Browsing by Author "Schmitt, Daniel"

For primates, and other arboreal mammals, adopting suspensory locomotion represents one of the strategies an animal can use to prevent toppling off a thin support during arboreal movement and foraging. While numerous studies have reported the incidence of suspensory locomotion in a broad phylogenetic sample of mammals, little research has explored what mechanical transitions must occur in order for an animal to successfully adopt suspensory locomotion. Additionally, many primate species are capable of adopting a highly specialized form of suspensory locomotion referred to as arm-swinging, but few scenarios have been posited to explain how arm-swinging initially evolved. This study takes a comparative experimental approach to explore the mechanics of below branch quadrupedal locomotion in primates and other mammals to determine whether above and below branch quadrupedal locomotion represent neuromuscular mirrors of each other, and whether the patterns below branch quadrupedal locomotion are similar across taxa. Also, this study explores whether the nature of the flexible coupling between the forelimb and hindlimb observed in primates is a uniquely primate feature, and investigates the possibility that this mechanism could be responsible for the evolution of arm-swinging.

To address these research goals, kinetic, kinematic, and spatiotemporal gait variables were collected from five species of primate (Cebus capucinus, Daubentonia madagascariensis, Lemur catta, Propithecus coquereli, and Varecia variegata) walking quadrupedally above and below branches. Data from these primate species were compared to data collected from three species of non-primate mammals (Choloepus didactylus, Pteropus vampyrus, and Desmodus rotundus) and to three species of arm-swinging primate (Hylobates moloch, Ateles fusciceps, and Pygathrix nemaeus) to determine how varying forms of suspensory locomotion relate to each other and across taxa.

From the data collected in this study it is evident the specialized gait characteristics present during above branch quadrupedal locomotion in primates are not observed when walking below branches. Instead, gait mechanics closely replicate the characteristic walking patterns of non-primate mammals, with the exception that primates demonstrate an altered limb loading pattern during below branch quadrupedal locomotion, in which the forelimb becomes the primary propulsive and weight-bearing limb; a pattern similar to what is observed during arm-swinging. It is likely that below branch quadrupedal locomotion represents a “mechanical release” from the challenges of moving on top of thin arboreal supports. Additionally, it is possible, that arm-swinging could have evolved from an anatomically-generalized arboreal primate that began to forage and locomote below branches. During these suspensory bouts, weight would have been shifted away from the hindlimbs towards forelimbs, and as the frequency of these boats increased the reliance of the forelimb as the sole form of weight support would have also increased. This form of functional decoupling may have released the hindlimbs from their weight-bearing role during suspensory locomotion, and eventually arm-swinging would have replaced below branch quadrupedal locomotion as the primary mode of suspensory locomotion observed in some primate species. This study provides the first experimental evidence supporting the hypothetical link between below branch quadrupedal locomotion and arm-swinging in primates.

Primates and many other animals that move in an arboreal environment often cling, sometimes for long periods, on vertical supports. Primates, however, face a special challenge in that almost all primates bear nails on the tips of their digits rather than claws. Squirrels and other arboreal animals possess claws and/or adhesive pads on their digits in order to hold their weight on vertical substrates. Assuming the ancestral primate was arboreal and lost claws prior to the radiation of primates this paradox has important implications and raises a significant question about living primates and early primate evolution: how can primates maintain vertical postures without claws and how did early primates meet this challenge? Primate vertically clinging and grasping postures (VCG) have been studied in the wild and theoretical models of VCG postures have been described. This dissertation builds on this work, by studying the biomechanics of VCG postures in primates. Based on mechanical models, it was hypothesized VCG posture in primates will vary in three ways.

Hypothesis 1: Species with different morphological features associated with different locomotor modes will vertically cling and grasp in different ways.

Hypothesis 2: As substrate size increases, primates will place their arms to the side of the support and adjust posture and muscle recruitment in order to maintain a necessary tangential to normal force ratio to resist gravity.

Hypothesis 3: On substrates of the same relative size, larger animals should be less effective at maintaining VCG postures due to scaling relationships between muscle strength and body mass.

The sample consisted of multiple individuals from eight strepsirrhine species at the Duke Lemur Center. The sample varied in locomotor mode--habitual vertical clinging and leaping (VCL) compared to less specialized arboreal quadrupeds--and body mass--100 to 4,000 grams. Subjects were videorecorded while holding VCG postures on substrates of increasing size. Substrate preference data were calculated based on frequency and duration of VCG postures on each substrate. Qualitative kinematic data were recorded for a maximum of thirty trials per individual, per substrate. Angular data were calculated for forelimbs and hindlimbs from these videos for ten trials per individual per substrate. In addition, kinetic data from an imbedded force transducer were collected for two species that vary in locomotor mode, but not body mass.

There are several significant and relevant results from this study that address both primate functional anatomy and locomotor evolution. Hypothesis one was supported by hand and hindlimb joint postures, shown to be highly sensitive to locomotor mode. VCL primates exhibited deeply flexed limbs and more hand grasping (wrapping around the substrate) versus parallel hand postures and use of bowed finger postures compared to less specialized primates. Kinetically, species were shown to bear the majority of their weight in their hindlimbs relative to their forelimbs. The forelimb joints and foot showed little variation by habitual locomotor mode. Hypothesis two found support in that species tend to prefer smaller substrates, clinging less frequently for shorter durations as substrate size increases. Hand posture changed as size increased, as primates (except for the slow lorises) in this study grasped with their pollex on smaller substrates, but the pollex disengaged in grasping on larger substrates. Hypothesis three was not supported; body mass did not influence VCG postures.

Taken together, the finding that the forelimb held a wide range of postures on each substrate size for all species and played a limited role in weight-bearing suggests the forelimb free to move (to adjust posture and or forage). The hindlimb plays a more specific role in weight-bearing and is more sensitive to variations in primate anatomy. Additionally, these findings lead to hypotheses concerning the relatively short pollexes of primates, and that the ancestral primate was smaller than 100g and preferred small substrates as found in a fine-branch niche.

There has been a trend toward decreasing skeletal robusticity in the genus Homo throughout the Pleistocene, culminating in the gracile postcrania of living modern humans. This change is typically attributed to changing tool technologies and subsistence patterns among human groups. However, other mammalian groups also experience a similar change in their postcranial strength over the same time period. It is proposed in this dissertation that ecological variables are correlated with measures of postcranial strength and may be a better explanation for Holocene skeletal gracilization in humans, as well as in other mammalian genera. This hypothesis is investigated through a close examination of the scaling patterns in two extant genera, Canis and Ursus, and a comparison of scaling patterns and relative strength of different species of Canis, including a fossil species that provides information about temporal change. Measurements of limb length, joint surface area, bone diameter, and strength measurements derived from radiographic images of long bone midshafts of North American specimens of Canis, (including the fossil Canis dirus) and Ursus were collected. Scaling patterns of the cross-sectional variables on limb length and joint surfaces were analyzed for the interspecific and intraspecific samples.

The first hypothesis tested was that Canis scales with geometric similarity of cross-sectional variables on bone length and body mass, and the Ursus scales with elastic similarity. Larger Canis have relatively stronger postcrania than smaller Canis. The primary way in which this strength is achieved in larger individuals is through a relatively shortening of the bone length. The second hypothesis tested was that postcranial strength is correlated with ecological variables. To investigate this hypothesis, scaling patterns of different species of Canis were compared, including the fossil dire wolf. The results show that the dire wolf is relatively stronger than its living congenerics. There is also a strong relationship between the ratio of prey body mass to predator body mass and relative strength for these species. Carnivores that are hunting animals much larger than themselves must have postcranial skeletons that are strong enough to withstand the loading of the skeleton that occurs during hunting, taking down, and processing large herbivores.

The relatively stiff gait of modern humans minimizes the muscular work done to move the lower limbs and the center of mass. Nonhuman primates, and perhaps our earliest ancestors, use a form of bipedalism in which the hip and knee are held in a flexed position. This thesis follows up on other studies examining loading and energetic costs of these compliant walking gaits by examining the effects of increased hip and knee flexion on kinetic, kinematic, and energy exchange variables. The bipedal gait of twelve human subjects using normal and bent hip and bent knee gait were compared. The subjects walked along force plates embedded in the ground while 3D kinematic data was simultaneously gathered. The data was then processed using EvaRT, Orthotrak, and Matlab to evaluate the variables used. During the bent hip and bent knee bipedal locomotion subjects demonstrated lower peak vertical and parallel ground reaction forces, much higher ankle flexion, less hip extension, and less energy recovery during a full stride. These data provide novel insight into the nature and costs of locomotion in bipedal primates and the earliest human ancestors.