Nonhuman primates, our closest biological relatives, play important roles in the livelihoods, cultures, and religions of many societies and offer unique insights into human evolution, biology, behavior, and the threat of emerging diseases. They are an essential component of tropical biodiversity, contributing to forest regeneration and ecosystem health. Current information shows the existence of 504 species in 79 genera distributed in the Neotropics, mainland Africa, Madagascar, and Asia. Alarmingly, ~60% of primate species are now threatened with extinction and ~75% have declining populations. This situation is the result of escalating anthropogenic pressures on primates and their habitats—mainly global and local market demands, leading to extensive habitat loss through the expansion of industrial agriculture, large-scale cattle ranching, logging, oil and gas drilling, mining, dam building, and the construction of new road networks in primate range regions. Other important drivers are increased bushmeat hunting and the illegal trade of primates as pets and primate body parts, along with emerging threats, such as climate change and anthroponotic diseases. Often, these pressures act in synergy, exacerbating primate population declines. Given that primate range regions overlap extensively with a large, and rapidly growing, human population characterized by high levels of poverty, global attention is needed immediately to reverse the looming risk of primate extinctions and to attend to local human needs in sustainable ways. Raising global scientific and public awareness of the plight of the world’s primates and the costs of their loss to ecosystem health and human society is imperative.

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Impending extinction crisis of the world's primates: why primates matter

The relationship between locomotor be- havior and long bone structural proportions is examined in 179 individuals and 13 species of hominoids and cerco- pithecoids. Articular surface areas, estimated from linear caliper measurements, and diaphyseal section moduli (strengths), determined from CT scans, were obtained for the femur, tibia, humerus, radius, and ulna. Both within- bone (articular to shaft) and between-bone (forelimb to hindlimb) proportions were calculated and compared be- tween taxa. It was hypothesized that: 1) species empha- sizing slow, cautious movement and/or more varied limb positioning (i.e., greater joint excursion) would exhibit larger articular to cross-sectional shaft proportions, and 2) species with more forelimb suspensory behavior would have relatively stronger/larger forelimbs, while those with more leaping would have relatively stronger/larger hind- limbs. The results of the analysis generally confirm both hypotheses. Several partial exceptions can be explained on the basis of more detailed structural-functional consid- erations. Associations between locomotion and structural proportions can be demonstrated both across major group- ings (hominoids and cercopithecoids) and between rela- tively closely related taxa, e.g., mountain and lowland gorillas, siamangs and gibbons, and Trachypithecus and other colobines. Furthermore, structure and function do not always covary with taxonomy. For example, compared to cercopithecoids, mountain gorillas have relatively larger joints, like other hominoids, but do not have rela- tively stronger forelimbs, unlike other hominoids. This is consistent with a locomotor repertoire emphasizing rela- tively slow movement but with very little forelimb suspen- sion. Proportions of Proconsul nyanzae, Proconsul he- seloni, Morotopithecus bishopi, and Theropithecus oswaldi are compared with modern distributions to illustrate the application of the techniques to fossil taxa. Am J Phys Anthropol 119:305-342, 2002. © 2002 Wiley-Liss, Inc.

Long bone articular and diaphyseal structure in old world monkeys and apes. I: locomotor effects.

The metabolic cost associated with locomotion represents a significant part of an animal's metabolic energy budget. Therefore understanding the ways in which animals manage the energy required for locomotion by controlling muscular effort is critical to understanding limb design and the evolution of locomotor behavior. The assumption that energetic economy is the most important target of natural selection underlies many analyses of steady animal locomotion, leading to the prediction that animals will choose gaits and postures that maximize energetic efficiency. Many quadrupedal animals, particularly those that specialize in long distance steady locomotion, do in fact reduce the muscular contribution required for walking by adopting pendulum-like center of mass movements that facilitate exchange between kinetic energy (KE) and potential energy (PE) [1]–[4]. However, animals that are not specialized for long distance steady locomotion may face a more complex set of requirements, some of which may conflict with the efficient exchange of mechanical energy. For example, the “stealthy” walking style of cats may demand slow movements performed with the center of mass close to the ground. Force plate and video data show that domestic cats (Felis catus, Linnaeus, 1758) have lower mechanical energy recovery than mammals specialized for distance. A strong negative correlation was found between mechanical energy recovery and diagonality in the footfalls and there was also a negative correlation between limb compression and diagonality of footfalls such that more crouched postures tended to have greater diagonality. These data show a previously unrecognized mechanical relationship in which crouched postures are associated with changes in footfall pattern which are in turn related to reduced mechanical energy recovery. Low energy recovery was not associated with decreased vertical oscillations of the center of mass as theoretically predicted, but rather with posture and footfall pattern on the phase relationship between potential and kinetic energy. An important implication of these results is the possibility of a tradeoff between stealthy walking and economy of locomotion. This potential tradeoff highlights the complex and conflicting pressures that may govern the locomotor choices that animals make.

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Whole body mechanics of stealthy walking in cats.

It is now well recognized that terrestrial mammals can maintain equivalent bone stresses despite dramatic differences in body size through the adoption of extended limb positions during locomotion. However, this dynamic solution is not available to all mammals. Medium- and large-bodied arboreal mammals, such as anthropoid primates, must maintain relatively gracile and mobile limbs in order to manoeuvre in a discontinuous arboreal environment. But they must also use flexed (i.e. crouched) limb positions in order to maintain balance on arboreal substrates, thus subjecting their gracile limbs to relatively high loads. To determine how primates resolve this conflict between their kinematics and their morphology, five species of Old World monkeys were videotaped with lateral, frontal, and overhead cameras while they walked at a range of natural speeds along a runway and raised horizontal poles instrumented with a force platform. Kinematic and kinetic data on the forelimb show that during arboreal quadrupedalism, Old World monkeys do crouch when travelling on arboreal supports compared to the ground. Simultaneously, they lower vertical peak reaction forces and thereby reduce and reorient the peak resultant substrate reaction force, so that moment arms and moments are roughly equivalent on poles and the ground. This is accomplished through the adoption of a compliant walking gait characterized by high degrees of forelimb protraction, substantial elbow yield, low vertical oscillations of the body, and long contact times. The use of a compliant walking gait appears to be extremely rare among mammals and is most likely related to an initial primate adaptation to quadrupedal locomotion on terminal branches. This gait represents a previously unrecognized dynamic postural mechanism for maintenance of similar bone stresses and safety factors in both arboreal and terrestrial environments.

Compliant walking in primates

An understanding of the evolution of human bipedalism can provide valuable insights into the biomechanical and physiological characteristics of locomotion in modern humans. The walking gaits of humans, other bipeds and most quadrupedal mammals can best be described by using an inverted-pendulum model, in which there is minimal change in flexion of the limb joints during stance phase. As a result, it seems logical that the evolution of bipedalism in humans involved a simple transition from a relatively stiff-legged quadrupedalism in a terrestrial ancestor to relatively stiff-legged bipedalism in early humans. However, experimental studies of locomotion in humans and nonhuman primates have shown that the evolution of bipedalism involved a much more complex series of transitions, originating with a relatively compliant form of quadrupedalism. These studies show that relatively compliant walking gaits allow primates to achieve fast walking speeds using long strides, low stride frequencies, relatively low peak vertical forces, and relatively high impact shock attenuation ratios. A relatively compliant, ape-like bipedal walking style is consistent with the anatomy of early hominids and may have been an effective gait for a small biped with relatively small and less stabilized joints, which had not yet completely forsaken arboreal locomotion. Laboratory-based studies of primates also suggest that human bipedalism arose not from a terrestrial ancestor but rather from a climbing, arboreal forerunner. Experimental data, in conjunction with anatomical data on early human ancestors, show clearly that a relatively stiff modern human gait and associated physiological and anatomical adaptations are not primitive retentions from a primate ancestor, but are instead recently acquired characters of our genus.

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A.R. Biknevicius

Papers

The kinetics of primate quadrupedalism: "hindlimb drive" reconsidered