Geometric characters of the radius and tibia in Macaca mulatta and Macaca fascicularis

Relative leg-to-arm skeletal strength proportions in orangutans by species and sex

Among extant great apes, orangutans climb most frequently. However, Bornean orangutans (Pongo pygmaeus) exhibit higher frequencies of terrestrial locomotion than do Sumatran orangutans (Pongo abelii). Variation in long bone cross-sectional geometry is known to reflect differential loading of the limbs. Thus, Bornean orangutans should show greater relative leg-to-arm strength than their Sumatran counterparts. Using skeletal specimens from museum collections, we measured two cross-sectional geometric measures of bone strength: the polar section modulus (Zpol) and the ratio of maximum to minimum area moments of inertia (Imax/Imin), at the midshaft of long bones in Bornean (n = 19) and Sumatran adult orangutans (n = 12) using medical CT and peripheral quantitative CT scans, and compared results to published data of other great apes. Relative leg-to-arm strength was quantified using ratios of femur and tibia over humerus, radius, and ulna, respectively. Differences between orangutan species and between sexes in median ratios were assessed using Wilcoxon rank sum tests. The tibia of Bornean orangutans was stronger relative to the humerus and the ulna than in Sumatran orangutans (p = 0.008 and 0.025, respectively), consistent with behavioral studies that indicate higher frequencies of terrestrial locomotion in the former. In three Zpol ratios, adult female orangutans showed greater leg-to-arm bone strength compared to flanged males, which may relate to females using their legs more during arboreal locomotion than in adult flanged males. A greater amount of habitat discontinuity on Borneo compared to Sumatra has been posited as a possible explanation for observed interspecific differences in locomotor behaviors, but recent camera trap studies has called this into question. Alternatively, greater frequencies of terrestriality in Pongo pygmaeus may be due to the absence of tigers on Borneo. The results of this study are consistent with the latter explanation given that habitat continuity was greater a century ago when our study sample was collected.

Coming to the Caribbean-acclimation of Rhesus macaques (Macaca mulatta) at Cayo Santiago

OBJECTIVES

To investigate whether the Cayo Santiago, Puerto Rico (Latitude: 18.1564°N; temperature range 19°C to 32°C) rhesus macaque population has acclimated to their tropical island conditions since arriving from Lucknow, India (Latitude: 26.8470°N; temperature range 8°C to 41°C) in 1938.

MATERIALS AND METHODS

Using the derived skeletal collection, measurements were taken of long bone lengths, diaphyseal circumference, and body weight using 635 (237 males and 398 females) skeletally mature individuals. Measurements sampled colony members born over a 51-year time span at Cayo Santiago, from 1951 to 2002.

RESULTS

Results demonstrated that body weights and diaphyseal circumferences significantly declined in both males and females. Long bone lengths relative to body weight and diaphyseal circumference also increased in females. Whereas body weight, long bone length and diaphyseal circumference declined at near parallel rates in males.

DISCUSSION

The population has acclimated to homogenous, tropical, conditions of the Caribbean island since their arrival over 80 years ago. Trends in both sexes aligned with Bergmann's rule, though females displayed a greater decline in body weight, as well as greater affinity with Allen's rule, than did males. Buffering effects related to male competition may be responsible for this discrepancy. Overall, the Cayo Santiago populations, as shown over a significant period (1951-2002) of their history, have acclimated to their island conditions by decreasing in size and altering body proportions.

Flexible intentions: An Active Inference theory

We present a normative computational theory of how the brain may support visually-guided goal-directed actions in dynamically changing environments. It extends the Active Inference theory of cortical processing according to which the brain maintains beliefs over the environmental state, and motor control signals try to fulfill the corresponding sensory predictions. We propose that the neural circuitry in the Posterior Parietal Cortex (PPC) compute flexible intentions-or motor plans from a belief over targets-to dynamically generate goal-directed actions, and we develop a computational formalization of this process. A proof-of-concept agent embodying visual and proprioceptive sensors and an actuated upper limb was tested on target-reaching tasks. The agent behaved correctly under various conditions, including static and dynamic targets, different sensory feedbacks, sensory precisions, intention gains, and movement policies; limit conditions were individuated, too. Active Inference driven by dynamic and flexible intentions can thus support goal-directed behavior in constantly changing environments, and the PPC might putatively host its core intention mechanism. More broadly, the study provides a normative computational basis for research on goal-directed behavior in end-to-end settings and further advances mechanistic theories of active biological systems.

Anthropogenic effects on body size and growth in lab-reared and free-ranging Macaca mulatta

The impact of anthropogenic pressures upon primates is increasingly prevalent, and yet the phenotypic aspects of these impacts remain understudied. Captive environments can pose unique pressures based on factors like physical activity levels and caloric availability; thus, maturation patterns should vary under differing captive conditions. Here, we evaluate the development and growth of two Macaca mulatta populations (N = 510) with known chronological ages between 9 months and 16 years, under different levels of captive management, to assess the impact of varying anthropogenic environments on primates. To track growth, we scored 13 epiphyseal fusion locales across long bones in a skeletal sample of lab-reared M. mulatta (n = 111), including the right tibia, femur, humerus, ulna, and radius. We employed a three-tier scoring system, consisting of "0" (unfused to diaphysis), "1" (fusing), and "2" (fused). To record body size, we collected five linear measures of these long bones, from the proximal and distal ends, and total lengths. Means and standard deviations were generated to compare samples; t-tests were used to determine significant differences between means. These values were compared to available data on the free-ranging, provisioned M. mulatta population of Cayo Santiago. The free-ranging monkeys (n = 274) were found to exhibit larger linear skeletal lengths (p < 0.05) than lab-reared specimens. Generally, the free-ranging macaques reached fusion at earlier chronological ages and exhibited an extended duration of the fusing growth stage. These observations may reflect the protein-rich diet provided to free-ranging monkeys and conversely, restricted movement and relaxed natural selection experienced by lab-reared monkeys.

The forearm and hand musculature of semi-terrestrial rhesus macaques (Macaca mulatta) and arboreal gibbons (fam.Hylobatidae). Part II. Quantitative analysis

Nonhuman primates have a highly diverse locomotor repertoire defined by an equally diverse hand use. Based on how primates use their hands during locomotion, we can distinguish between terrestrial and arboreal taxa. The 'arboreal' hand is likely adapted towards high wrist mobility and grasping, whereas the 'terrestrial' hand will show adaptations to loading. While the morphology of the forearm and hand bones have been studied extensively, functional adaptations in the forearm and hand musculature to locomotor behaviour have been documented only scarcely. In this paper, we investigate the forelimb musculature of the highly arboreal gibbons (including Hylobates lar,Hylobates pileatus,Nomascus leucogenys,Nomascus concolor and Symphalangus syndactylus) and compare this with the musculature of the semi-terrestrial rhesus macaques (Macaca mulatta). Anatomical data from previous dissections on knuckle-walking bonobos (Pan paniscus) and bipedal humans (Homo sapiens) are also included to further integrate the analyses in the scope of catarrhine hand adaptation. This study indicates that the overall configuration of the arm and hand musculature of these primates is very similar but there are some apparent differences in relative size which can be linked to differences in forelimb function and which might be related to their specific locomotor behaviour. In macaques, there is a large development of wrist deviators, wrist and digital flexors, and m. triceps brachii, as these muscles are important during the different phases of palmi- and digitigrade quadrupedal walking to stabilize the wrist and elbow. In addition, their m. flexor carpi ulnaris is the most important contributor to the total force-generating capacity of the wrist flexors and deviators, and is needed to counteract the adducting torque at the elbow joint during quadrupedal walking. Gibbons show a relatively high force-generating capacity in their forearm rotators, wrist and digital flexors, which are important muscles in brachiation to actively regulate forward movement of the body. The results also stress the importance of the digital flexors in bonobos, during climbing and clambering, and in humans, which is likely linked to our advanced manipulation skills.

The relative efficacy of the cranium and os coxa for taxonomic assessment in macaques

OBJECTIVES

The cranium is generally considered more reliable than the postcranium for assessing primate taxonomy, although recent research suggests that pelvic shape may be equally reliable. However, little research has focused on intrageneric taxonomic discrimination. Here, we test the relative taxonomic efficacy of the cranium and os coxa for differentiating two macaque species, with and without considering sexual dimorphism.

MATERIALS AND METHODS

Geometric morphometric analyses were performed on cranial and os coxa landmarks for 119 adult Macaca fascicularis, M. mulatta, and Chlorocebus pygerythrus. Among-group shape variation was examined using canonical variates analyses. Cross-validated discriminant function analysis provided rates of correct group classification. Additionally, average morphological distances were compared with neutral genetic distances.

RESULTS

Macaque species were clearly differentiated, both cranially and pelvically, when sex was not considered. Males were more often correctly classified based on the os coxa, while female classification rates were high for both morphologies. Female crania and male os coxa were differentiated approximately the same as genetic distance, while male crania were more similar (convergent), and female os coxa were more divergent than expected based on genetic distance.

DISCUSSION

The hypothesis that cranial and os coxal shape can be used to discriminate among macaque species was supported. The cranium was better at differentiating females, while the os coxa was better at differentiating male macaques. Hence, there is no a priori reason for preferring the cranium when assessing intragenetic taxonomic relationships, but the effects of high levels of sexual dimorphism must be corrected for to accurately assess taxonomic signatures.

Relationship between humeral geometry and shoulder muscle power among suspensory, knuckle-walking, and digitigrade/palmigrade quadrupedal primates

Shoulder morphology is functionally related to different patterns of locomotion in primates. To investigate this we performed a quantitative analysis of the relationship between cortical bone thickness (Cbt) of the muscle/tendon attachment site on the humerus and physiological cross-sectional area (PCSA) of the shoulder muscle in primates with different locomotory habits. The deltoid, subscapularis, supraspinatus, and infraspinatus were investigated. A chimpanzee, a gibbon, a baboon, two species of macaque, a lutong, a capuchin, and a squirrel monkey were included in the study. The total length of the humerus was measured and the values were converted into three-dimensional reconstructed data on a computer by computed tomography. The Cbt values were obtained from the volumes divided by the areas of the muscle/tendon attachment sites of the humerus by computer analysis. Muscle mass, muscle fascicle length, and muscle pennation angle were measured and PCSA was calculated using these parameters. A relatively high Cbt and small PCSA were characteristic of the gibbon. The gibbon's high Cbt suggests that passive tension in the muscle/tendon attachment site of suspensory primates (brachiators) may be greater than that of quadrupedal primates, whereas the relatively small PCSA indicates an association with a large amount of internal muscle fascia to endure the passive stress of brachiation. Although chimpanzees undertake some suspensory locomotion, the results for this species resemble those of the digitigrade/palmigrade quadrupedal primates rather than those of the suspensory primate. However, the deltoid and subscapularis in chimpanzee differ from those of the other primates and appear to be affected by the peculiar locomotion of knuckle-walking, i.e. the moment arm of forelimb in chimpanzees is relatively longer than that of digitigrade/palmigrade quadrupedal primates. Hence, a large PCSA in the deltoid and subscapularis may contribute to sustaining the body weight during locomotion. On the other hand, a thin cortical bone relative to a large PCSA was a feature of the cercopithecids, indicating that digitigrade/palmigrade quadrupedal locomotion produces less tension at the muscle/tendon attachment sites compared with that produced by brachiators.

Regional, ontogenetic, and sex-related variations in elastic properties of cortical bone in baboon mandibles

Understanding the mechanical features of cortical bone and their changes with growth and adaptation to function plays an important role in our ability to interpret the morphology and evolution of craniofacial skeletons. We assessed the elastic properties of cortical bone of juvenile and adult baboon mandibles using ultrasonic techniques. Results showed that, overall, cortical bone from baboon mandibles could be modeled as an orthotropic elastic solid. There were significant differences in the directions of maximum stiffness, thickness, density, and elastic stiffness among different functional areas, indicating regional adaptations. After maturity, the cortical bone becomes thicker, denser, and stiffer, but less anisotropic. There were differences in elastic properties of the corpus and ramus between male and female mandibles which are not observed in human mandibles. There were correlations between cortical thicknesses and densities, between bone elastic properties and microstructural configuration, and between the directions of maximum stiffness and bone anatomical axes in some areas. The relationships between bone extrinsic and intrinsic properties bring us insights into the integration of form and function in craniofacial skeletons and suggest that we need to consider both macroscopic form, microstructural variation, and the material properties of bone matrix when studying the functional properties and adaptive nature of the craniofacial skeleton in primates. The differences between baboon and human mandibles is at variance to the pattern of differences in crania, suggesting differences in bone adaption to varying skeletal geometries and loading regimes at both phylogenetic and ontogenetic levels.