Reevaluation of the lumbosacral region of Oreopithecus bambolii

Functional interpretations of the postcranium of the late Miocene ape Oreopithecus bambolii are controversial. The claim that Oreopithecus practiced habitual terrestrial bipedalism is partly based on restored postcranial remains originally recovered from Baccinello, Tuscany ( Köhler and Moyà-Solà, 1997). The lower lumbar vertebrae of BA#72 were cited as evidence that Oreopithecus exhibits features indicative of a lordotic lumbar spine, including dorsal wedging of the vertebral bodies and a caudally progressive increase in postzygapophyseal interfacet distance. Here, we demonstrate why the dorsal wedging index value obtained by Köhler and Moyà-Solà (1997) for the BA#72 last lumbar vertebra is questionable due to distortion in that region, present a more reliable way to measure postzygapophyseal interfacet distance, and include an additional metric (laminar width) with which to examine changes in the transverse dimensions of the neural arches. We also quantify the external morphology of the BA#72 proximal sacrum, which, despite well-documented links between sacral morphology and bipedal locomotion, and excellent preservation of the sacral prezygapophyses, first sacral vertebral body, and right ala, was not evaluated by Köhler and Moyà-Solà (1997). Measures of postzygapophyseal interfacet distance and laminar width on the penultimate and last lumbar vertebrae of BA#72 reveal a pattern encompassed within the range of living nonhuman hominoids and unlike that of modern humans, suggesting that Oreopithecus did not possess a lordotic lumbar spine. Results further show that the BA#72 sacrum exhibits relatively small prezygapophyseal articular facet surface areas and mediolaterally narrow alae compared with modern humans, indicating that the morphology of the Oreopithecus sacrum is incompatible with the functional demands of habitual bipedal stance and locomotion. The Oreopithecus lumbosacral region does not exhibit adaptations for habitual bipedal locomotion.     

Placement of the diaphragmatic vertebra in catarrhines: implications for the evolution of dorsostability in hominoids and bipedalism in hominins

A fundamental adaptation to orthograde posture and locomotion amongst living hominoid primates is a numerically reduced lumbar column, which acts to stiffen the lower back and reduce injuries to the intervertebral discs. A related and functionally complementary strategy of spinal stability is a caudal position of the diaphragmatic vertebra relative to the primitive condition found in nonhominoid primates and most other mammals. The diaphragmatic vertebra marks the transition in vertebral articular facet (zygapophysis) orientation, which either resists (prediaphragmatic) or allows (postdiaphragmatic) trunk movement in the sagittal plane (i.e., flexion and extension). Unlike most mammals, which have dorsomobile spines (long lumbar columns and cranially placed diaphragmatic vertebrae) for running and leaping, hominoids possess dorsostable spines (short lumbar columns and caudally placed diaphragmatic vertebrae) adapted to orthogrady and antipronogrady. In contrast to humans and other extant hominoids, all known early hominin partial vertebral columns demonstrate cranial displacement of the diaphragmatic vertebra. To address this difference, variation in diaphragmatic placement is assessed in a large sample of catarrhine primates. I show that while hominoids are characterized by modal common placement of diaphragmatic and last rib-bearing vertebrae in general, interspecific differences in intraspecific patterns of variation exist. In particular, humans and chimpanzees show nearly identical patterns of diaphragmatic placement. A scenario of hominin evolution is proposed in which early hominins evolved cranial displacement from the ancestral hominid condition of common placement to achieve effective lumbar lordosis during the evolution of bipedal locomotion.     

Lumbar lordosis of extinct hominins

The lordotic curvature of the lumbar spine (lumbar lordosis) in humans is a critical component in the ability to achieve upright posture and bipedal gait. Only general estimates of the lordotic angle (LA) of extinct hominins are currently available, most of which are based on the wedging of the vertebral bodies. Recently, a new method for calculating the LA in skeletal material has become available. This method is based on the relationship between the lordotic curvature and the orientation of the inferior articular processes relative to vertebral bodies in the lumbar spines of living primates. Using this relationship, we developed new regression models in order to calculate the LAs in hominins. The new models are based on primate group-means and were used to calculate the LAs in the spines of eight extinct hominins. The results were also compared with the LAs of modern humans and modern nonhuman apes. The lordotic angles of australopithecines (41° ± 4), H. erectus (45°) and fossil H. sapiens (54° ± 14) are similar to those of modern humans (51° ± 11). This analysis confirms the assumption that human-like lordotic curvature was a morphological change that took place during the acquisition of erect posture and bipedalism as the habitual form of locomotion. Neandertals have smaller lordotic angles (LA = 29° ± 4) than modern humans, but higher angles than nonhuman apes (22° ± 3). This suggests possible subtle differences in Neandertal posture and locomotion from that of modern humans.     

Evolution of the sacrum in hominoids

In order to study the formation of the sacrum during the primate evolution, a new way of numbering mammalian vertebrae is presented; this demonstrates that the thoracolumbosacral complex is fixed at 22 vertebrae in 80% and at 22 +/- 1 in 100% of the cases. The shift of a vertebra from one type to another occurs either at the thoracolumbar or at the lumbosacral junction and not at the cervicothoracic junction. Rarely does the shift take place at the sacrococcygeal junction. Data from 318 primates reveal that the seven original lumbar vertebrae of the Old World monkeys are reduced in the great apes by a caudad "thoracization" of one to two lumbar vertebrae and a cephalad sacralization of one to four lumbar vertebrae. In the apes, sacralization is not total and different stages that are intermediate between lumbar and sacral are described. In Homo sapiens there is a total sacralization of the last two original lumbar vertebrae. In addition, development of the sacral wings (alae) is minimal in apes and reaches its maximum in hominids. The tendency of the hominoid sacrum to incorporate the last lumbar vertebrae and to widen markedly provides for an enhanced articulation of the sacrum with the ilium and offers a firm base of support for the trunk during erect posture. This is necessary for the support of the weight of the trunk above the sacrum and for the stabilization of the body during bipedal posture and locomotion. Encephalization did not play any major role in the widening of the sacrum since the former by far preceded the latter.     

Effect of posture and locomotion on energy expenditure

Energy expenditure for human adults and infants and for dogs was measured in resting (supine or lateral) posture, in bipedal posture and locomotion, and in quadrupedal posture and locomotion. Variations in respiratory and heart rate and in body temperature were utilized in this comparative study. Oxygen consumption was also measured in human adults. In human adults, bipedal posture and locomotion were shown to be much less energy-consuming than corresponding quadrupedal posture and locomotion. The opposite was observed in adult dogs, where bipedalism was shown to be much more energy-consuming than quadrupedalism. In addition, this study demonstrated, for human adults in their natural erect posture, an energy expenditure barely higher than in supine or lateral resting posture, while the dogs in their natural quadrupedal stance, the energy expenditure is much higher than in their resting posture. With respect to energy, therefore, humans are more adapted to bipedalism than dogs to quadrupedalism. Human children, at the transitional stage between quadrupedalism and bipedalism, have high and almost equal requirements for all postures and locomotions. This demonstrates, in term of energy, their incomplete adaptation to erect behavior.     

Foramen magnum position in bipedal mammals

The anterior position of the human foramen magnum is often explained as an adaptation for maintaining balance of the head atop the cervical vertebral column during bipedalism and the assumption of orthograde trunk postures. Accordingly, the relative placement of the foramen magnum on the basicranium has been used to infer bipedal locomotion and hominin status for a number of Mio-Pliocene fossil taxa. Nonetheless, previous studies have struggled to validate the functional link between foramen magnum position and bipedal locomotion. Here, we test the hypothesis that an anteriorly positioned foramen magnum is related to bipedalism through a comparison of basicranial anatomy between bipeds and quadrupeds from three mammalian clades: marsupials, rodents and primates. Additionally, we examine whether strepsirrhine primates that habitually assume orthograde trunk postures exhibit more anteriorly positioned foramina magna compared with non-orthograde strepsirrhines. Our comparative data reveal that bipedal marsupials and rodents have foramina magna that are more anteriorly located than those of quadrupedal close relatives. The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines. Among the primates sampled, humans exhibit the most anteriorly positioned foramina magna. The results of this analysis support the utility of foramen magnum position as an indicator of bipedal locomotion in fossil hominins.     

Positional behavior and substrate use in wild adult bearded capuchin monkeys (Sapajus libidinosus)

Natural selection for positional behavior (posture and locomotion) has at least partially driven the evolution of anatomical form and function in the order Primates. Examination of bipedal behaviors associated with daily activity patterns, foraging, and terrestrial habitat use in nonhuman primates, particularly those that adopt bipedal postures and use bipedal locomotion, allows us to refine hypotheses concerning the evolution of bipedalism in humans. This study describes the positional behavior of wild bearded capuchins (Sapajus libidinosus), a species that is known for its use of terrestrial substrates and its habitual use of stones as tools. Here, we test the association of terrestrial substrate use with bipedal posture and locomotion, and the influence of sex (which co‐varies with body mass in adults of this species) on positional behavior and substrate use. Behavior and location of 16 wild adult bearded capuchins from two groups were sampled systematically at 15 s intervals for 2 min periods for 1 year (10,244 samples). Despite their different body masses, adult males (average 3.5 kg) and females (average 2.1 kg) in this study did not differ substantially in their positional behaviors, postures, or use of substrates for particular activities. The monkeys used terrestrial substrates in 27% of samples. Bipedal postures and behaviors, while not a prominent feature of their behavior, occurred in different forms on the two substrates. The monkeys crouched bipedally in trees, but did not use other bipedal postures in trees. While on terrestrial substrates, they also crouched bipedally but occasionally stood upright and moved bipedally with orthograde posture. Bearded capuchin monkeys' behavior supports the suggestion from anatomical analysis that S. libidinosus is morphologically better adapted than its congeners to adopt orthograde postures.     

Morphological variation of the thoracolumbar vertebrae in Macropodidae and its functional relevance

The present study was designed to investigate how the form of the marsupial thoracolumbar vertebrae varies to cope with the particular demands of diverse loading and locomotor behaviors. The vertebral columns of 10 species of Macropodidae, with various body masses and modes of locomotion, together with two other arboreal marsupials, koala and cuscus, were selected. Seventy-four three-dimensional landmark coordinates were acquired on each of the 10 last presacral vertebrae of the 70 vertebral columns. The interspecific variations of the third lumbar vertebra (L3, which approximates the mean) and the transitional patterns of the thoracolumbar segments were examined using the combined approach of generalized Procrustes analysis (GPA) and principal components analysis (PCA). The results of analyses of an individual vertebra (L3) and of the transitional patterns indicate significant interspecific differences. In the L3 study the first PC shows allometric shape variation, while the second PC seems to relate to adaptation for terrestrial versus arboreal locomotion. When the L3 vertebrae of the common spotted cuscus and koala are included for comparison, the vertebra of the tree kangaroo occupies an intermediate position between the hopping kangaroo and these arboreal marsupials. The L3 vertebrae in the arboreal marsupials possess a distinct dorsoventrally expanded vertebral body, and perpendicularly orientated spinous and transverse processes. The results of the present study suggest that vertebral shape in the kangaroo and wallaroos provides a structural adaptation to hopping through a relatively enlarged loading area and powerful lever system. In contrast, the small-sized bettongs (or rat kangaroos) have a relatively flexible column and elongated levers for the action of back muscles that extend and laterally flex the spine. The complex pattern of vertebral shape transition in the last 10 presacral vertebrae was examined using PCAs that compare between species information about vertebral shape variation along the thoracolumbar column. The results reinforce and emphasize important aspects of the patterns of variation seen in the detailed analysis of the third lumbar vertebra. The results also imply that size, spinal loading pattern, and locomotor behavior exert an influence on shaping the vertebra. Further, the morphological adaptations are consistent within these marsupials and this opens up the possibility that this kind of analysis may be useful in making functional inferences from fossil material.     

Evolution of the hominoid vertebral column: The long and the short of it

The postcranial axial skeleton exhibits considerable morphological and functional diversity among living primates. Particularly striking are the derived features in hominoids that distinguish them from most other primates and mammals. In contrast to the primitive catarrhine morphotype, which presumably possessed an external (protruding) tail and emphasized more pronograde trunk posture, all living hominoids are characterized by the absence of an external tail and adaptations to orthograde trunk posture. Moreover, modern humans evolved unique vertebral features that satisfy the demands of balancing an upright torso over the hind limbs during habitual terrestrial bipedalism. Our ability to identify the evolutionary timing and understand the functional and phylogenetic significance of these fundamental changes in postcranial axial skeletal anatomy in the hominoid fossil record is key to reconstructing ancestral hominoid patterns and retracing the evolutionary pathways that led to living apes and modern humans. Here, we provide an overview of what is known about evolution of the hominoid vertebral column, focusing on the currently available anatomical evidence of three major transitions: tail loss and adaptations to orthograde posture and bipedal locomotion.     

Arboreal bipedalism in wild chimpanzees: implications for the evolution of hominid posture and locomotion

Field observations of bipedal posture and locomotion in wild chimpanzees (Pan troglodytes) can serve as key evidence for reconstructing the likely origins of bipedalism in the last prehominid human ancestor. This paper reports on a sample of bipedal bouts, recorded ad libitum, in wild chimpanzees in Bwindi Impenetrable National Park in southwestern Uganda. The Ruhija community of chimpanzees in Bwindi displays a high rate of bipedal posture. In 246.7 hr of observation from 2001-2003, 179 instances of bipedal posture lasting 5 sec or longer were recorded, for a rate of 0.73 bouts per observation hour. Bipedalism was observed only on arboreal substrates, and was almost all postural, and not locomotor. Bipedalism was part of a complex series of positional behaviors related to feeding, which included two-legged standing, one-legged standing with arm support, and other intermediate postures. Ninety-six percent of bipedal bouts occurred in a foraging context, always as a chimpanzee reached to pluck fruit from tree limbs. Bipedalism was seen in both male and female adults, less frequently among juveniles, and rarely in infants. Both the frequency and duration of bipedal bouts showed a significant positive correlation with estimated substrate diameter. Neither fruit size nor nearest-neighbor association patterns were significantly correlated with the occurrence of bipedalism. Bipedalism is seen frequently in the Bwindi chimpanzee community, in part because of the unusual observer conditions at Bwindi. Most observations of bipedalism were made when the animals were in treetops and the observer at eye-level across narrow ravines. This suggests that wild chimpanzees may engage in bipedal behavior more often than is generally appreciated. Models of the likely evolutionary origins of bipedalism are considered in the light of Bwindi bipedalism data. Bipedalism among Bwindi chimpanzees suggests the origin of bipedal posture in hominids to be related to foraging advantages in fruit trees. It suggests important arboreal advantages in upright posture. The origin of postural bipedalism may have preceded and been causally disconnected from locomotor bipedalism.     

Vertebrae numbers of the early hominid lumbar spine

General doctrine holds that early hominids possessed a long lumbar spine with six segments. This is mainly based on Robinson's (1972) interpretation of a single partial Australopithecus africanus skeleton, Sts 14, from Sterkfontein, South Africa. As its sixth last presacral vertebra exhibits both thoracic and lumbar characteristics, current definitions of lumbar vertebrae and lumbar ribs are discussed in the present study. A re-analysis of its entire preserved vertebral column and comparison with Stw 431, another partial A. africanus skeleton from Sterkfontein, and the Homo erectus skeleton KNM-WT 15000 from Nariokotome, Kenya, did not provide strong evidence for the presence of six lumbar vertebrae in either of these early hominids. Thus, in Sts 14 the sixth last presacral vertebra has on one side a movable rib. In Stw 431, the corresponding vertebra shows indications for a rib facet. In KNM-WT, 15000 the same element is very fragmentary, but the neighbouring vertebrae do not support the view that it is L1. Although in all three fossils the transitional vertebra at which the articular facets change orientation seems to be at Th11, this is equal to a large percentage of modern humans. Indeed, a modal number of five lumbar vertebrae, as in modern humans, is more compatible with evolutionary principles. For example, six lumbar vertebrae would require repetitive shortening and lengthening not only of the lumbar, but also of the entire precaudal spine. Furthermore, six lumbar vertebrae are claimed to be biomechanically advantageous for early hominid bipedalism, yet an explanation is lacking as to why the lumbar region should have shortened in later humans. All this raises doubts about previous conclusions for the presence of six lumbar vertebrae in early hominids. The most parsimonious explanation is that they did not differ from modern humans in the segmentation of the vertebral column.     

Kinematics of bipedalism in Propithecus verreauxi

Bipedalism is rare in primates and has evolved in two distantly related groups: hominoids and indrids. Although copious data are available on the mechanics of bipedal locomotion in hominoids and vertical clinging and leaping (VCL) in indrids, no research has addressed the unique mode of bipedal locomotion exhibited by select indrid primates. Propithecus verreauxi is a highly specialized indrid vertical clinger and leaper that uses a peculiar form of bipedalism on the ground. The objectives of this study were to describe the bipedal gait of Propithecus , to assess the influence of VCL specializations on the kinematic patterns and propulsion mechanisms used by Propithecus during bipedalism, and to compare Propithecus bipedalism with the bipedal gaits of other primates capable of using bipedalism. Video was collected of five adult P. verreauxi moving bipedally in a seminatural setting at the Duke University Primate Center. Duty factor, footfall patterns, joint angles and center of mass movement were quantified in the sagittal plane for 73 steps. Propithecus uses a bipedal gallop, a gait unique to Propithecus . The kinematic similarities (e.g. large hip and knee angular excursions and preparatory countermovements) between bipedal galloping and VCL lead us to suggest that Propithecus takes advantage of specializations for VCL to conserve energy during bipedal galloping. Propithecus also walks bipedally at slower speeds. When Propithecus walks, it utilizes a relatively compliant gait similar to that of other primate facultative bipeds ( Pan , Hylobates ). During bipedal walking, energy conservation may be sacrificed for increased balance and reduced joint loads.     

The energetic cost of locomotion: humans and primates compared to generalized endotherms

A wide range of selective pressures have been advanced as possible causes for the adoption of bipedalism in the hominin lineage. One suggestion has been that because modern human walking is relatively efficient compared to that of a typical quadruped, the ancestral quadruped may have reaped an energetic advantage when it walked on two legs. While it has become clear that human walking is relatively efficient and human running inefficient compared to "generalized endotherms", workers differ in their opinion of how the cost of human bipedal locomotion compares to that of a generalized primate walking quadrupedally. One view is that human walking is particularly efficient in comparison to other primates. The present study addresses this by comparing the cost of human walking and running to that of the eight primate species for which data are available and by comparing cost in primates to that of a "generalized endotherm". There is no evidence that primate locomotion is more costly than that of a generalized endotherm, although more data on adult Old World monkeys and apes would be useful. Further, human locomotion does not appear to be particularly efficient relative to that of other primates.     

Apparent density of the primate calcaneo‐cuboid joint and its association with locomotor mode,foot posture,and the “midtarsal break”

Primates use a range of locomotor modes during which they incorporate various foot postures. Humans are unique compared with other primates in that humans lack a mobile fore‐ and midfoot. Rigidity in the human foot is often attributed to increased propulsive and stability requirements during bipedalism. Conversely, fore‐ and midfoot mobility in nonhuman primates facilitates locomotion in arboreal settings. Here, we evaluated apparent density (AD) in the subchondral bone of human, ape, and monkey calcanei exhibiting different types of foot loading. We used computed tomography osteoabsorptiometry and maximum intensity projection (MIP) maps to visualize AD in subchondral bone at the cuboid articular surface of calcanei. MIPs represent 3D volumes (of subchondral bone) condensed into 2D images by extracting AD maxima from columns of voxels comprising the volumes. False‐color maps are assigned to MIPs by binning pixels in the 2D images according to brightness values. We compared quantities and distributions of AD pixels in the highest bin to test predictions relating AD patterns to habitual locomotor modes and foot posture categories of humans and several nonhuman primates. Nonhuman primates exhibit dorsally positioned high AD concentrations, where maximum compressive loading between the calcaneus and cuboid likely occurs during “midtarsal break” of support. Humans exhibit less widespread areas of high AD, which could reflect reduced fore‐ and midfoot mobility. Analysis of the internal morphology of the tarsus, such as subchondral bone AD, potentially offers new insights for evaluating primate foot function during locomotion. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Functional aspects of strepsirrhine lumbar vertebral bodies and spinous processes

The relationship between form and function in the lumbar vertebral column has been well documented among platyrrhines and especially catarrhines, while functional studies of postcranial morphology among strepsirrhines have concentrated predominantly on the limbs. This morphometric study investigates biomechanically relevant attributes of the lumbar vertebral morphology of 20 species of extant strepsirrhines. With this extensive sample, our goal is to address the influence of positional behavior on lumbar vertebral form while also assessing the effects of body size and phylogenetic history. The results reveal distinctions in lumbar vertebral morphology among strepsirrhines in functional association with their habitual postures and primary locomotor behaviors. In general, strepsirrhines that emphasize pronograde posture and quadrupedal locomotion combined with leaping (from a pronograde position) have the relatively longest lumbar regions and lumbar vertebral bodies, features promoting sagittal spinal flexibility. Indrids and galagonids that rely primarily on vertical clinging and leaping with orthograde posture share a relatively short (i.e., stable and resistant to bending) lumbar region, although the length of individual lumbar vertebral bodies varies phylogenetically and possibly allometrically. The other two vertical clingers and leapers, Hapalemur and Lepilemur, more closely resemble the pronograde, quadrupedal taxa. The specialized, suspensory lorids have relatively short lumbar regions as well, but the lengths of their lumbar regions are influenced by body size, and Arctocebus has dramatically longer vertebral bodies than do the other lorids. Lumbar morphology among galagonids appears to reflect a strong phylogenetic signal superimposed on a functional one. In general, relative length of the spinous processes follows a positively allometric trend, although lorids (especially the larger-bodied forms) have relatively short spinous processes for their body size, in accordance with their positional repertoire. The results of the study broaden our understanding of postcranial adaptation in primates, while providing an extensive comparative database for interpreting vertebral morphology in fossil primates.     

Vertebral age changes in Japanese macaques

This study deals with maturation and aging of the vertebrae in Japanese macaques (Macaca fuscata fuscata) of known chronological age. The samples used were 103 skeletons of captive raised Japanese macaques varying in age from 6-23 years. Epiphyseal union between the vertebral body and the epiphyseal disk (epiphyseal ring, annular epiphysis) and degenerative changes of the vertebrae were macroscopically examined. It was revealed that vertebral epiphyseal union develops comparatively rapidly in the sacral and cervical vertebrae, moderately in the lumbar vertebrae, and slowly in the thoracic vertebrae. It was found that, as a central tendency, the vertebral epiphyseal union begins at about 6 years of age, progresses lineally in proportion to age, and completes at about 23 years of age. However, considerable variation in developmental states of union was observed among individuals of the same age. Concerning vertebral degenerative changes, few were observed among the present samples. Compared with the other primates with regard to the timing of vertebral maturation, shortening of duration of maturation was found among humans. Human vertebrae may have become an early-maturing organ in order to sustain the increased loading that is accompanied by the adoption of habitual erect posture and bipedal locomotion.     

Vertebral morphology of Nacholapithecus kerioi based on KNM-BG 35250

This paper describes the morphology of the vertebral remains of the KNM-BG 35250 Nacholapithecus kerioi individual from the Middle Miocene of Kenya. Cervical vertebrae are generally large relative to presumed body mass, suggesting a heavy head with large jaws and well-developed neck muscles. The atlas retains the lateral and posterior bridges over the vertebral artery. The axis has a robust dens and a large angle formed by superior articular surfaces. The thoracic vertebral specimens include the diaphragmatic vertebra and one post-diaphragmatic vertebra. The thoracic vertebral bodies are much smaller that those of male Papio cynocephalus, whereas many of the dorsal elements are large and robust, exceeding those of male P. cynocephalus. Lumbar vertebral bodies are small relative to body mass, craniocaudally moderately long, and have a median ventral keel. The transverse process is craniocaudally long and arises from the widest part of the body cranially and the pedicle above the inferior vertebral notch caudally. Anapophyses are present in one of the preserved lumbar vertebrae. The postzygapophyses are thick dorsoventrally. These lumbar features are broadly shared with Proconsul. However, the base of the spinous process is longer and more caudally positioned in N. kerioi compared to Proconsul, and is more similar to the condition in Pongo. They are not dorsally (or moderately caudally) directed as is seen in P. nyanzae, Pan, and most other extant primates. A caudally directed spinous process does not permit a broad range of spinal dorsiflexion. The presumed stiff back in N. kerioi suggests a different locomotor repertoire than in Proconsul. Morotopithecus bishopi, although not possessing the same features, exhibits another morphological suite of characters for lumbar stiffness. Diverse functional adaptations of the lumbar spine were present in African hominoids during the Early to Middle Miocene.     

Speed modulation in hylobatid bipedalism: a kinematic analysis

Gibbons are highly arboreal apes, and it is expected that their bipedal locomotion will show some particularities related to the arboreal environment. Previous research has shown that, during hylobatid bipedalism, unsupported phases are rare and stride frequencies are relatively low. This study confirms previous findings, and we suggest that low stride frequencies and the absence of unsupported phases are ways to reduce disadvantageous branch oscillations during arboreal travel. Despite these restrictions, gibbons are able to locomote at a wide range of speeds, implying that they likely exploit other mechanisms to modulate their locomotor speed. To investigate this possibility, we collected video images of a large number of spontaneous bipedal bouts of four untrained white-handed gibbons by using an instrumented walkway with four synchronized cameras. These video images were digitized to obtain a quantification of the 3D kinematics of hylobatid bipedalism. We defined a large number of spatiotemporal and kinematic gait variables, and the relationship between these gait variables and (dimensionless) speed was statistically tested. It was found that gibbons mainly increase stride length to increase their locomotor speed; the main speed-modulating mechanisms are hip and ankle excursion and coupled knee and ankle extension at toe-off. Although aerial phases are rare, gibbons generally adopt a bipedal bouncing gait at most speeds and a clear-cut gait transition, as seen in human locomotion, is absent. Comparison with human and bonobo bipedalism showed that the variability of the 3D joint angles of the hind limb are comparable during human and gibbon bipedalism, and much lower than during bonobo bipedalism. The low variability found in gibbons might be related to constraints imposed by the arboreal environment. These arboreal constraints clearly affect the bipedal gait characteristics of gibbons, but do not constrain the ability to adopt a bipedal bouncing gait during terrestrial locomotion.     

Brief communication: arboreal bipedalism in Bwindi chimpanzees

Evidence of the form and function of bipedal behavior in nonhuman primates provides critical evidence to test theories about the origins of hominid bipedalism. Bipedalism has long been considered an evolutionarily interesting but rare behavior in wild chimpanzees. During May 2001, chimpanzees of the Ruhija community in the Bwindi Impenetrable National Park, Uganda, engaged in an exceptional frequency of arboreal bipedalism when feeding in large Ficus trees. Seventy-eight bipedal bouts of at least 5 sec duration were recorded for the entire community (0.49 bouts/hr), with a mean duration of 13.7 sec (+/-1.6 sec). The animals employed many variations on the bipedal postural theme, ranging from erect standing on the largest substrates while grasping overhead limbs for support, to standing on one leg while suspending the other leg in space, to extended-lean standing, in which bipedal standing transitioned into horizontal arm-leg suspension as the animal reached for more distant fruits. Bipedalism was used as part of a behavioral repertoire that integrated brachiation, four-limbed suspension, and forelimb-supported standing for effective small-fruit foraging. These observations suggest that under certain ecological conditions, arboreal bipedalism can be an important posture for wild chimpanzees, and may have been an important behavioral precursor to full terrestrial bipedalism.