Locomotor Habits Research Articles

Trophic Niche Differentiation in Rodents and Marsupials Revealed by Stable Isotopes.

Tropical rainforests support the greatest diversity of small mammals in the world, yet we have little understanding about the mechanisms that promote the coexistence of species. Diet partitioning can favor coexistence by lessening competition, and interspecific differences in body size and habitat use are usually proposed to be associated with trophic divergence. However, the use of classic dietary methods (e.g. stomach contents) is challenging in small mammals, particularly in community-level studies, thus we used stable isotopes (δ13C and δ15N) to infer about trophic niche. We investigated i) how trophic niche is partitioned among rodent and marsupial species in three Atlantic forest sites and ii) if interspecific body size and locomotor habit inequalities can constitute mechanisms underlying the isotopic niche partitioning. We found that rodents occupied a broad isotopic niche space with species distributed in different trophic levels and relying on diverse basal carbon sources (C3 and C4 plants). Surprisingly, on the other hand, marsupials showed a narrow isotopic niche, both in δ13C and δ15N dimensions, which is partially overlapped with rodents, contradicting their description as omnivores and generalists proposed classic dietary studies. Although body mass differences did not explained the divergence in isotopic values among species, groups of species with different locomotor habit presented clear differences in the position of the isotopic niche space, indicating that the use of different forest strata can favor trophic niche partitioning in small mammals communities. We suggest that anthropogenic impacts, such as habitat modification (logging, harvesting), can simplify the vertical structure of ecosystems and collapse the diversity of basal resources, which might affect negatively small mammals communities in Atlantic forests.

Body mass and locomotor habits of the smallest darter, Anhinga minuta (Aves, Anhingidae)

During the Neogene of South America, Anhingidae was represented by several species, mainly with greater sizes than the extant members. In the present contribution, body mass and locomotor habits of Anhinga minuta, the smallest known darter, were inferred. Body mass was estimated using two methods, one with measures of a tibiotarsus (the holotype) and the other, with measurements of a humerus; locomotor habits were inferred through muscular reconstructions and wing parameters (wing span, wing area and wing loading). Estimates of wing span and wing area were based on the length of humerus, assuming a condition of isometry with respect to Anhinga anhinga; wing loading was obtained through a relation formula between wing area and body mass. The results obtained indicate a body mass of about 729 g, a wing span of 0.958 m, a wing area of 0.117 m2 and a corresponding wing loading of 61 N/m2. These values and also the proximal insertion of the musculus pectoralis are consistent with those of a soaring bird but with more frequent flapping than extant anhingids. Furthermore, the inferred musculature for tibiotarsus indicates abilities for swimming, climbing and moving through the vegetation as in extant representatives.

Observations of terrestrial locomotion in wild Polypterus senegalus from Lake Albert, Uganda

Polypterids, the most basal actinopterygians, are a group of fish long-considered living fossils and holding a key position for understanding fish and tetrapod evolution. Knowledge of the natural history of Polypterus is limited, their having been studied in little detail since the early 1900s. The locomotory habits of wild Polypterus senegalus from Lake Albert, Uganda, were investigated in 2014. High-speed videography demonstrated the capability of large Polypterus to move overland successfully. Contrary to previous evidence, field observations found that terrestrial locomotion in Polypterus is not inherently restricted by body size. Evidence that Polypterus exhibit this behaviour as part of their natural life history can be found in the existence of environmental challenges and the presence of adaptations for amphibious life.

Hind limb morphometry of terror birds (Aves, Cariamiformes, Phorusrhacidae): functional implications for substrate preferences and locomotor lifestyle

ABSTRACTThe hind limbs of birds have long been considered a key feature in the conquest of different environments. However, the high level of morphological diversity encountered complicates the foundation of a good theoretical correlation between morphology, locomotor habits and substrate preference and this, in turn, complicates palaeobiological interpretations. Phorusrhacids (Aves, Cariamiformes) are a good example, since they have been unequivocally categorised as terrestrial birds due to their reduced forelimbs; and as apex predators with the ability to pursue prey based only on their hind limb morphology. Multivariate techniques (PCA and discriminant analysis), based on traditional metrics and geomorphometrics of the hind limb and pelvis, were applied in order to explore terrestriality and cursoriality in phorusrhacids. Although several groups of birds could be identified, when looking solely at hind limb metrics, some phorusrhacids appear to be associated with walking birds, while others are associated with cursorial birds. However, the pelvis separates cursorial birds and phorusrhacids from walking and wading birds. This scenario is complicated further by a lack of clear definition of the different locomotor modes and substrate preferences in extant birds, and this makes it difficult to confirm phorusrhacid cursoriality based solely on morphometrics. However, some qualitative features of the pelvis and foot make the picture a little clearer. To study limb adaptations in fossil birds, a more holistic study, with an emphasis on qualitative features of the whole posterior locomotor module, is necessary, since morphometrics leaves some issues unresolved. A comparison with the wings is also needed, in order to make a more complete analysis of locomotor behaviour.

Do functional demands associated with locomotor habitat, diet, and activity pattern drive skull shape evolution in musteloid carnivorans?

A major goal of evolutionary studies is to better understand how complex morphologies are related to the different functions and behaviours in which they are involved. For example, during locomotion and hunting behaviour, the head and the eyes have to stay at an appropriate level in order to reliably judge distance as well as to provide postural information. The morphology and orientation of the orbits and cranial base will have an impact on eye orientation. Consequently, variation in orbital and cranial base morphology is expected to be correlated with aspects of an animal's lifestyle. In this study, we investigate whether the shape of the skull evolves in response to the functional demands imposed by ecology and behaviour using geometric morphometric methods. We test if locomotor habitats, diet, and activity pattern influence the shape of the skull in musteloid carnivorans using (M)ANOVAs and phylogenetic (M)ANOVAs, and explore the functional correlates of morphological features in relation to locomotor habitats, diet, and activity pattern. Our results show that phylogeny, locomotion and, diet strongly influence the shape of the skull, whereas the activity pattern seems to have a weakest influence. We also show that the locomotor environment is highly integrated with foraging and feeding, which can lead to similar selective pressures and drive the evolution of skull shape in the same direction. Finally, we show similar responses to functional demands in musteloids, a super family of close related species, as are typically observed across all mammals suggesting the pervasiveness of these functional demands.

Interpreting locomotor biomechanics from the morphology of human footprints

Fossil hominin footprints offer unique direct windows to the locomotor behaviors of our ancestors. These data could allow a clearer understanding of the evolution of human locomotion by circumventing issues associated with indirect interpretations of habitual locomotor patterns from fossil skeletal material. However, before we can use fossil hominin footprints to understand better the evolution of human locomotion, we must first develop an understanding of how locomotor biomechanics are preserved in, and can be inferred from, footprint morphologies.In this experimental study, 41 habitually barefoot modern humans created footprints under controlled conditions in which variables related to locomotor biomechanics could be quantified. Measurements of regional topography (depth) were taken from 3D models of those footprints, and principal components analysis was used to identify orthogonal axes that described the largest proportions of topographic variance within the human experimental sample. Linear mixed effects models were used to quantify the influences of biomechanical variables on the first five principal axes of footprint topographic variation, thus providing new information on the biomechanical variables most evidently expressed in the morphology of human footprints. The footprint's overall depth was considered as a confounding variable, since biomechanics may be linked to the extent to which a substrate deforms. Three of five axes showed statistically significant relationships with variables related to both locomotor biomechanics and substrate displacement; one axis was influenced only by biomechanics and another only by the overall depth of the footprint. Principal axes of footprint morphological variation were significantly related to gait type (walking or running), kinematics of the hip and ankle joints and the distribution of pressure beneath the foot. These results provide the first quantitative framework for developing hypotheses regarding the biomechanical patterns reflected by fossil hominin footprints by demonstrating the statistically significant effects of specific kinematic variables on patterns of variation in footprint topography.

Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents

ABSTRACTEstimating body mass and locomotor habits of fossil rodents is challenging for taxa without available postcranial material. Although equations exist to estimate body mass from dental dimensions based on extant rodents, the applicability of such equations is doubtful given that modern rodents have evolved highly specialized teeth, whereas fossil taxa often exhibit a much less derived condition. For the present study, 11 cranial variables from a sample of 203 extant rodents of known body mass representing a range of taxonomic groups were assessed using reduced major axis (RMA) regression. The results show a strong correlation between body mass and each of the 11 cranial variables. The best estimators for body mass are skull length and cheek-tooth area, whereas the least reliable measures are palate length, and foramen magnum, as well as braincase and occipital condyle dimensions. We estimated body mass for specimens of five fossil Ischyromyidae rodents for which body mass had never been estimated (Paramys copei, P. delicatus, Reithroparamys delicatissimus, Rapamys atramontis, and Ischyromys typus). Principal components and canonical variates analyses based on 10 cranial dimensions for 103 members of Sciuromorpha demonstrate that a relationship exists between locomotor habits and cranial variables in this suborder. In these analyses, early ischyromyids are all placed in the terrestrial group with Aplodontia, Marmota, Cynomys, and Spermophilus. This contradicts previous hypotheses regarding early rodent locomotion, which suggested that they were arboreal or more generalized in their habits.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Bertrand, O. C., M. A. Schillaci, and M. T. Silcox. 2015. Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1014905.

Sexual Dimorphisms of Appendicular Musculoskeletal Morphology Related to Social Display in Cuban Anolis Lizards.

In Anolis lizards, sexual dimorphism has been reported in morphological and ecological traits. Males show larger body size and longer limbs related to territorial combat and courtship display with the dewlap. Although functional-anatomical traits are closely related to locomotor behaviors, differences between sexes in musculoskeletal traits on limbs remain unclear. We explored the relationships among sexual dimorphisms in musculoskeletal morphology, habitat, and locomotor traits in Anolis lizards. Specifically, we examined appendicular musculoskeletal morphology in three species of Cuban Anolis by measuring muscle mass and lengths of moment arms. Through comparisons of crossing locomotion, we found that the runner species possessed larger extensors in hindlimbs, which are advantageous for running, whereas the masses of the humeral and femoral retractors were larger in climber species, allowing these lizards to hold up their bodies and occupy tree substrates. Comparisons between the sexes showed different trends among the three species. Males of A. porcatus, which inhabit narrow branches or leaves, had stronger elbow extensors that maintain the display posture. In contrast, males of A. sagrei, which occupy broad surfaces, did not show sexual differences that affected social display. Moreover, A. bartschi indicated sexual differences despite the absence of dewlapping behavior. Our findings suggest that both sexes show fundamentally similar relationships between muscular morphology and locomotor habits to adapt arboreal or terrestrial substrates, and yet sexual dimorphism in forelimb muscles may additionally affected by male specific display with the dewlap.

Reconciling the convergence of supraspinous fossa shape among hominoids in light of locomotor differences

Differences in scapular morphology between modern humans and the African and lesser apes are associated with the distinct locomotor habits of these groups. However, several traits, particularly aspects of the supraspinous fossa, are convergent between Homo and Pongo-an unexpected result given their divergent locomotor habits. Many morphological assessments of the scapula rely on the limited number of static landmarks available, and traditional approaches like these tend to oversimplify scapular shape. Here, we present the results of two geometric morphometric (GM) analyses of hominoid supraspinous fossa shape-one employing five homologous landmarks and another with 83 sliding semilandmarks-alongside those of traditional methods to evaluate if three-dimensional considerations of fossa shape afford more comprehensive insights into scapular shape and functional morphology. Traditional measures aligned Pongo and Homo with narrow and transversely oriented supraspinous fossae, whereas African ape and Hylobates fossae are broader and more obliquely situated. However, our GM results highlight that much of the convergence between Homo and Pongo is reflective of their more medially positioned superior angles. These approaches offered a more complete assessment of supraspinous shape and revealed that the Homo fossa, with an intermediate superior angle position and moderate superoinferior expansion, is actually reminiscent of the African ape shape. Additionally, both Pongo and Hylobates were shown to have more compressed fossae, something that has not previously been identified through traditional analyses. Thus, the total morphological pattern of the Pongo supraspinous fossa is unique among hominoids, and possibly indicative of its distinctive locomotor habits.

Analysis of the forearm rotational efficiency in extant hominoids: New insights into the functional implications of upper limb skeletal structure

The greatly diversified locomotor behaviors in the Hominoidea impose different mechanical requirements in the upper limb of each species. As forearm rotation has a major role in locomotion, the skeletal structures involved in this movement may display differences among taxa that reflect functional adaptations. To test this, we use a biomechanical model that quantifies the rotatory capacity of pronator teres (rotational efficiency) from skeletal measurements. Using a large sample of hominoids, we aim to identify the morphological adaptations that confer differences in the mechanics of forearm motion and to assess the functional advantage of these adaptations. Forearm positions along the pronation–supination range where rotational efficiency is maximal depend on the orientation of the humeral medial epicondyle and differ among taxa. Our results indicate that these are related to locomotor mode. Knuckle-walkers exhibit a medial epicondyle more posteriorly directed, which, in elbow angles close to extension, causes rotational efficiency to be maximal in pronated positions of the forearm. Species with a significant amount of arboreal locomotion, such as vertical climbing, i.e., Pongo spp., Pan troglodytes and Gorilla gorilla, display more proximally oriented epicondyles, which, in elbow flexion, leads to maximum rotational efficiencies in supinated positions of the forearm. Hylobatidae, with the less posteriorly and proximally oriented epicondyle, show their maximum rotational efficiencies closer to the forearm neutral position throughout most of the flexion–extension range, which may be linked to brachiation in this taxon. In humans, the epicondylar orientation and thus the positions of the maximum rotational efficiencies fall between arboreal and terrestrial hominoids. This may be related to the enhanced manipulative skills of the upper limb. In conclusion, the current analysis indicates that the orientation of the humeral medial epicondyle is linked to the locomotor habits of extant hominoids and therefore can be used for locomotor inferences in fossil taxa.

Community Structure of South American Carnivores in the Past and Present

Fil: Prevosti, Francisco Juan. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales Bernardino Rivadavia; Argentina. Universidad Nacional de Lujan; Argentina

Functional morphology and comparative anatomy of appendicular musculature in Cuban Anolis lizards with different locomotor habits.

We examined the diversity of the musculoskeletal morphology in the limbs of Anolis lizards with different habitats and identified variations in functional and morphological adaptations to different ecologies or behaviors. Dissection and isolation of 40 muscles from the fore- and hindlimbs of five species of Anolis were performed, and the muscle mass and length of the moment arm were compared after body size effects were removed. Ecologically and behaviorally characteristic morphological differences were observed in several muscles. Well-developed hindlimb extensors were observed in ground-dwelling species, A. sagrei and A. bremeri, and were considered advantageous for running, whereas adept climber species possessed expanded femoral retractors for weight-bearing during climbing. Moreover, morphological variations were observed among arboreal species. Wider excursions of the forelimb joint characterized A. porcatus, presumably enabling branch-to-branch locomotion, while A. equestris and A. angusticeps possessed highly developed adductor muscles for grasping thick branches or twigs. These findings suggest divergent evolution of musculoskeletal characteristic in the limbs within the genus Anolis, with correlations observed among morphological traits, locomotor performance, and habitat uses.

Reconsidering locomotor habits and life style of the Balearic insular giant rodent Hypnomys Bate, 1918 from the allometry of the limb long bones

The genus Hypnomys Bate, 1918 includes some endemic Neogene chronospecies from Mallorca and Menorca, evolved in insularity conditions from the Lower Pliocene to the human arrival at the archipelago. The study of the allometric proportions (functional length and sagittal diameter) of the limbs’ long bones of Hypnomys eliomyoides Agusti, 1980 (Lower Pleistocene of Menorca), Hypnomys sp. (Pleistocene of Menorca), Hypnomys onicensis (Reumer, 1994) (Upper Pliocene-Lower Pleistocene of Mallorca) and Hypnomys morpheus Bate, 1918 (Upper Pleistocene of Mallorca) only indicates small differences with the garden dormouse (Eliomys quercinus Linnaeus, 1776) and other mainland rodents and insectivores. The study about the locomotion type by means of Similarity Matrix of Bou indicates that 1) Hypnomys spp. presents the greater similarities with the arboreal, jumping and gliding species; 2) the jumping and digging life style of Hypnomys spp. are more developed (25%) than in E. quercinus; 3) the walking locomotion of Hypnomys spp. is only a 7% greater than in E. quercinus; and 4) the degree of similarity with the gliding locomotion of Hypnomys sp. is greater (17%) than in E. quercinus. Some of this data indicates that Hypnomys spp. was better adapted to the arboreal life than E. quercinus, due to the relationship between the jumping and gliding capacities with the arboreal locomotion. The greater similarity of Hypnomys spp. with the walking locomotion does not necessarily involve more terrestrial habits. Our interpretation contrasts with earlier conclusions of Hypnomys life style (Hypnomys cf. onicensis and Hypnomys morpheus) where the terrestrial locomotion was overestimated in front of the arboreal locomotion. The locomotion type of Hypnomys would be related with the presence of important forest masses on the Balearic Islands during the Neogene, a different degree of environmental stress from that of the mainland ecosystems and a probable expansion of the ecological niche occupied by these species.

Heterochrony and post‐natal growth in mammals – an examination of growth plates in limbs

Mammals display a broad spectrum of limb specializations coupled with different locomotor strategies and habitat occupation. This anatomical diversity reflects different patterns of development and growth, including the timing of epiphyseal growth plate closure in the long bones of the skeleton. We investigated the sequence of union in 15 growth plates in the limbs of about 400 specimens, representing 58 mammalian species: 34 placentals, 23 marsupials and one monotreme. We found a common general pattern of growth plate closure sequence, but one that is universal neither between species nor in higher-order taxa. Locomotor habitat has no detectable correlation with the growth plate closure sequence, but observed patterns indicate that growth plate closure sequence is determined more strongly through phylogenetic factors. For example, the girdle elements (acetabulum and coracoid process) always ossify first in marsupials, whereas the distal humerus is fused before the girdle elements in some placentals. We also found that heterochronic shifts (changes in timing) in the growth plate closure sequence of marsupials occur with a higher rate than in placentals. This presents a contrast with the more limited variation in timing and morphospace occupation typical for marsupial development. Moreover, unlike placentals, marsupials maintain many epiphyses separated throughout life. However, as complete union of all epiphyseal growth plates is recorded in monotremes, the marsupial condition might represent the derived state.

The Structure of the Mammalian Predator Guild in the Santa Cruz Formation (Late Early Miocene)

The Santa Cruz Formation (late early Miocene, Santacrucian age) registers 11 species of mammalian predators (Metatheria, Sparassodonta). Together with large carnivorous flightless birds, they comprised the terrestrial predator guild. The Santacrucian sparassodonts were diverse in body size, had different locomotory habits, and were primarily hypercarnivores. The objective of this work is to analyze the guild structure of the sparassodonts of the Santa Cruz Formation, using the variables of body mass, diet, and locomotion as proxies. Furthermore, we analyze the interaction with other predators and potential prey. The univariated test V of Poole and Rathcke and the multivariated test of Clark-Evans were used to construct the models. In the multivariate test, we made a Principal Component Analysis to resume and standardize the variables. With body mass and locomotion we obtained an evenly spaced pattern of segregation for the sparassodont species, being non-significant and significant, respectively. The pattern was aggregated and significant only with diet. The analysis of all variables together resulted in an evenly spaced and significant pattern, which is consistent with character displacements (segregation of species throughout the morphospace) that would help to diminish interspecific competition during the Santacrucian age and would allow selection of prey species of different sizes and substrate specializations. When the body size pattern of predator birds and sparassodonts were plotted together, the pattern is evenly spaced and non-significant. Other factors, including locomotion, would differentiate these species and their ecological niches.

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.

Rotator cuff muscle function and its relation to scapular morphology in apes

It is widely held that many differences among primate species in scapular morphology can be functionally related to differing demands on the shoulder associated with particular locomotor habits. This perspective is largely based on broad scale studies, while more narrow comparisons of scapular form often fail to follow predictions based on inferred differences in shoulder function. For example, the ratio of supraspinous fossa/infraspinous fossa size in apes is commonly viewed as an indicator of the importance of overhead use of the forelimb, yet paradoxically, the African apes, the most terrestrial of the great apes, have higher scapular fossa ratios than the more suspensory orangutan. The recent discovery of several nearly complete early hominin scapular specimens, and their apparent morphological affinities to scapulae of orangutans and gorillas rather than chimpanzees, has led to renewed interest in the comparative analysis of human and extant ape scapular form. To facilitate the functional interpretation of differences in ape scapulae, particularly in regard to relative scapular fossa size, we used electromyography (EMG) to document the activity patterns in all four rotator cuff muscles in orangutans and gibbons, comparing the results with previously published data for chimpanzees.The EMG results indicate that the distinctive contributions of each cuff muscle to locomotion are the same in the three ape species, failing to support inferences of differences in rotator cuff function based on relative scapular fossa size comparisons. It is also shown that relative scapular fossa size is not in fact a good predictor of either the relative masses or cross-sectional areas of the rotator cuff muscles in apes, and relative fossa size gives a false impression of the importance of individual cuff muscles to locomotor differences among apes. A possible explanation for the disparity between fossa and muscle size relates to the underappreciated role of the scapular spine in structural reinforcement of the blade.

Striatal ensembles continuously represent animals kinematics and limb movement dynamics during execution of a locomotor habit

The sensorimotor striatum contributes to the normal execution of motor habits but the mechanisms underlying this function are largely unknown[1]. Motor habits are stereotyped sequences of movements learned through a long trial-and-error process, automatically triggered by a set of sensory cues and that tend to persist despite outcome degradation (e.g. reward omission). We found that rats running on a treadmill become proficient in a fixed time interval estimation task by developing a highly stereotyped locomotor routine. Consistently with the definition of habits, the routine was acquired slowly (at least 2 months of daily practice), and once learned, it persisted for several sessions when the rewarding outcome was omitted. We took advantage of this unexpected behavior and used tetrode arrays to record the spiking activity of dorsolateral striatal ensembles while rats perform the locomotor habit. We report sequential activations of striatal neurons during the entire execution of the task. Importantly, we found that the firing rate of a large fraction of neurons was either locked to the locomotor limb movements or correlated with the kinematics of the habit (running speed, acceleration, position and time). These results contrast with the long-standing view that striatum is mainly concerned with action selection[2]. Rather movements and task kinematics encoding suggest that the striatum continuously control the execution of habitual action. Additional experiments are currently being performed to further investigate this hypothesis.

Trabecular bone anisotropy and orientation in an Early Pleistocene hominin talus from East Turkana, Kenya

Among the structural properties of trabecular bone, the degree of anisotropy is most often found to separate taxa with different habitual locomotor modes. This study examined the degree of anisotropy, the elongation, and primary orientation of trabecular bone in the KNM-ER 1464 Early Pleistocene hominin talus as compared with extant hominoid taxa. Modern human tali were found to have a pattern of relatively anisotropic and elongated trabeculae on the lateral aspect, which was not found in Pan, Gorilla, Pongo, or KNM-ER 1464. Trabecular anisotropy in the fossil talus most closely resembled that of the African apes except for a region of high anisotropy in the posteromedial talus. The primary orientation of trabeculae in the anteromedial region of KNM-ER 1464 was strikingly different from that of the great apes and very similar to that of modern humans in being directed parallel to the talar neck. These results suggest that, relative to that of modern humans, the anteromedial region of the KNM-ER 1464 talus may have transmitted body weight to the midfoot in a similar manner while the lateral aspect may have been subjected to more variable loading conditions.

Humeral Cross‐Sectional Shape in Suspensory Primates and Sloths

Studies on the cross‐sectional geometry of long bones in African apes have documented that shape ratios derived from second moments of area about principle axes (e.g., Imax/Imin) are often correlated with habitual locomotor behaviors. For example, humeral cross‐sections tend to appear more circular in more arboreal and forelimb suspensory chimpanzees compared with terrestrial quadrupedal gorillas. These data support the hypothesis that cross‐sections that are more circular in shape are adapted for multidirectional loading regimes and bending moments encountered when using acrobatic locomotor behaviors. Whether a more circular humerus reflects greater use of forelimb suspension in other primates and nonprimate mammals is unknown. In this study, cross‐sections at or near midshaft of the humerus were obtained from anthropoid primates that differ in their use of forelimb suspension, as well as from two genera of suspensory sloths. Imax/Imin ratios were compared within and between groups, and correlations were made with behavioral data. In broad comparisons, observed differences in morphology follow predicted patterns. Humeri of suspensory sloths are circular. Humeri of the more suspensory hominoids tend to be more circular than those of quadrupedal taxa. Humeri of the suspensory atelines are similar to hominoids, while those of Cebus are more like nonsuspensory cercopithecoids. There is, however, considerable overlap between taxa and within finer comparisons variation between species are not in the predicted direction. Thus, although Imax/Imin ratios of the humerus are informative for characterizing generalized locomotor modes (i.e., forelimb suspensory vs. quadrupedal), additional structural information is needed for more fine‐grained assessments of locomotion. Anat Rec, 296:545–556, 2013. © 2013 Wiley Periodicals, Inc.