Influences of limb proportions and body size on locomotor kinematics in terrestrial primates and fossil hominins.

During locomotion, mammalian limb postures are influenced by many factors including the animal's limb length and body mass. Polk (2002) compared the gait of similar-sized cercopithecine monkeys that differed limb proportions and found that longer-limbed monkeys usually adopt more extended joint postures than shorter-limbed monkeys in order to moderate their joint moments. Studies of primates as well as non-primate mammals that vary in body mass have demonstrated that larger animals use more extended limb postures than smaller animals. Such extended postures in larger animals increase the extensor muscle mechanical advantage and allow postures to be maintained with relatively less muscular effort (Polk, 2002; Biewener 1989). The results of these previous studies are used here to address two anthropological questions. The first concerns the postural effects of body mass and limb proportion differences between australopithecines and members of the genus Homo. That is, H. erectus and later hominins all have larger body mass and longer legs than australopithecines, and these anatomical differences suggest that Homo probably used more extended postures and probably required relatively less muscular force to resist gravity than the smaller and shorter-limbed australopithecines. The second question investigates how animals with similar size but different limb proportions differ in locomotor performance. The effects of limb proportions on gait are relevant to inferring postural and locomotor differences between Neanderthals and modern Homo sapiens which differ in their crural indices and relative limb length. This study demonstrates that primates with relatively long limbs achieve higher walking speeds while using lower stride frequencies and lower angular excursions than shorter-limbed monkeys, and these kinematic differences may allow longer-limbed taxa to locomote more efficiently than shorter-limbed species of similar mass. Such differences may also have characterized the gait of Homo sapiens in comparison to Neanderthals, but more experimental data on humans that vary in limb proportions are necessary in order to evaluate this question more thoroughly.

Adaptive and phylogenetic influences on musculoskeletal design in cercopithecine primates.

Broad allometric studies of the musculoskeletal system have frequently sought to explain how locomotor variables have been influenced by body mass. To examine animals that vary widely in body mass, these studies have included taxa that differ in their locomotor adaptations and phylogenetic relatedness. Because these sources of diversity could obscure the effects of body mass, this study was designed to test the effects of adaptive differences in limb proportions and phylogeny, as well as body mass, on locomotor kinematics and extensor muscle mechanical advantage. More specifically, two hypotheses were tested in a sample of closely related animals: (i) that, among animals with similar body mass, those with longer limb segments should adopt more extended limb postures to moderate the joint and midshaft bending moments that they experience, and (ii) that body mass will have similar influences on joint posture and joint moments in closely related and diverse mammalian samples. Three-dimensional kinematic and synchronous force-platform data were collected for six individual cercopithecine monkeys ranging in mass from 4kg to 24kg and at a range of walking speeds. Comparisons among three monkeys with similar body mass but different limb segment lengths reveal a significant effect of limb proportion on posture. That is, animals with longer limbs frequently use more extended limb postures and can have correspondingly lower joint moments. The scaling of locomotor variables across the entire sample of closely related monkeys was generally similar to published results for a diverse sample of mammals, with larger monkeys having more extended limb postures, lower joint moments and greater effective mechanical advantage (EMA) for their limb extensor musculature. Ankle EMA, however, did not increase with body mass in the primate sample, suggesting that clade-specific adaptive differences (e.g. the use of arboreal supports by primates) may constrain the effects of body mass.

Comparing measures of travel distances in primates: methodological considerations and socioecological implications.

Travel costs can influence numerous aspects of the lives of primates, including net energy balance (and therefore reproductive success of females) and maximum group size. Despite their potential impact, there has been no systematic comparison of different measures of travel distance. We compared three measures of travel distance in 30 min (actual distance of individuals, straight-line distance of individuals, and straight-line distance of groups) and their ratios in a small group and a large group of vervet monkeys (Cercopithecus aethiops) and between the large group of vervets and a group of patas monkeys (Erythrocebus patas) of roughly similar size. The large group of vervets traveled farther than the small group regardless of the measure used, but the ratios of the different measures were not significantly different between those groups. Patas monkeys traveled significantly farther than the large group of vervets regardless of the measure used. In both vervets and patas, straight-line distances of individuals (ISLD) and groups (GSLD) underestimated actual distances traveled by individuals (IAD), but the degree to which they did so differed between species. IAD is more accurate than the other two measures and is preferred for studies of energetics and individual reproductive success, although ISLD or GSLD may be substituted when the ratios of IAD/ISLD or IAD/GSLD do not differ between groups or species. The ratio of IAD/ISLD was larger in vervets than in patas, suggesting that individual vervets meander more over short periods of time than patas. The ratio of ISLD/GSLD was larger in patas than in vervets, suggesting that patas move at angles or across the group's center-of-mass whereas vervets move more consistently along with others in their group. This has implications for the formation of spatial subgroups and alliances within groups.