Knuckle-walking and the evolution of hominoid hands

Knuckle-walking is a pattern of digitigrade locomotion unique to African apes among Primates. Only chimpanzees and gorillas are specially adapted for supporting weight on the dorsal aspects of middle phalanges of flexed hand digits II–V. When forced to the ground, most orangutans assume one of a variety of flexed hand postures, but they cannot knuckle-walk. Some orangutans place their hands in palmigrade postures which are impossible to African apes. The knuckle-walking hands and plantigrade feet of African apes are both morphologically and adaptively distinct from those of Pongo, their nearest relative among extant apes. These features are associated with a common adaptive shift to terrestrial locomotion and support placing chimpanzees and gorillas in the same genus Pan. It is further suggested than Pan comprises the subgenera (a) Pan, including P. troglodytes and pygmy chimpanzees, and (b) Gorilla, including mountain and lowland populations of P. gorilla. African apes probably diverged from ancestral pongids that were specially adapted for distributing their weight in terminal branches of the forest canopy. Early adjustments to terrestrial locomotion may have involved fist-walking which later evolved into knuckle-walking. Orangutans continued to adapt to feeding and locomotion in the forest canopy and their hands and feet became highly specialized for four-digit prehension. Although chimpanzees retained arboreal feeding and nesting habits, they moved from tree to tree by terrestrial routes and became less restricted in habitat. While adapting to a diet of ground plants gorillas increased in size to the point that arboreal nesting is less frequent among them than among chimpanzees and orangutans. Early hominids probably diverged from pongids that had not developed prospective adaptations to knuckle-walking, and therefore did not evolve through a knuckle-walking stage. Initial adjustments to terrestrial quadrupedal locomotion and resting stance probably included palmigrade hand posturing. Their thumbs may have been already well developed as an adaptation for grasping during arboreal climbing. A combination of selection pressures for efficient terrestrial locomotor support and for object manipulation further advanced early hominid hands toward modern human configuration.     

Australopithecines: Ancestors of the African Apes?

Since australopithecines display humanlike traits such as short ilia, relatively small front teeth and thick molar enamel, they are usually assumed to be related toHomo rather than toPan orGorilla. However, this assumption is not supported by many other of their features. This paper briefly surveys the literature concerning craniodental comparisons of australopith species with those of bonobos, common chimps, humans and gorillas, adult and immature. It will be argued, albeit on fragmentary data, that the large australopiths of East Africa were in many instances anatomically and therefore possibly also evolutionarily nearer toGorilla than toPan orHomo, and the South African australopiths nearer toPan andHomo than toGorilla. An example of a possible evolutionary tree is provided. It is suggested that the evidence concerning the relation of the different australopithecines with humans, chimpanzees and gorillas should be re-evaluated.     

Ontogeny of body size variation in African apes

Size variation in African apes (Gorilla gorilla [gorilla], Pan paniscus [pygmy chimpanzee], and Pan troglodytes [“common” chimpanzee]) is substantial, both within and between species. We investigate the possible evolutionary significance of this variation through an analysis of the ontogeny of size variation in this group. In addition, we highlight possible areas of future endocrinological research, and evaluate recently proposed alternative models that attempt to account for ontogenetic variation in apes. The present study shows that intergeneric variation in size is largely a consequence of differences among species in the rate of body weight growth. Interspecific size variation in Pan is a product of both rate and duration differences in growth. The ontogenetic bases of sexual dimorphism vary in this group. Dimorphism is largely a result of sex differences in the duration of body weight growth in gorillas and pygmy chimpanzees, but results from differences in the rate of growth in common chimpanzees. Ontogenetic analyses largely confirm earlier interpretations, but with better data and methods. The great degree of ontogenetic variation within and among these species, especially in the timing and magnitude of “pubertal” growth spurts, implies that studies of endocrine growth control in African apes could be a productive line of future research. We also suggest that ontogenetic variation can be understood with respect to ecological risks. Growth rates seem to be negatively correlated with ecological risk in African apes, suggesting links between ontogenetic patterns and social and ecological variables. High growth rates in gorillas compared to Pan are most consistent with this model. Variation between chimpanzees and pygmy chimpanzees (especially females) also seem to fit predictions of this model. © 1996 Wiley-Liss, Inc.     

Ontogeny of the hominoid scapula: The influence of locomotion on morphology

Primate shoulder morphology has been linked with locomotor habits, oftentimes irrespective of phylogenetic heritage. Among hominoids, juvenile African apes are known to climb more frequently than adults, while orangutans and gibbons maintain an arboreal lifestyle throughout ontogeny. This study examined if these ontogenetic locomotor differences carry a morphological signal, which should be evident in the scapulae of chimpanzees and gorillas but absent in taxa that do not display ontogenetic behavioral shifts. The scapular morphology of five hominoid primates and one catarrhine outgroup was examined throughout ontogeny to evaluate if scapular traits linked with arboreal activities are modified in response to ontogenetic behavioral shifts away from climbing. Specifically, the following questions were addressed: 1) which scapular characteristics distinguish taxa with different locomotor habits; and 2) do these traits show associated changes during development in taxa known to modify their behavioral patterns? Several traits characterized suspensory taxa from nonsuspensory forms, such as cranially oriented glenohumeral joints, obliquely oriented scapular spines, relatively narrow infraspinous fossae, and inferolaterally expanded subscapularis fossae. The relative shape of the dorsal scapular fossae changed in Pan, Gorilla, and also Macaca in line with predictions based on reported ontogenetic changes in locomotor behavior. These morphological changes were mostly distinct from those seen in Pongo, Hylobates, and Homo and imply a unique developmental pattern, possibly related to ontogenetic locomotor shifts. Accordingly, features that sorted taxa by locomotor habits and changed in concert with ontogenetic behavioral patterns should be particularly useful for reconstructing the locomotor habits of fossil forms. Am J Phys Anthropol 152:239–260, 2013. © 2013 Wiley Periodicals, Inc.     

Comparative and functional morphology of hominoid fingers.

Comparisons of hominoid metacarpals and phalanges reveal differences, many of which are closely linked to locomotor hand postures. The African apes display features of the metacarpals and phalanges which distinguish them from the other Hominoidea. These features are most evident in digits III and IV. The orangutan hand is demonstrably less well adapted to knuckle-walking and is distinctive in its adaptation to power and hook grasping of vertical and horizontal supports, respectively. Orangutan fingers possess a "double-locking" mechanism (Napier, '60), and a slight ulnad shift in the axis of the hand which results in lengthened phalanges of ray IV. Hylobatid apes are more like orangutans in their finger morphology than any of the other Hominoidea, but exhibit unique features of their own. These include elongate phalanges of fingers II-V. Human metacarpals II-V form two sets composed of II-III, and IV-V. The heads of both metacarpals II and III are characterized by axial torsion. This reflects the enhanced manipulatory role of the third finger in humans. Human distal phalanges are unique in the development of pronounced apical tufts. Multivariate analysis of metacarpal III and proximal III yields variables that array the extant apes along an arboreal-terrestrial axis, from hylobatid apes to male gorillas. The positions of taxa on this discriminant concur with observations on the locomotion of free-ranging apes.     

Volumetric and lateralized differences in selected brain regions of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

The two species of Pan, bonobos and common chimpanzees, have been reported to have different social organization, cognitive and linguistic abilities and motor skill, despite their close biological relationship. Here, we examined whether bonobos and chimpanzee differ in selected brain regions that may map to these different social and cognitive abilities. Eight chimpanzees and eight bonobos matched on age, sex and rearing experiences were magnetic resonance images scanned and volumetric measures were obtained for the whole brain, cerebellum, striatum, motor‐hand area, hippocampus, inferior frontal gyrus and planum temporale. Chimpanzees had significantly larger cerebellum and borderline significantly larger hippocampus and putamen, after adjusting for brain size, compared with bonobos. Bonobos showed greater leftward asymmetries in the striatum and motor‐hand area compared with chimpanzees. No significant differences in either the volume or lateralization for the so‐called language homologs were found between species. The results suggest that the two species of Pan are quite similar neurologically, though some volumetric and lateralized differences may reflect inherent differences in social organization, cognition and motor skills. Am. J. Primatol. 71:988–997, 2009. © 2009 Wiley‐Liss, Inc.     

Ontogenetic changes in limb bone structural proportions in mountain gorillas (Gorilla beringei beringei)

Behavioral studies indicate that adult mountain gorillas (Gorilla beringei) are the most terrestrial of all nonhuman hominoids, but that infant mountain gorillas are much more arboreal. Here we examine ontogenetic changes in diaphyseal strength and length of the femur, tibia, humerus, radius, and ulna in 30 Virunga mountain gorillas, including 18 immature specimens and 12 adults. Comparisons are also made with 14 adult western lowland gorillas (Gorilla gorilla gorilla), which are known to be more arboreal than adult mountain gorillas. Infant mountain gorillas have significantly stronger forelimbs relative to hind limbs than older juveniles and adults, but are nonsignificantly different from western lowland gorilla adults. The change in inter-limb strength proportions is abrupt at about two years of age, corresponding to the documented transition to committed terrestrial quadrupedalism in mountain gorillas. The one exception is the ulna, which shows a gradual increase in strength relative to the radius and other long bones during development, possibly corresponding to the gradual adoption of stereotypical fully pronated knuckle-walking in older juvenile gorillas. Inter-limb bone length proportions show a contrasting developmental pattern, with hind limb/forelimb length declining rapidly from birth to five months of age, and then showing no consistent change through adulthood. The very early change in length proportions, prior to significant independent locomotion, may be related to the need for relatively long forelimbs for climbing in a large-bodied hominoid. Virunga mountain gorilla older juveniles and adults have equal or longer forelimb relative to hind limb bones than western lowland adults. These findings indicate that both ontogenetically and among closely related species of Gorilla, long bone strength proportions better reflect actual locomotor behavior than bone length proportions.     

Ontogeny and the evolution of adult body size dimorphism in apes

This analysis investigates the ontogeny of body size dimorphism in apes. The processes that lead to adult body size dimorphism are illustrated and described. Potential covariation between ontogenetic processes and socioecological variables is evaluated. Mixed-longitudinal growth data from 395 captive individuals (representing Hylobates lar [gibbon], Hylobates syndactylus [siamang], Pongo pygmaeus [orangutan], Gorilla gorilla [gorilla], Pan paniscus [pygmy chimpanzee], and Pan troglodytes [“common” chimpanzee]) form the basis of this study. Results illustrate heterogeneity in the growth processes that produce ape dimorphism. Hylobatids show no sexual differentiation in body weight growth. Adult body size dimorphism in Pongo can be largely attributed to indeterminate male growth. Dimorphism in African apes is produced by two different ontogenetic processes. Both pygmy chimpanzees (Pan paniscus) and gorillas (Gorilla gorilla) become dimorphic primarily through bimaturism (sex differences in duration of growth). In contrast, sex differences in rate of growth account for the majority of dimorphism in common chimpanzees (Pan troglodytes). Diversity in the ontogenetic pathways that produce adult body size dimorphism may be related to multiple evolutionary causes of dimorphism. The lack of sex differences in hylobatid growth is consistent with a monogamous social organization. Adult dimorphism in Pongo can be attributed to sexual selection for indeterminate male growth. Interpretation of dimorphism in African apes is complicated because factors that influence female ontogeny have a substantial effect on the resultant adult dimorphism. Sexual selection for prolonged male growth in gorillas may also increase bimaturism relative to common chimpanzees. Variation in female growth is hypothesized to covary with foraging adaptations and with differences in female competition that result from these foraging adaptations. Variation in male growth probably corresponds to variation in level of sexual selection. © 1995 Wiley-Liss, Inc.     

Behavioral Ecology of Sympatric Chimpanzees and Gorillas in Bwindi Impenetrable National Park,Uganda: Diet

Via a field study of chimpanzees (Pan troglodytes schweinfurthii) and gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, Uganda, we found that their diets are seasonally similar, but diverge during lean seasons. Bwindi chimpanzees fed heavily on fruits of Ficus sp., which were largely ignored by the gorillas. Bwindi gorilla diet was overall more folivorous than chimpanzee diet, but was markedly more frugivorous than that of gorillas in the nearby Virunga Volcanoes. During 4 mo of the year Bwindi gorilla diet included more food species than that of the chimpanzees. Three factors in particular—seasonal consumption of fibrous foods by gorillas, interspecific differences in preferred fruit species, and meat consumption by chimpanzees—contributed to dietary divergence between the two species. When feeding on fruits, gorillas ate Myrianthus holstii more frequently than chimpanzees did, while chimpanzees included more figs in their annual diet. Chimpanzee diet included meat of duikers and monkeys; gorilla frequently consumed decaying wood.     

Skeletal evidence of trauma in African apes, with special reference to the gombe chimpanzees

Incidence of cranial and postcranial skeletal trauma was investigated in samples of chimpanzees (Pan troglodytes troglodytes, P. troglodytes schweinfurthii), lowland gorillas (Gorilla gorilla gorilla), and bonobos (P. paniscus). The larger (adult) samples of chimpanzees (N=127 crania, 92 postcrania) and gorillas (N=136 crania, 62 postcrania) are curated at the Powell-Cotton Museum, Birchington, U.K. The bonobo collection (N=71 crania, 15 postcrania) is housed the Musée Royal de l'Afrique Centrale in Tervuren, Belgium. In addition, data were collected on the small but extremely well-documented skeletal sample from Gombe National Park (N=14 crania, 13 postcrania — including adults and adolescents). Cranial injuries, including healed fractures and bite wounds, were fairly frequent in the museum collection of chimpanzees (5.5% of individuals), but were twice as frequent in gorillas (11.0%). In the Gombe sample an even higher incidence was observed (28.6% of individuals). Bonobos, however, showed the lowest incidence of cranial trauma found among any of the African ape samples (1.4% of individuals). Postcranial trauma, documented most clearly by healed fractures, was seen in 21.7% of the Powell-Cotton chimpanzees, 30.8% of Gombe chimpanzees, 17.7% of gorillas, and in 13.3% of bonobos. Most of these lesions were found in the upper appendage. Nevertheless, highly debilitating healed fractures of the femur were also noted, most frequently and severe in female gorillas. The pattern of injuries suggests serious risks of falling in all free-ranging African apes, but also (in chimpanzees and gorillas) considerable risk from interindividual aggression, especially for males.     

Relative growth,ontogeny, and sexual dimorphism in gorilla(Gorilla gorilla gorilla and G. g. beringei): Evolutionary and ecological considerations

Gorillas are the largest and among the most sexually dimorphic of all extant primates. While gorillas have been incorporated in broad-level comparisons among large-bodied hominoids or in studies of the African apes, comparisons between gorilla subspecies have been rare. During the past decade, however, behavioral, morphological, and molecular data from a number of studies have indicated that the western lowland (Gorilla gorilla gorilla) and eastern mountain (Gorilla gorilla beringei) subspecies differ to a greater extent than has been previously believed. In this study I compare patterns of relative growth of the postcranial skeleton to evaluate whether differences between subspecies result from the differential extension of common patterns of relative growth. In addition, patterns of ontogeny and sexual dimorphism are also examined. Linear skeletal dimensions and skeletal weight were obtained for ontogenetic series of male and female G.g. gorilla (n = 315) and G.g. beringei (n = 38). Bivariate and multivariate methods of analysis were used to test for differences in patterns of relative growth, ontogeny, and sexual dimorphism between sexes of each subspecies and in same-sex comparisons between subspecies. Results indicate males and females of both subspecies are ontogenetically scaled for postcranial proportions and that females undergo an earlier skeletal growth spurt compared to males. However, results also indicate that the onset of the female growth spurt occurs at different dental stages in lowland and mountain gorillas and that mountain gorillas may be characterized by higher rates of growth. Finally, data demonstrate lowland and mountain gorilla females do not differ significantly in adult body size, but mountain gorilla males are significantly larger than lowland gorilla males, suggesting mountain gorillas are characterized by a higher degree of sexual dimorphism in body size. Thus, although lowland and mountain gorillas do not appear to have evolved novel adaptations of the postcranium which correlate with differences in locomotor behavior, the present investigation establishes subspecies differences in ontogeny and sexual dimorphism which may be linked with ecological variation. Specifically, these findings are evaluated in the context of risk aversion models which predict higher growth rates and increased levels of sexual dimorphism in extreme folivores. Am. J. Primatol. 43:1–31, 1997. © 1997 Wiley-Liss, Inc.     

Understanding Language Evolution: Beyond Pan-Centrism

Language does not fossilize but this does not mean that the language's evolutionary timeline is lost forever. Great apes provide a window back in time on our last prelinguistic ancestor's communication and cognition. Phylogeny and cladistics implicitly conjure Pan (chimpanzees, bonobos) as a superior (often the only) model for language evolution compared with earlier diverging lineages, Gorilla and Pongo (orangutans). Here, in reviewing the literature, it is shown that Pan do not surpass other great apes along genetic, cognitive, ecologic, or vocal traits that are putatively paramount for language onset and evolution. Instead, revived herein is the idea that only by abandoning single-species models and learning about the variation among great apes, there might be a chance to retrieve lost fragments of the evolutionary timeline of language.     

Interspecific and Ontogenetic Variation in Proximal Pedal Phalangeal Curvature of Great Apes (Gorilla gorilla, Pan troglodytes, and Pongo pygmaeus)

Considerable attention has been devoted to understanding phalangeal curvature in primates, particularly with regard to locomotion. Previous work has found that increased phalangeal curvature may be indicative of increased grasping during suspensory and climbing behaviors, but the details of this relationship, particularly as regards feet, is still unclear. Using behavioral studies to predict an interspecific gradient of variation in pedal phalangeal curvature, I collected digital data from the third and fifth digit proximal pedal phalanges in adult Gorilla gorilla, Pan troglodytes, and Pongo pygmaeus and calculated included angles of phalangeal curvature to assess the appropriateness of pooling digits within taxa and evaluate the association between variation in pedal phalangeal curvature and frequency of climbing behavior. I also used an ontogenetic sample of Pan troglodytes to evaluate the postnatal relationship between variation in phalangeal curvature and grasping behaviors. I found intraspecific variation in phalangeal curvature suggesting among-digit variation in grasping behaviors. Curvature of Pongo was significantly greater than of both Pan and Gorilla. In contrast, Pan was significantly more curved than Gorilla only in comparison of third digits. Ontogenetic decreases in pedal phalangeal curvature among Pan troglodytes accorded well with postnatal decreases in documented climbing frequency. These findings largely support earlier work regarding the association between arboreal grasping and phalangeal curvature, and provide a unique intraspecific analysis that illuminates a number of areas where our knowledge of the behavioral and biomechanical determinants of phalangeal curvature should be explored further, particularly with respect to the role of among-digit variation in phalangeal curvature.     

Morphological integration and the evolution of knuckle-walking

The evolution of knuckle-walking has profound implications for our understanding of the emergence of bipedalism. The modern debate surrounding its evolution is concerned with whether or not it is homologous in chimpanzees and gorillas. Here, this problem is approached using the methods of morphological integration to test hypotheses of patterns and magnitudes of integration in the third manual ray and capitate. If knuckle-walking morphologies are highly integrated and evolve in a correlated bundle (i.e., comprising a functional complex), it seems reasonable that they could have been recruited independently relatively easily in gorillas and chimpanzees, thus increasing the likelihood of homoplasy. If, however, there is no evidence for a knuckle-walking complex, then it seems less likely that chimpanzees and gorillas would have evolved knuckle-walking independently. Results indicate that chimpanzees and gorillas are not characterized by high magnitudes of integration or unique patterns of integration that distinguish them from non-knuckle-walking taxa. This does not support the hypothesis of a knuckle-walking complex, nor does it support the contention that knuckle-walking could have been easily evolved independently in chimpanzees and gorillas. Implications for trait analysis and the evolution of bipedalism are discussed, as are recent analyses supporting the independent origins of knuckle-walking.     

A comparative study of the variety and complexity of object manipulation in captive chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

Four types of specific objects: wooden spoons, metal bowls, plastic boxes, and cotton towels were introduced in a similar setting to two captive groups of different species in the genusPan, the bonobo and the chimpanzee. In total, 582 unique manipulation forms were distinguished by a set of variables: types of objects, motor patterns, body-parts used, the number of objects manipulated, and types of orienting manipulation. In sum, chimpanzees and bonobos were not so different in the variety and the complexity of object manipulation forms. However, comparison of the two species revealed significant differences as follows: (1) chimpanzees preferred to use only one hand during manipulation of both single- and multiple-objects, whereas in the case of multiple-objects bonobos used both hands significantly more often; (2) chimpanzees performed more orienting manipulations in single-object manipulations than did bonobos, whereas the reverse was the case in multiple-object manipulations; and (3) chimpanzees' object manipulations were overall more substrate-oriented than were bonobos'. The factors producing these differences are discussed in relation to positional behaviors and habitual tool use in the two species.     

Comparing infant and juvenile behavior in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): a preliminary study

The dichotomy between the two Pan species, the bonobo (Pan paniscus) and chimpanzee (Pan troglodytes) has been strongly emphasized until very recently. Given that most studies were primarily based on adult individuals, we shifted the “continuity versus discontinuity” discussion to the infant and juvenile stage. Our aim was to test quantitatively, some conflicting statements made in literature considering species differences between immature bonobos and chimpanzees. On one hand it is suggested that infant bonobos show retardation in motor and social development when compared with chimpanzees. Additionally it is expected that the weaning process is more traumatic to chimpanzee than bonobo infants. But on the other hand the development of behaviors is expected to be very similar in both species. We observed eight mother–infant pairs of each species in several European zoos. Our preliminary research partially confirms that immature chimpanzees seem spatially more independent, spending more time at a larger distance from their mother than immature bonobos. However, the other data do not seem to support the hypothesis that bonobo infants show retardation of motor or social development. The development of solitary play, environmental exploration, social play, non-copulatory mounts and aggressive interactions do not differ between the species. Bonobo infants in general even groom other group members more than chimpanzee infants. We also found that older bonobo infants have more nipple contact than same aged chimpanzees and that the weaning process seems to end later for bonobos than for immature chimpanzee. Additionally, although immature bonobos show in general more signs of distress, our data suggest that the weaning period itself is more traumatic for chimpanzees.     

A Faithful Record of Stressful Life Events Recorded in the Dental Developmental Record of a Juvenile Gorilla

The pattern and rate of dental development are critical components of the life history of primates. Much recent research has focused on dental development in chimpanzees and other hominoids, but comparatively little is known about dental development in Gorilla. To date, dental chronologies for Gorilla are based on a sample of 1 and information about variations in the time and timing of crown initiation and completion is lacking. We provide data on dental development in 1 captive, juvenile, female, western lowland Gorilla gorilla gorilla of known age, sex, life events, and date of death (carefully documented as part of zoo records) that experienced various physical insults during her first year of life. The perfect natural experiment allowed us to test the association of the timing of accentuated stress lines in teeth with significant physiological and psychological events during ontogeny of this juvenile gorilla. We analyzed histological sections from 14 permanent teeth (maxillary and mandibular I1-M2) and assessed crown initiation (CI) and crown formation times (CFT) using short- and long-period incremental lines in both enamel and dentine; they are advanced for all teeth compared to previously published chronology. The data suggest a relatively accelerated pace of dental development in gorillas compared to chimpanzees and fit an emerging pattern of an accelerated life history schedule in gorillas. Data on the timing of major accentuated lines in the developing dentition are tightly associated with exact dates of surgical procedures and follow-up hospital visits as recorded on zoo medical records. Our data highlight the importance of captive individuals with well-documented medical records for studying life history.

Manual pressure distribution patterns of knuckle‐walking apes

Differences in how the hands of gorillas and chimpanzees contact the ground while knuckle walking have been noted but generally not quantified. It is widely believed that gorillas maintain a pronated arm and contact the ground with digits 2–5 consistently, while chimpanzees have variable arm position and digit contact. To further test these generalizations, distribution of pressure across the manus, peak digital pressures, and hand position were quantified using a pressure mat in eight captive chimpanzees (Pan troglodytes) and seven gorillas (Gorilla gorilla). Chimpanzees and gorillas make initial ground contact with the ulnar aspect of the hand and pressure moves radially. They differ in which digit usually makes final contact and receives maximum pressure, and hand position during contact. Gorillas regularly use a palm‐back hand position and touch‐off with digit 2. They show less variation in pressure application across the digits. Chimpanzees are more variable in hand position and pressure application. In both, hand position plays a key role in determining which digit acts as the final touch‐off element. Am J Phys Anthropol 152:44–50, 2013. © 2013 Wiley Periodicals, Inc.     

Trabecular Architecture of the Manual Elements Reflects Locomotor Patterns in Primates

The morphology of trabecular bone has proven sensitive to loading patterns in the long bones and metacarpal heads of primates. It is expected that we should also see differences in the manual digits of primates that practice different methods of locomotion. Primate proximal and middle phalanges are load-bearing elements that are held in different postures and experience different mechanical strains during suspension, quadrupedalism, and knuckle walking. Micro CT scans of the middle phalanx, proximal phalanx and the metacarpal head of the third ray were used to examine the pattern of trabecular orientation in Pan, Gorilla, Pongo, Hylobates and Macaca. Several zones, i.e., the proximal ends of both phalanges and the metacarpal heads, were capable of distinguishing between knuckle-walking, quadrupedal, and suspensory primates. Orientation and shape seem to be the primary distinguishing factors but differences in bone volume, isotropy index, and degree of anisotropy were seen across included taxa. Suspensory primates show primarily proximodistal alignment in all zones, and quadrupeds more palmar-dorsal orientation in several zones. Knuckle walkers are characterized by having proximodistal alignment in the proximal ends of the phalanges and a palmar-dorsal alignment in the distal ends and metacarpal heads. These structural differences may be used to infer locmotor propensities of extinct primate taxa.