New pedal remains of Megaladapis and their functional significance

New remains of Megaladapis from the caves within the Ankarana Range of northern Madagascar and the cave site of Ankilitelo near Toliara in southwestern Madagascar add considerably to the present sample of pedal remains for this genus. Here we describe and analyze the new pedal material and discuss the function of the Megaladapis foot in terms of positional behavior and substrate use. The northern specimens belong to the M. madagascariensis /M. grandidieri group in terms of size and morphology whereas the new southwestern fossils are assigned to M. madagascariensis. The new specimens demonstrate that the small and intermediate sized M. madagascariensis and M. grandidieri were very similar in anatomy and inferred locomotor function, findings that also support the prior suggestion that they belong to a single widespread subgenus (Megaladapis). The new fossils provide the first examples of many pedal elements and present the first opportunity to analyze the whole pedal complex from associated remains. The foot of Megaladapis is distinctive among primates in numerous features. Intrinsic proportions of the hindlimb indicate that the foot is relatively longer than that of any other primate. The first complete calcanei reveal a large and highly modified hindfoot. The calcaneus is reduced distally, indicating an emphasis on climbing over leaping or quadrupedal walking and running. Proximally, a large, medially directed calcaneal tuberosity suggests both a strong inversion component to plantarflexion and a well-developed abductor mechanism and recalls the calcaneal morphology of the larger lorisines in some respects. Talar shape is consistent with considerable tibial rotation during plantarflexion and dorsiflexion. The subtalar joint is designed to emphasize supination/pronation and medial/lateral rotation over proximodistal translation. The distal tarsals are extremely reduced in length, and they form a high transverse arch and a serial tarsus; this configuration promotes inversion/eversion at the transverse tarsal joint. The phalanges are long and moderately curved, and the hallux is very long, robust, and abducted. Pedal morphology suggests that Megaladapis (subgenus Megaladapis) was well adapted to exploit an arboreal environment.(ABSTRACT TRUNCATED)

Subfossil Indri indri from the Ankarana Massif of northern Madagascar

Subfossil specimens of Indri indri have been recovered recently from the Ankarana Massif cave system in the far north of Madagascar. Taken together with material from the central highland site of Ampasambazimba, the range of this species appears to have once included much of the northern half of the island and to have extended north and west beyond the eastern rainforest (not unlike Hapalemur simus). It is probable that forest corridors connected the subfossil localities to the current range at some time in the past. Climatic desiccation (fluctuating or long-term) and/or human degradation of the environment may have created the disjunct distributions of living and subfossil I. indri. It is also possible that I. indri once included populations or subspecies that were better adapted to dry forest, woodland, or mosaic environments, habitats very different from those occupied by their living conspecifics. Such adaptive diversity would have been similar to that of Propithecus diadema which today has subspecies in the montane forests and one (P.d. perrieri) in the dry forests of the northeast. These discoveries add new information on range extensions to the distributional database for the primates of Madagascar, and illustrate the piecemeal process of their extinctions.

Fossil Homo femur from Berg Aukas, northern Namibia

The proximal half of a hominid femur was recovered from deep within a paleokarst feature at the Berg Aukas mine, northern Namibia. The femur is fully mineralized, but it is not possible to place it in geochronological context. It has a very large head, an exceptionally thick diaphyseal cortex, and a very low collodiaphyseal angle, which serve to differentiate it from Holocene homologues. The femur is not attributable to Australopithecus, Paranthropus, or early Homo (i.e., H. habilis sensu lato). Homo erectus femora have a relatively longer and AP flatter neck, and a shaft that exhibits less pilaster than the Berg Aukas specimen. Berg Aukas also differs from early modern femora in several features, including diaphyseal cortical thickness and the degree of subtrochanteric AP flattening. The massive diaphyseal cortex of Berg Aukas finds its closest similarity within archaic H. sapiens (e.g., Castel di Guido) and H. erectus (e.g., KNM-ER 736) samples. It has more cortical bone at midshaft than any other specimen, although relative cortical thickness and the asymmetry of its cross-sectional disposition at this level are comparable with those of other Pleistocene femora. The closest morphological comparisons with Berg Aukas are in archaic (i.e., Middle Pleistocene) H. sapiens and Neandertal samples.

Quantifying phalangeal curvature: an empirical comparison of alternative methods

It has been generally assumed and theoretically argued that the curvature of finger and toe bones seen in some nonhuman primates is associated with cheiridial use in an arboreal setting. Assessment of such curvature in fossil primates has been used to infer the positional behavior of these animals. Several methods of quantifying curvature of bones have been proposed. The measure most commonly applied to phalanges is that of included angle, but this has come under some criticism. We consider various other approaches for quantifying phalangeal curvature, demonstrating that some are equivalent to use of included angle, but that one--normalized curvature moment arm (NCMA)--represents a true alternative. A comparison of NCMA to included angle, both calculated on manual and pedal proximal phalanges of humans, apes, some monkeys, and the Hadar fossils, revealed that these two different measures of curvature are highly correlated and result in very similar distributional patterns.

Kinetics of leaping primates: influence of substrate orientation and compliance

Our current knowledge about the forces leapers generate and absorb is very limited and based exclusively on rigid force platform measurements. In their natural environments, however, leapers take off and land on branches and tree trunks, and these may be compliant. We evaluated the influence of substrate properties on leaping kinetics in prosimian leapers by using a combined field and laboratory approach. Tree sway and the timing of takeoffs relative to the movements of trees were documented for animals under natural conditions in Madagascar. Field data collected on three species (Indri indri, Propithecus diadema, Propithecus verreauxi) indicate that in the majority of takeoffs, the substrate sways and the animals takeoff before the elastic rebound of the substrate. This implies that force is "wasted" to deform supports. Takeoff and landing forces were measured in an experimental setting with a compliant force pole at the Duke University Primate Center. Forces were recorded for 2 Propithecus verreauxi and 3 Hapalemur griseus. Peak takeoff forces were 9.6 (P. verreauxi) and 10.3 (H. griseus) times body weight, whereas peak landing forces were 6.7 (P. verreauxi) and 8.4 (H. griseus) times body weight. As part of the impulse generated does not translate into leaping distance but is used to deform the pole, greater effort is required to reach a given target substrate, and, consequently, takeoff forces are high. The landing forces, on the other hand, are damped by the pole/substrate yield that increases the time available for deceleration. Our results contrast with previous studies of leaping forces recorded with rigid platform measuring systems that usually report higher landing than takeoff forces. We conclude that 1) Leapers generate and are exposed to exceptionally high locomotory forces. The takeoff forces are higher than the landing forces when using compliant supports, indicating that the takeoff rather than the landing may be critical in interpreting leaping behavior and related aspects of musculoskeletal design. 2) Large-bodied vertical clingers and leapers do not usually take advantage of the elastic energy stored in substrates. Rather, force (and energy) is wasted to deform compliant supports. 3) A compliant force pole approximates the conditions faced by large-bodied vertical clingers and leapers in the wild more closely than do rigid force platforms.

A re-evaluation of subspecific variation and canine dimorphism in woolly spider monkeys (Brachyteles arachnoides)

A recent study suggests that differing populations of woolly spider monkeys exhibit a substantial degree of morphological, cytogenetic, and behavioral variation. We re-evaluate the differences between populations in the degree of canine tooth height sexual dimorphism and in the frequency of thumbs. Statistical analysis of variation in the degree of canine sexual dimorphism between these populations fails to provide strong evidence for subspecific variation: differences in the degree of canine dimorphism cannot be considered statistically significant. Differences between populations in the frequency of thumbs are, however, statistically significant. The lack of clear distinctions between populations in the degree of canine dimorphism complicates assessments of behavioral variation between these populations. We suggest that the level of geographic variation in woolly spider monkey canine dimorphism is not consistent with subspecific status.

Electromyography of back muscles during quadrupedal and bipedal walking in primates

Despite the extensive electromyographic research that has addressed limb muscle function during primate quadrupedalism, the role of the back muscles in this locomotor behavior has remained undocumented. We report here the results of an electromyographic (EMG) analysis of three intrinsic back muscles (multifidus, longissimus, and iliocostalis) in the baboon (Papio anubis), chimpanzee (Pan troglodytes), and orangutan (Pongo pygmaeus) during quadrupedal walking. The recruitment patterns of these three back muscles are compared to those reported for the same muscles during nonprimate quadrupedalism. In addition, the function of the back muscles during quadrupedalism and bipedalism in the two hominoids is compared. Results indicate that the back muscles restrict trunk movements during quadrupedalism by contracting with the touchdown of one or both feet, with more consistent activity associated with touchdown of the contralateral foot. Moreover, despite reported differences in their gait preferences and forelimb muscle EMG patterns, primates and nonprimate mammals recruit their back muscles in an essentially similar fashion during quadrupedal walking. These quadrupedal EMG patterns also resemble those reported for chimpanzees, gibbons and humans (but not orangutans) walking bipedally. The fundamental similarity in back muscle function across species and locomotor behaviors is consistent with other data pointing to conservatism in the evolution of the neural control of tetrapod limb movement, but does not preclude the suggestion (based on forelimb muscle EMG and spinal lesion studies) that some aspects of primate neural circuitry are unique.

Taxonomic affinity of the early Homo cranium from Swartkrans, South Africa

A quantitative analysis that employs randomization methods and distance statistics has been undertaken in an attempt to clarify the taxonomic affinities of the partial Homo cranium (SK 847) from Member 1 of the Swartkrans Formation. Although SK 847 has been argued to represent early H. erectus, exact randomization tests reveal that the magnitude of differences between it and two crania that have been attributed to that taxon (KNM-ER 3733 and KNM-WT 15000) is highly unlikely to be encountered in a modern human sample drawn from eastern and southern Africa. Some of the variables that differentiate SK 847 from the two early H. erectus crania (e.g., nasal breadth, frontal breadth, mastoid process size) have been considered to be relevant characters in the definition of that taxon. Just as the significant differences between SK 847 and the two early H. erectus crania make attribution of the Swartkrans specimen to that taxon unlikely, the linkage of SK 847 to KNM-ER 1813, and especially Stw 53, suggests that the Swartkrans cranium may have its closest affinity with H. habilis sensu lato. Differences from KNM-ER 1813, however, hint that the South African fossils may represent a species of early Homo that has not been sampled in the Plio-Pleistocene of eastern Africa. The similarity of SK 847 and Stw 53 may support faunal evidence which suggests that Sterkfontein Member 5 and Swartkrans Member 1 are of similar geochronological age.

Proximal femur of Australopithecus africanus from Member 4, Makapansgat, South Africa

A left proximal femur (MLD 46) from Member 4, Makapansgat, South Africa is described and analyzed. It consists of the head, neck, and a small segment of the shaft that extends to just below the lesser trochanter. The femur exhibits degenerative joint disease in the form of marginal osteophyte formation and thus its taxonomic identity has been somewhat obscured. Consideration of all like-sized mammalian femora from Makapansgat suggests that the femur is that of either a felid or hominid. Comparison of MLD 46 to femora of extent and extinct felids reveals that MLD 46 does not possess two morphological features that are characteristic of felids, namely a deep, prolonged trochanteric fossa and a high neck-shaft angle. Simple shape variables (ratios) and multivariate analyses consistently place MLD 46 with modern and fossil hominids, and most closely align it with the australopithecines. We conclude that the femur is most reasonably attributable to Australopithecus africanus, which is the only hominid yet identified from Makapansgat. Despite its pathological condition, MLD 46 is the most complete proximal femur known for A. africanus, thereby permitting further morphological comparisons with homologues of A. afarensis and Paranthropus. Marginal osteophytes of mammalian femoral heads characteristically occur in individuals of advanced age, suggesting that MLD 46 may have lived some time with the disease. Finally, MLD 46 is considerably larger than the previously described specimen, Sts 14, from Sterkfontein Member 4. There may be as great a contrast in body size in A. africanus as there is between the large and small specimens of A. afarensis.

Body size, locomotion, and long bone cross-sectional geometry in indriid primates

The geometry of the midshaft cross-sections of the femur and humerus of five indriid species was analysed. Internal (marrow cavity) and external diameters were measured on X-rays in the anteroposterior (a-p) and mediolateral (m-l) planes; cross-sectional areas, second moments of area, and section moduli were calculated using formulae for a hollow ellipse. Cortical thickness, robusticity indices (relating external diameters to the length of the bones), and a-p/m-l shape variables were also calculated. Model II regression was supplemented by analyses of correlation between size and shape. Indriids are saltatory, i.e., their locomotion is dominated by the hind limbs. Accordingly, the femur is more rigid than the humerus, and it shows a consistent difference between the a-p and m-l planes in measures related to bending strength. Cortical thickness varies considerably both within and across species. The type specimen of the new species Propithecus tattersalli is virtually indistinguishable from P. verreauxi on the basis of its long bone cross-sectional geometry. Femoral robusticity is uncorrelated with size, but humeral robusticity decreases significantly with increasing size. Femoral shape variables (a-p/m-l) are all negatively correlated with body size, indicating that m-l dimensions of the femur increase at a faster rate than do a-p dimensions. The highly loaded plane of movement seems to be more reinforced in the smaller species. Contrary to static biomechanical scaling predictions of positive allometry, all cross-sectional parameters scale relatively close to isometry. It is concluded that either changes in locomotor performance must compensate for the weight-related increase in forces and moments or that the larger-bodied animals operate appreciably closer to the limits of their safety margins.

Phylogenetic and functional affinities of Babakotia (primates), a fossil lemur from northern Madagascar

Recent paleontological expeditions to the Ankarana range of northern Madagascar have recovered the partial remains of four individuals of a newly recognized extinct lemur, Babakotia radofilai. Craniodental and postcranial material serve to identify Babakotia as a member of the palaeopropithecids (also including the extinct genera Palaeopropithecus, Archaeoindris, and Mesopropithecus). Living indrids form the sister group to this fossil clade. The postcranial anatomy indicates that Babakotia was a medium-sized (approximately 15 kg) indroid whose inferred positional behaviors were primarily slow climbing and hanging. Although it is probable that a leaping component typified the ancestral positional repertoire of all Malagasy lemurs, the mosaic nature of the locomotor skeleton of Babakotia further suggests that vertical climbing and hang-feeding rather than ricochetal leaping were primitive for indrids and palaeopropithecids and that the dramatic saltatory adaptations of the living indrids postdate the divergence of these two lineages.

EMG of serratus anterior and trapezius in the chimpanzee: scapular rotators revisited

The importance of arm-raising has been a major consideration in the functional interpretation of differences in shoulder morphology among species of nonhuman primates. Among the characters that have been associated with enhancement of the arm-raising mechanism in hominoid primates are the relative enlargement of cranial trapezius and caudal serratus anterior, as the main scapular rotators, as well as changes in scapular morphology associated with their improved leverage for scapular rotation. Yet in an EMG study of cranial trapezius and caudal serratus anterior function in the great apes, Tuttle and Basmajian (Yrbk. Phys. Anthropol. 20:491-497, 1977) found these muscles to be essentially inactive during arm-raising. Although Tuttle and Basmajian suggest that the cranial orientation of the glenoid fossa in apes has reduced the demand for scapular rotation during arm-raising, subsequent EMG studies on other primate species suggest that these muscles do play a significant role in arm motion during active locomotion. This paper presents a reexamination of muscle recruitment patterns for trapezius and caudal serratus anterior in the chimpanzee. All but the lowest parts of caudal serratus anterior were found to be highly active during arm-raising motions, justifying earlier morphological interpretations of differences in caudal serratus anterior development. The lowest digitations of this muscle, while inactive during arm-raising, displayed significant activity during suspensory postures and locomotion, presumably to control the tendency of the scapula to shift cranially relative to the rib cage. Cranial trapezius did not appear to be involved in arm-raising; instead, its recruitment was closely tied to head position.

The capitular joint of the first rib in primates: a re-evaluation of the proposed link to locomotion

Evidence exists that in Australopithecus afarensis the head of the first rib articulated with the body of the first thoracic vertebra but not with the body of the seventh cervical vertebra (Ohman, 1983, 1986). Thus, the Hadar hominid would have differed from most primates, in which both these vertebrae are involved in formation of the first costal capitular joint. Indeed, Ohman (1986) has claimed that a univertebral pattern is unique to modern and fossil hominids among primates. He offered various theories on the adaptive significance of this trait, chief among which was a link to freeing the upper limb from any role in locomotion. Believing that Ohman's statement about the distribution of the univertebral pattern in living forms was based on inadequate samples, we have compiled data on the first costal capitular joint in a wider range of primate genera. Our observations demonstrate that the univertebral pattern, rather than being unique to hominids, is common among siamangs, occurs in an occasional gibbon, and is typical of the larger indriids. Consequently, one can no longer accept any contention that the univertebral first costal capitular joint of A. afarensis implies that it did not use its upper limbs for locomotion. Rather, the formation of this joint is correlated with orthogrady and body size. We discuss a possible explanation of this correlation in terms of movement of the first rib during breathing in an orthograde primate, be it one that stands on two legs, swings by two arms, or clings to trunks using all four appendages.

Functional differentiation of long bones in lorises

The external dimensions of the limb bones and the geometry of their midshaft cross-sections were determined for Loris tardigradus and Nycticebus coucang. Relative cortical thickness, cortical area, and second moment of area were calculated and contrasted with locomotor stresses. The difference in shape-related strength of the bones between the smaller- and the larger-bodied species is more pronounced than can be expected from stresses acting during normal locomotion. The Nycticebus skeleton has a much higher safety margin overall and seems to be dimensioned for infrequent but critical stresses of high magnitude. Lorisine gaits in general are characterized by low ground reaction forces, great mobility in all joints, and a nearly equal share in propulsion and weight-bearing by the fore- and hindlimb. Accordingly, the long bones of lorises (especially those of L. tardigradus) tend to be less rigid than those of other mammalian species (including other primates), they lack a preferential plane of higher bending strength, and femur and humerus do not differ markedly in their capacity to withstand mechanical stresses. External dimensions of the humerus and femur of the two African lorisine species parallel and corroborate these results. Some more general implications for the relationships between bone shape and locomotor stresses are also discussed.

Back muscle function during bipedal walking in chimpanzee and gibbon: implications for the evolution of human locomotion

The evolution of erect posture and locomotion continues to be a major focus of interest among paleoanthropologists and functional morphologists. To date, virtually all of our knowledge about the functional role of the back muscles in the evolution of bipedalism is based on human experimental data. In order to broaden our evolutionary perspective on the vertebral region, we have undertaken an electromyographic (EMG) analysis of three deep back muscles (multifidus, longissimus thoracis, iliocostalis lumborum) in the chimpanzee (Pan troglodytes) and gibbon (Hylobates lar) during bipedal walking. The recruitment patterns of these three muscles seen in the chimpanzee closely parallel those observed in the gibbon. The activity patterns of multifidus and longissimus are more similar to each other than either is to iliocostalis. Iliocostalis recruitment is clearly related to contact by the contralateral limb during bipedal walking in both species. It is suggested that in both the chimpanzee and gibbon, multifidus controls trunk movement primarily in the sagittal plane, iliocostalis responds to and adjusts movement in the frontal plane, while longissimus contributes to both of these functions. In many respects, the activity patterns shared by the chimpanzee and gibbon are quite consistent with recent human experimental data. This suggests a basic similarity in the mechanical constraints placed on the back during bipedalism among these three hominoids. Thus, the acquisition of habitual bipedalism in humans probably involved not so much a major change in back muscle action or function, but rather an improvement in the mechanical advantages and architecture of these muscles.

Lucy's length: stature reconstruction in Australopithecus afarensis (A.L.288-1) with implications for other small-bodied hominids

New stature estimates are provided for A.L.288-1 (Australopithecus afarensis) based on (1) the relationship between femur length and stature in separate samples of human pygmies and pygmy chimpanzees and (2) model II regression alternatives to standard least-squares methods. Estimates from the two samples are very similar and converge on a value of approximately 3'6" for "Lucy." These results are compared to prior estimates and extended to other small-bodied hominids such as STS-14 and O.H.62. A new foot-to-stature ratio is also estimated for A.L.288-1, and its potential biomechanical significance for gait is evaluated in comparison to other groups.

Telemetered Electromyography of the Fast and Slow Extensors of the Leg of the Brown Lemur (Lemur Fulvus)

Quantitative telemetered electromyography from the four heads of the quadriceps femoris (vastus medialis, rectus femoris, vastus intermedius and vastus lateralis) during normal postures and locomotion (quadrupedal resting, walk/run, gallop and leaping) is presented for the brown lemur, Lemur fulvus. The vastus intermedius is the sole contributor to muscle recruitment during quadrupedal resting postures. It exhibits consistently high levels of electrical activity during all locomotory behaviour. The vastus medialis is recruited least among these muscles during walk/run and tends to be ‘saved’ for galloping and leaping. The rectus femoris and vastus lateralis are recruited at similarly high levels during all phasic activities. The rectus femoris appears to be used eccentrically, storing ‘elastic strain energy’ during all phasic activities. The vastus lateralis combines exceptionally high potential effective force with relatively high potential velocity and resistance to fatigue and probably develops the majority of force in all phasic activities. These results support previously documented structural and histochemical data that imply a functional division of labour among these muscle synergists.

Analysis of individual, intraspecific and interspecific variability in quantitative parameters of caprine tooth enamel structure

Qualitative and quantitative features of mammalian tooth enamel structure are increasingly being used in taxonomic and phylogenetic analyses, although the variability shown by these traits has not received adequate consideration. This study evaluates the variability displayed by nine quantitative parameters in deep, intermediate, and superficial molar enamel in the closely related bovids Ovis aries and Capra hircus. These parameters are assessed in terms of the absolute and/or relative variability evinced at a given depth within a single individual, among conspecific individuals, and between species samples. The degrees of relative variability expressed at a given depth are comparable among conspecific individuals and between taxonomic samples. Nevertheless, in many instances, there are significant differences in absolute variability amongst individuals. Also, in four parameters for which individual specimen averages could be calculated, the equality of these means among conspecifics can be rejected. Variability is not equivalent at different enamel depths. The null hypothesis of equality of individual, conspecific variances can be rejected most commonly for parameters measured in deep and superficial enamel, and coefficients of variation also tend to be higher for deep and superficial enamel than for enamel of intermediate depth. The greater variability displayed by deep and especially superficial enamel may be related to the initial onset and the terminal phase of ameloblastic secretory activity. Taxonomic and phylogenetic analyses that utilize quantitative data on enamel structure are valid only if comparisons have been made at equivalent enamel depths.

Architectural and histochemical diversity within the quadriceps femoris of the brown lemur (Lemur fulvus)

Physiologically related features of muscle morphology are considered with regard to functional adaptation for locomotor and postural behavior in the brown lemur (Lemur fulvus). Reduced physiological cross-sectional area, estimated maximum excursion of the tendon of insertion, length of tendon per muscle fasciculus, and areal fiber type composition were examined in the quadriceps femoris in order to assess the extent of a "division of labor" among four apparent synergists. Each of these four muscles in this prosimian primate displays a distinguishing constellation of morphological features that implies functional specialization during posture and normal locomotion (walk/run, galloping, leaping). Vastus medialis is best suited for rapid whole muscle recruitment and may be reserved for relatively vigorous activities such as galloping and leaping (e.g., small cross-sectional area per mass, long excursion, predominance of fast-low oxidative fibers, relatively little tendon per fasciculus). In theory, rectus femoris could be employed isometrically in order to store elastic strain energy during all phasic activities (e.g., large cross-sectional area per mass, short excursion, predominance of fast-high oxidative fibers, large amount of tendon per fasciculus). Vastus intermedius exhibits an overall morphology indicative of a typical postural muscle (e.g., substantial cross-sectional area, short excursion, predominance of slow-high oxidative fibers, large amount of tendon per fasciculus). The construction of vastus lateralis reflects an adaptation for high force, relatively high velocity, and resistance to fatigue (e.g., large cross-sectional area, long excursion, most heterogeneous distribution of fiber types, large amount of tendon per fasciculus); this muscle is probably the primary contributor to a wide range of locomotor behaviors in lemurs. Marked dramatic architectural disparity among the four bellies, coupled with relative overall fiber type heterogeneity, suggests the potential for exceptional flexibility in muscle recruitment within this mass. One interpretation of this relatively complex neuromuscular organization in the brown lemur is that it represents an adaptation for the exploitation of a three-dimensional arboreal environment by rapid quadrupedalism and leaping among irregular and spatially disordered substrates.

Variability of biological similarity criteria

The "allometric cancellation" technique for determining similarity criteria (dimensionless numbers) insures that these ratios are essentially free from size-dependent variation. However, with the traditional methods of calculating such values, other important sources of variation are not examined. A correlation analysis of residuals demonstrates that many similarity criteria are actually highly variable relationships among organisms and frequently have questionable empirical validity.

Structural and mechanical indicators of limb specialization in primates

The structural mechanics of femora and humeri from primates representing a wide spectrum of habitual locomotor activities were examined to determine how cross-sectional properties vary with functional specializations of the extremities. Average bending rigidities of the midshaft of humerus and femur were measured in 60 individuals of four nonhuman primate species (Macaca nemestrina, Macaca fascicularis, Presbytis cristata, Hylobates lar) using single-beam photon absorptiometry. Linear regression analyses of the loge transformed data were used to assess the relative usage of the forelimb and hindlimb in propulsion and weight bearing, and to evaluate deviations from generalized mammalian quadrupedalism. The results suggest that average bending rigidities of the humerus and femur in primates reflect the extent to which the forelimb and hindlimb are used differently in locomotion; deviations of average bending rigidity from geometric similarity indicate functional variations from generalized mammalian quadrupedalism and the ratio of humeral to femoral bending rigidity can be used to identify trends towards hindlimb or forelimb dominance in locomotion and can be employed in general to determine how the limb was used.

Arboreality and bipedality in the Hadar hominids

Numerous studies of the locomotor skeleton of the Hadar hominids have revealed traits indicative of both arboreal climbing/suspension and terrestrial bipedalism. These earliest known hominids must have devoted part of their activities to feeding, sleeping and/or predator avoidance in trees, while also spending time on the ground where they moved bipedally. In this paper we offer new data on phalangeal length and curvature, morphology of the tarsus and metatarsophalangeal joints, and body proportions that further strengthen the argument for arboreality in the Hadar hominids. We also provide additional evidence on limb and pedal proportions and on the functional anatomy of the hip, knee and foot, indicating that the bipedality practiced at Hadar differed from that of modern humans. Consideration of the ecology at Hadar, in conjunction with modern primate models, supports the notion of arboredality in these earliest australopithecines. We speculate that selection for terrestrial bipedality may have intensified through the Plio-Pleistocene as forests and woodland patches shrunk and the need arose to move increasingly longer distances on the ground. Only with Homo erectus might body size, culture and other factors have combined to 'release' hominids from their dependence on trees.

Function of the subclavius muscle in a nonhuman primate, the spider monkey (Ateles)

Within the primate order, the morphology of the subclavius muscle is known to be unique among the prehensile-tailed South American monkeys. 3 spider monkeys, Ateles, were monitored electromyographically to determine the recruitment of this muscle during various locomotor and postural activities. Rather than indicating a static stabilizing function, which has typically been inferred from classical anatomical studies, results from this study suggest that the subclavius performs more as a dynamic element in movements of the pectoral girdle during brachiation, vertical climbing, pronograde quadrupedalism and leaping. Complementary activity patterns were also identified between the subclavius and the caudal fibers of the trapezius indicating that the subclavius is used when the animal must depress, or resist cranial displacement, of the protracted shoulder girdle, while the caudal trapezius is recruited when the girdle is retracted on the chest wall.

The functional morphology of the accessory interosseous muscle in the gibbon hand: determination of locomotor and manipulatory compromises

The evidence for two functional roles of M. accessorius interosseus can be adduced as follows: (1) abduction of the whole finger is clearly required to resist the force of the thumb against the index during pinch grasp (Fig. 4) and when greater resistance is applied to the food, activity increases in the muscle. (2) The muscle also flexes the metacarpophalangeal joint at the onset of grasp on the ladder rung. In hanging from the finger tips or from the cage top, with the metacarpophalangeal joints extended, the muscle goes silent. From the functional point of view, the name given by Huxley (1871) to the M. accessorius interosseus ('abductor tertii internodii secundi digiti') is perhaps the most appropriate one. For reasons of economy, however, we favour continued use of the nomen Musculus accessorius interosseus (Fitzwilliams, 1910) or accessory interosseous muscle. The name coined by Keith (1894; p. 299) which implies that this muscle is an extensor of the distal interphalangeal joint, and any suggestions that the muscle functions primarily to flex the proximal interphalangeal joint are less appropriate or in error. The EMG data reveal that the M. accessorius interosseus is primarily an abductor of the index finger in gibbons, and we suggest that it is a unique feature of lesser apes that has evolved in compensation for a deep thumb-index cleft and the loss of the radial moiety of the first dorsal interosseous muscle. The primary role of this specialized muscle is in thumb-index pinch grasping.