7000* Colobus polykomos 9000* Nasalis larvatus (4) Presbytis melalophos (2) Pygathrix nemaeus (4) 8000* Trachypithecus obscurus 6000* Scandentia Tupaia glis (2) Tupaia glis belangeri (2) Tupaia minor Tupaia tana Marsupialia Chironectes minimus 0* Dasyuroides byrnei Didelphis marsupialis 1500* Didelphis virginiana (2) Isoodon obesulus 600* Marmosa robinsoni (2) Monodelphis domestica Philander opossum 800* Caluromys philander 0* Spilocuscus maculatus (2) Trichosurus vulpecula (3) 3500* Carnivora Nasua nasua 6000* Potos flavus (3) 00* Procyon lotor Felis nigripes (2) 2500* Felis geoffroyi 2500* Felis planiceps 2500* Felis sylvestris (2) Mustela putorius (4) Martes martes (2) Genetta genetta (2) Genetta tigrina Paradoxurus hermaphrod. 3500* Viverricula indica 2500* Rodentia Atlantoxerus getulus Callosciurus prevosti (2) Callosciurus notatus Cynomys ludovicianus 900* Ratufa indica 1500* Sciurus carolinensis Sciurus vulgaris (3) 0* Spermophilus citellus 0* Spermophilus lateralis (2) Tamias sibiricus Glis glis (2) Muscardinus avellanarius Acomys minous Mus musculus Rattus norvegicus (3) Apodemus flavicollis Lemmus lemmus *The asterisk denotes that body weight is compiled from one of the following sources: Grzimek, 1987; Rowe, 1996; Garbutt, 1999; Nowak, 1999.

5 Forelimb kinematics and proportions in primates 3779 between different sized adults of a species is determined by ontogenetic development (Wayne, 1986; Lammers and German, 02; Schilling and Petrovitch, 05), interspecific allometry reflects size-related mechanical adaptations. Accordingly, the limb proportions of different sized conspecifics do not scale isometrically and can be very different. The taxa included and the sample representing each taxon can be seen in Table2, along with the corresponding body mass values and the measured lengths of scapula, humerus and radius. Those specimens labelled with an asterisk denote specimens for which body masses were compiled from the literature. The available head trunk length in those specimens was used to decide whether the mean or the maximum body mass values were more appropriate in estimating the unknown mass (Grzimek, 1987; Rowe, 1996; Garbutt, 1999; Nowak, 1999). All other body mass values relate to the skeletal specimens. The majority of taxa included in the primate sample consist of arboreal quadrupedal primates. The members of the Cheirogaleidae, Lemuridae, Callitrichidae and Cebidae prefer to walk and run quadrupedally along narrow branches but also use other modes of progression such as climbing and leaping. However, none of these taxa exhibits distinct specializations for leaping (e.g. extremely elongated hind limbs) (Rowe, 1996; Fleagle, 1999). Included Galagonidae are mostly such species that prefer to walk and run quadrupedally but do not show the morphological specializations of vertical clingers and leapers with the exception of the Northern lesser bush baby. Loridae walk and climb with large limb excursions but none of these primates has been observed to leap (Walker, 1979; Demes et al., 1990; Schmitt and Lemelin, 04). Quadruped climbing is along with walking and leaping a preferred mode of locomotion in Colobinae (Napier, 1963; Morbeck, 1979; Isler and Grüter, 05). Cercopithecine Old World monkeys (baboons, macaques, patas monkeys, guenons) are primarily adapted to semiterrestrial and terrestrial quadrupedalism (Napier, 1967; Rollinson and Martin, 1981; McCrossin et al., 1998). Still, most guenons and some macaques have returned to arboreality. Re-adaptations to arboreality in guenons have been observed to affect the morphology of the autopodia more than that of proximal limb elements (Meldrum, 1991; Schmitt and Larson, 1995). The marsupial, rodent and carnivore samples mostly include small arboreal and terrestrial species. The majority of these mammals tend to move in a roughly similar fashion characterized by a crouched limb posture (Jenkins, 1971; Fischer et al., 02). Cursorial specializations were attributed to the larger carnivores (racoon, cats and viverrids) (Jenkins and Camazine, 1977; Nowak, 1999). In order to evaluate the proportions of a three-segmented limb structure, intralimb proportions in this study are expressed as the percentage each segment length represents of the sum of the lengths of the segments. The hand is omitted due to its negligible quantitative contribution to forelimb protraction and retraction in palmigrade mammals (Fischer et al., 02; Schmidt, 05b). Only a few species in the sample use their hands in a digitigrade posture (some terrestrial cercopithecines and some carnivores) but for comparative reasons their hand proportions were not considered in this study. Interlimb ratio is calculated for the three-segmented limbs using the following formula: scapula+humerus+radius/femur+tibia+ tarsometatarsus in percent. The morphometric data of the hindlimb for this sample were taken from a previous publication (Schmidt, 05a). Original data on hindlimb length for the new specimens in the sample (Loridae, Galagonidae, Daubentoniidae and Colobinae) can be provided on request. A one-way fixed-factor analysis of variance (ANOVA) was used to determine the degree of variance of forelimb proportions. Comparison took place on the lowest taxonomic level of families, among primates at least. The lower sample size of tree-shrews, marsupials, rodents and carnivores made a further subdivision into families less appropriate. Because sample sizes are unequal across the taxa, the GT2 method was employed (Hochberg, 1974; Sokal and Rohlf, 1995) to compare group means and to calculate lower and upper comparison limits for each sample mean. Means are significantly different if their comparison intervals do not overlap (Hochberg, 1974; Sokal and Rohlf, 1995). The comparison interval is different from the confidence interval because its computation uses the critical values of the studentized maximum modulus distribution for the comparison of multiple means instead of the Student s t-distribution used to calculate confidence intervals. It was investigated whether the allometric scaling of the relative segment lengths is a significant source of their variation. Relative segment lengths and body mass values were log-transformed (ln) to produce log shape variables. Bivariate regressions were derived using the reduced major axis (RMA) line-fitting technique. The coefficient of determination r 2 was calculated in order to estimate the portion of variation in relative segment length that can be explained by the variation of body mass (Sokal and Rohlf, 1995). RESULTS The first part of this section describes forelimb kinematics in four small arboreal quadruped primates (the grey mouse lemur, the brown lemur, the cotton-top tamarin and the squirrel monkey) with regard to the kinematic principles displayed by other small mammals: the predominance of scapula excursion in limb protraction and retraction, the parallel motion of scapula and forearm and the function of the intrinsic limb joints in providing limb compliance. The body mass of the animals ranges from 100 to 00g, thus allowing some conclusions to be drawn regarding the influence of size on forelimb kinematics. Walking speed varies considerably in all four species (grey mouse lemur, ms 1 ; brown lemur, m s 1 ; cotton-top tamarin, m s 1 ; squirrel monkey, ms 1 ) but its influence of limb kinematics is lower than one might expect. Like various other small non-cursorial mammals (Fischer et al., 02; Schilling and Fischer, 1999), the primates modify their walking speed mainly by changing temporal gait parameters. Contact phase duration decreases with increasing speed and thereby step frequency increases. Spatial gait parameters like step length and body progression during the contact phase were not or only to a minor degree modified to increase velocity. Only the cotton-top tamarin increases its walking speed by increasing both step frequency and step length. Fig.2 shows the variation of step duration and step length over the range of dimensionless speed. The majority of steps of the grey mouse lemur, the brown lemur and the tamarin overlap with respect to Froude numbers but the squirrel monkeys moved somewhat slower. At Froude numbers equal to those of the other primates, squirrel monkeys preferred to run. Although kinematic parameters vary considerably in all species, on average less than % of this variation results from variation in speed. Speed-dependence is, therefore, considered in the description only for those kinematic parameters that consistently and to a higher percentage change with increasing walking velocity. r 2 values are given to characterize the strength of the relationship between walking speed and the respective parameter. The second part of this section focuses on intra- and interlimb proportions in primates and other mammals. In the primate-specific diagonal footfall sequence, the relationship between limb length and body size and the ratio between fore- and hindlimb length can act as constraints on limb kinematics. Therefore, the scaling pattern of

6 3780 M. Schmidt Step duration (s) Step length (m) A Microcebus murinus r 2 =0.674 Saguinus oedipus r 2 =0.603 B Froude number Saimiri sciureus r 2 =0.860 Eulemur fulvus r 2 = Microcebus murinus r 2 =0.175 Saguinus oedipus r 2 =0.481 Froude number Saimiri sciureus r 2 =0.216 Eulemur fulvus r 2 =0.180 Fig. 2. Influence of walking velocity on step duration (A) and step length (B). Walking velocity is transformed to the dimensionless Froude number. forelimb length, the length ratio between fore- and hindlimbs and the intralimb proportions of the forelimb are examined. In this morphometric section of the paper, a broader sample of quadrupeds is considered in an attempt to test whether primates in general differ from other mammals. Forelimb kinematics in grey mouse lemurs, brown lemurs, cotton-top tamarins and squirrel monkeys Angular excursion of the forelimb The proximal pivot of the forelimb is the instantaneous centre of scapular rotation, held and guided by muscles. This point is on the same height level as the ipsilateral hip joint providing fore- and hindlimbs the same functional length. Lemurs, however, seem to have unequal functional limb lengths, judging by the strong downward incline of their trunks when they walk, meaning that their proximal scapular border is lower than their hip joint. The scapula excursions of the brown lemur are very large and the two spines hardly overlap at touchdown or lift-off indicating that the point of zero velocity is situated outside the body (Fig.3). The measured angle of total forelimb excursion in the brown lemur (86±3 deg.), therefore, is not only significantly larger than that of the other primates but is also larger than its hindlimb excursion angle (74deg.) (Schmidt, 05a). With the exception of the brown lemur, total forelimb excursion is fairly similar among the primates (Table 3). Angular excursion hardly changes with increasing speed. Variations in step length are often accompanied by variations of limb stiffness and functional limb length. Therefore, angular excursion does not necessarily increase with increasing step length. By drawing a vertical line through the point of ground contact, the total angular excursion of the forelimb can be split into a retraction angle and a protraction angle. The protraction angle of the forelimb is always larger than the retraction angle. The retraction angle is fairly constant in the grey mouse lemur, the cotton-top tamarin and the squirrel monkey but larger in the brown lemur (Table 3). Accordingly, protraction is more variable. The forelimb of the brown lemur is the most protracted; the forelimb of the squirrel monkey is the least protracted. Obviously, body size has no significant effect on angular excursion in the three smaller primates but the brown lemur exhibits a higher degree of forelimb protraction. Kinematics of limb segments As previously shown for the hindlimbs in these species (Schmidt, 05a), highly uniform limb excursion can be the result of quite different segment and joint kinematics. This is also the case for the forelimb. Fig. 4 shows the typical excursion of scapula, humerus, radius and hand during the support phase of a step cycle. Table4 lists mean values, standard deviations and the overall range of the touchdown and lift-off angles, as well as the amplitude of excursion during the stance phase. The movement of the scapula is the most similar factor among the species (Fig. 4). Mean angles at touchdown and lift-off and the mean amplitude of scapula retraction hardly differ among the species (Table 4). Scapula retraction starts at an angle of approximately 45 deg., continues more or less regularly throughout the support phase and ends at an angle of approximately 90deg. No yield has been observed in the scapulo thoracic joint, and in this respect the most proximal forelimb joint is comparable with the hip joint of the hindlimb. Humeral excursion differs much more among the species and in such a way that body size seems to influence the degree of humeral protraction. The brown lemur exhibits the greatest Fig. 3. Scapula position of the brown lemur at touchdown (A) and lift-off (B).

7 Forelimb kinematics and proportions in primates 3781 humeral protraction and the largest amplitude of humeral excursion. The lowest mean touchdown angle was measured in the grey mouse lemur at less than 90 deg. Cotton-top tamarins and squirrel monkeys protract their humeri to a similarly larger degree. The average touchdown angle is approximately 100 deg. but increases with increasing walking velocity in both species (Saguinus, r 2 =0.245; Saimiri, r 2 =0.516). It should be noted, however, that despite the lower degree of humeral protraction, the forelimb in the mouse lemur exhibits the same degree of protraction as in the cotton-top tamarin and the squirrel monkey. In all four species, the angular velocity of humeral protraction is higher in the first half of the support phase and slows down to near zero during the last 10% of the Table 3. Protraction angle, retraction angle and total excursion of the forelimb Protraction angle (deg.) Retraction angle (deg.) Total excursion (deg.) N Means±s.d. Range Means±s.d. Range Means±s.d. Range Microcebus murinus 25 42± ± ± Eulemur fulvus 47± ± ± Saguinus oedipus 43± ± ± Saimiri sciureus 22 38± ± ± stance phase when the humerus reaches a more or less horizontal position. This positioning is influenced by walking speed in the brown lemur (r 2 =0.241). At higher speeds, the distal end of the humerus is raised upon the level of the shoulder. It might be affected by the overall slower walking speed that the humerus of the squirrel monkey is markedly less retracted and seldom, if ever, reaches a horizontal position Shoulder blade Fig. 4. Angular excursion of forelimb segments and forelimb joints during the support phase of the limb. 80 Microcebus murinus Eulemur fulvus 50 Saguinus oedipus Saimiri sciureus Upper arm Shoulder joint Angle (deg.) Forearm Elbow joint Hand Stance duration (%) Wrist joint

8 3782 M. Schmidt Table 4. Forelimb segments: angles at touchdown and lift-off, and the amplitude of excursion Touchdown angle (deg.) Lift-off angle (deg.) Amplitude (deg.) Means±s.d. (N) Range Means±s.d. (N) Range Means±s.d. (N) Range Shoulder blade Microcebus murinus 41±7 (76) ±6 (92) ±6 (76) Eulemur fulvus 46±6 (60) ±9 (60) ±9 (60) 69 Saguinus oedipus 42±3 (46) ±5 (52) ±6 (25) Saimiri sciureus 43±5 (60) ±6 (60) ±8 (60) Upper arm Microcebus murinus 78±9 (76) ±8 (92) ±8 (76) Eulemur fulvus 125±9 (60) ±9 (60) ±9 (60) Saguinus oedipus 102±8 (47) ±5 (57) ±8 (31) Saimiri sciureus 100±7 (73) ±6 (73) ±9 (73) Forearm Microcebus murinus 11±9 (72) ±6 (84) ±8 (72) Eulemur fulvus 24±4 (35) ±9 (35) ±9 (35) Saguinus oedipus 21±5 (46) ±9 (50) ±9 (36) Saimiri sciureus 36±3 (65) ±4 (65) ±5 (65) Hand Microcebus murinus 10±8 (59) ±9 (54) ±9 (38) Eulemur fulvus 16±7 () ±9 () ±9 () Saguinus oedipus 16±5 (26) ±9 (33) ±9 (24) Saimiri sciureus 13±7 (45) ±9 (45) ±9 (45) Throughout most of the support phase, the forearm moves exactly in parallel to the scapula (Fig. 4). This matched-motion pattern of the first and the third segment is said to be typical of a three-segmented leg and can also be seen in the hindlimb between thigh and foot (Fischer and Witte, 1998; Fischer et al., 02). The matched-motion pattern is only broken at the beginning and end of the support phase, when forearm excursions exceed scapula excursions. The variability of forearm excursion among the four species does not appear to be related to size. In the cotton-top tamarin, the degree of forearm retraction is influenced by speed (r 2 =0.274) in such a way that step length increases by an increasing lift-off angle of the forearm. While the upper arm and forearm undergo large angular excursions and the scapula dominates limb retraction due to its high pivot, the hand plays a minor role in forelimb excursion. All four species place their hands in a palmigrade posture. The touchdown angle deviates from zero only because of the thickness of the palmar patches. Carpus and metacarpus are lifted from the support during the second half of the stance phase. The angles at lift-off vary widely in each species but their mean values are similar (Table4). Kinematics of forelimb joints Almost all quadrupedal mammals flex their limbs to a certain degree during the support phase. This means that the anatomical limb length, i.e. the sum of the lengths of limb segments, does not correspond to the functional limb length, i.e. the distance between the point of ground contact and the proximal pivot of a limb. The ratio between functional limb length and anatomical limb length expresses the degree of overall limb flexion and normally varies throughout the support phase. Functional limb length is at its minimum when the hand passes under the scapula pivot. Several authors term this posture mid-stance or mid-support regardless of its timing relative to stance duration because it marks the transition from the braking phase to the propulsive phase in limb retraction. Table 5 gives the mean angles at mid-stance of the limb joint illustrating how each joint contributes to overall limb flexion and, thus, to the compliance of the limb. Of the four primates in this study, the forelimb of the grey mouse lemur is the most flexed throughout the support phase. At touchdown, it forms 82% of the anatomical limb length and at lift-off, 74%. In the most flexed posture, functional forelimb length is 62% of the anatomical length. The most extended forelimbs are exhibited by the squirrel monkey (ratio: touchdown 96%, lift-off 91%). Although the ratio between functional and anatomical limb length is very similar to that in the brown lemur (touchdown 95%, lift-off 94%), limb flexion at mid-stance is less pronounced in the squirrel monkey (77%) than in the brown lemur (73%). Generally, the forelimb is most extended at the beginning of the step cycle. The grey mouse lemur and the brown lemur significantly decrease limb compliance with increasing speed. In the grey mouse lemur, the amount of shoulder flexion (r 2 =0.338) and elbow flexion (r 2 =0.327) during the contact phase decreases. Limb compliance in the brown lemur is reduced due to a decrease of elbow flexion (r 2 =0.429). Fig. 4 depicts the joint excursions for the shoulder, elbow and wrist joint of the four species during the support phase of a step cycle. Maximum shoulder joint extension occurs at the beginning of the cycle. The shoulder joint is almost fully extended in the brown lemur but only moderately extended in the grey mouse lemur. A significant yield followed by a re-extension phase was observed only in the squirrel monkey. In the other three species, shoulder flexion lasts until mid-stance, from whence on the shoulder joint seems to be frozen at a constant angle and the humerus is further displaced only by scapular retraction. The flexion and re-extension pattern of the elbow joint reveals the prominent role it plays in yielding (Fig.4; Table5). With the exception of in the grey mouse lemur, the elbow joint is at its most extended at touchdown. Maximum flexion occurs at mid-stance. While the hand is resting on the support, the wrist joint extends continuously (dorsiflexion) as a result of the retraction of the forearm during the first half of the stance phase. Maximum extension occurs when the hand passes under the elbow joint. Then, the hand is subsequently lifted from the ground by the flexion of the wrist. This motion can be fairly rapid, as observed in the grey mouse lemur and the squirrel monkey.

9 Forelimb kinematics and proportions in primates 3783 Table 5. Forelimb joints: angles at touchdown and lift-off, the amplitude of excursion and angle at mid-stance Touchdown angle (deg.) Lift-off angle (deg.) Amplitude (deg.) Mid-stance (deg.) Means±s.d. (N) Means±s.d. (N) Means±s.d. (N) Means±s.d. (N) Range Range Range Range Shoulder joint Microcebus murinus 1±9 (76) 82±6 (92) 49±8 (75) 80±8 (75) Eulemur fulvus 165±9 (60) 89±9 (60) 84±9 (60) 82±9 (60) Saguinus oedipus 144±8 (44) 94±7 (51) 51±8 (27) 98±7 (25) Saimiri sciureus 1±7 (73) 106±6 (92) 39±9 (73) 105±9 (73) Elbow joint Microcebus murinus 85±9 (74) 101±9 (89) ±8 (74) 63±6 (74) Eulemur fulvus 153±9 (35) 126±9 (35) 60±7 (35) 76±9 (35) Saguinus oedipus 122±8 (42) 106±9 (46) 36±6 (35) 86±7 (25) Saimiri sciureus 135±4 (65) 132±3 (65) ±6 (65) 104±7 (65) Wrist joint Microcebus murinus 187±7 (63) 215±9 (89) 76±9 (63) 225±9 (74) Eulemur fulvus 195±6 () 223±9 () 61±8 () 228±8 () Saguinus oedipus 186±4 (26) 2±9 (33) 43±9 (25) 211±9 (25) Saimiri sciureus 194±3 (45) 212±6 (45) 50±9 (45) 219±9 () Forelimb length and limb proportions in quadrupedal primates and other mammals The evaluation of limb proportions focuses on the basic difference between primates and other mammals of small body size. Greater effort was, therefore, made to obtain large samples of small-sized taxa, in order to permit comparison between those animals thought to be closest to the presumed ancestral morphometric pattern of each phylogenetic lineage. Biewener emphasizes that scaling analyses in a large and phylogenetically diverse sample are often marred by the fact that body size-related effects cannot accurately be distinguished from phylogenetic signals and other functional determinants of skeletal form (evolutionary ancestry, life style, locomotor behaviour) (Biewener, 05). Therefore, morphometry in this study focuses on comparison at the family level within primates. Differences in forelimb length and proportion can be expected to reflect size-related effects much more accurately on this lower taxonomic level due to the greater similarity of locomotor behaviours. The cercopithecid Old World monkeys were divided into the two subfamilies Cercopithecinae and Colobinae because the colobus monkeys and leaf monkeys generally use more quadrupedal climbing, suspensory behaviour and leaping in progression than the macaques, baboons and guenons. The lower sample size of tree-shrews, marsupials, rodents and carnivores made a further subdivision into families less appropriate. Scaling of forelimb length to body mass Fig.5 shows the scaling pattern of forelimb length to body mass for the entire sample of quadrupedal mammals included in this study. Body mass ranges between 15 g (dormouse Muscardinus avellanarius) and 23.5 kg (baboon Papio hamadryas). Because subdivision of the primate sample into the eight families would be less illustrative, primates were subdivided into Strepsirhini, Platyrrhini and Catarrhini for graphical reasons. Allometry coefficients are shown in Table 6 along with the corresponding confidence intervals at the various taxonomic levels, which indicate that the scaling pattern strongly depends on the degrees of relationship between the taxa considered. Slopes were considered to deviate significantly from isometry if the 95% confidence interval did not include the isometric expectation (0.33). The F-value indicates that body mass influences forelimb length significantly in all groups but the Galagonidae and Colobinae. Forelimb length tends to scale with positive allometry in most primate families, tree-shrews, rodents and carnivores but only in ln Forelimb Primates: Strepsirhini Platyrrhini Scandentia Marsupialia Rodentia Catarrhini Carnivora ln Body mass Fig. 5. Scaling of forelimb length to body mass on logarithmic coordinates in primates and other groups of quadruped mammals.

10 3784 M. Schmidt Table 6. Scaling of forelimb length to body mass F-value* r 2 y-intercept±95% C.I. RMA-slope±95% C.I. N Marsupialia *** ± ± Rodentia *** ± ± Carnivora ** ± ± Scandentia * ± ± Primates 994.9*** ± ± Cheirogaleidae * ± ± Lemuridae 4.157*** ± ± Loridae * ± ± Galagonidae ± ± Callitrichidae *** ± ± Cebidae 35.0*** ± ± Cercopithecinae *** ± ± Colobinae ± ± *Significance level of F: *** P<0.001; ** P<0.01; * P<0.05. C.I., confidence interval; RMA, reduced major axis. primates does the confidence interval of the allometry coefficient fail to overlap with the isometric expectation of However, this is not the case for primate families. The y-intercept and its 95% confidence interval indicate that primates as a group do not have significantly longer forelimbs than the other mammals. Adaptive differences among primates are reflected by the huge variation in y-intercepts but the confidence intervals do widely overlap (Table6). Among the smallest species of all groups, where body mass is below 150 g, forelimb lengths are equal (Fig. 5). A clear distinction between primates and other mammals appears if body mass exceeds 0 g. The forelimbs of primates, then, are relatively longer than those of other mammals, regardless of the locomotor habitat or phylogenetic position of the latter. Interlimb proportions Almost all species considered here have shorter forelimbs than hindlimbs (Fig. 6). Interlimb ratio is calculated for the threesegmented limbs using the following formula: scapula+ humerus+radius/femur+tibia+tarsometatarsus in percent. The majority of specimens up to a body size of about 5kg have interlimb ratios below 90, except in the case of marsupials. No distinction can be made between rodents, carnivores, primates and tree-shrews. Scaling effects emerge if body mass exceeds 2.5kg but they are significant only for the Cebidae and Cercopithecinae, in which the interlimb ratio increases with increasing body size. The four Forelimb / hindlimb ratio Primates: Scandentia Strepsirhini Platyrrhini Catarrhini Marsupialia Rodentia + Carnivora ln Body mass Fig. 6. Forelimb / hindlimb ratio over body mass (ln-transformed) in primates and other groups of quadruped mammals. primates investigated in the kinematic study exhibit the following interlimb ratios: grey mouse lemur 75, brown lemur 73, cotton-top tamarin 70 and squirrel monkey 77. Intralimb proportions of the forelimb Fig.7 shows the mean values of the relative segment lengths and their associated comparison intervals at the taxonomic level of families (primates) or higher phylogenetic levels in the other mammals. Primates differ significantly from other mammals in the relative lengths of their scapula and radius, with Cheirogaleidae being intermediate. Scapula proportion in primates has reduced to approximately % of forelimb length whereas radius proportion has increased to approximately 38 %. The relative length of the middle segment, however, is fairly constant. In most cases, the comparison intervals of the humerus mutually overlap among the groups. Changes in forelimb proportion are, thus, mainly brought about by alterations in the relative lengths of the outer segments. The intermediate position of the Cheirogaleidae results from sizerelated differences between the grey mouse lemurs and the dwarf % Scapula % Humerus % Radius Marsupialia Carnivora Rodentia Scandentia Cheirogaleidae Lemuridae Loridae Galagonidae Daubentoniidae Callitrichidae Cebidae Cercopithecinae Colobinae Fig. 7. Intralimb proportions of the forelimb: Analysis of variance. Mean values of the relative segment lengths and their comparison intervals are compared across the sample. Primates are subdivided into families.