Hip extensor mechanics and the evolution of walking and climbing capabilities in humans, apes, and fossil hominins.

The evolutionary emergence of humans' remarkably economical walking gait remains a focus of research and debate, but experimentally validated approaches linking locomotor capability to postcranial anatomy are limited. In this study, we integrated 3D morphometrics of hominoid pelvic shape with experimental measurements of hip kinematics and kinetics during walking and climbing, hamstring activity, and passive range of hip extension in humans, apes, and other primates to assess arboreal-terrestrial trade-offs in ischium morphology among living taxa. We show that hamstring-powered hip extension during habitual walking and climbing in living apes and humans is strongly predicted, and likely constrained, by the relative length and orientation of the ischium. Ape pelves permit greater extensor moments at the hip, enhancing climbing capability, but limit their range of hip extension, resulting in a crouched gait. Human pelves reduce hip extensor moments but permit a greater degree of hip extension, which greatly improves walking economy (i.e., distance traveled/energy consumed). Applying these results to fossil pelves suggests that early hominins differed from both humans and extant apes in having an economical walking gait without sacrificing climbing capability. Ardipithecus was capable of nearly human-like hip extension during bipedal walking, but retained the capacity for powerful, ape-like hip extension during vertical climbing. Hip extension capability was essentially human-like in Australopithecus afarensis and Australopithecus africanus, suggesting an economical walking gait but reduced mechanical advantage for powered hip extension during climbing.

Strength properties of extant hominoid hallucal and pollical metapodials.

Functional comparisons of cortical bone strength properties between hominoid hallucal and pollical metapodials (Mt1 and Mc1, respectively) are lacking. Determining which of these two elements is stronger, and by how much, could be informative because the hallux and pollex are used differently both within and among extant hominoids during locomotion and manipulation (i.e., functional differentiation between autopod pairs). Here, we compare Mt1 and Mc1 midshaft cortical area, polar section modulus, and polar second moment of area, calculated from high-resolution computed tomography images in humans (n = 21), chimpanzees (n = 47), gorillas (n = 24), orangutans (n = 20), siamangs (n = 8), and gibbons (n = 21). Intraindividual comparisons between bones within species were made using paired t-tests. Log10-transformed Mt1:Mc1 ratios were created to assess relative strength asymmetry between bones, and interspecific comparisons of these proportions were made using analyses of variance. Absolute strength differences between the Mt1 and Mc1 for all variables were significantly larger in the Mt1 for all species (p < 0.05). Significant differences across species in Mt1:Mc1 proportions were also found, thereby demonstrating that strength asymmetry between bones differs among taxa (p < 0.05); asymmetry was lowest in orangutans, intermediate in gorillas, and greatest in humans, chimpanzees, siamangs, and gibbons. These findings support the hypothesis that the Mt1 is better adapted structurally than the Mc1 for bearing mechanical loads during weight support of locomotion in all extant hominoids and that pedal hallucal grasping likely engenders higher loads than manual pollical grasping in nonhuman hominoids. Thus, functional differentiation in autopod use within and among hominoids is reflected in hallucal and pollical metapodial strength properties.

Comparison of point transect distance and traditional acoustic point-count sampling of hoolock gibbons in Htamanthi Wildlife Sanctuary, Myanmar.

Effective conservation demands more accurate and reliable methods of survey and monitoring of populations. Surveys of gibbon populations have relied mostly on mapping of groups in "listening areas" using acoustical point-count data. Traditional methods of estimating density in have usually used counts of gibbon groups within fixed-radius areas or areas bounded by terrain barriers to sound transmission, and have not accounted for possible decline in detectability with distance. In this study we sampled the eastern hoolock gibbon (Hoolock leucogenys) population in Htamanthi Wildlife Sanctuary (WS), Myanmar, using two methods: the traditional point-count method with fixed-radius listening areas, and a newer method using point-transect Distance analysis from a sample point established in the center of each listening point array. The basic data were obtained by triangulating on singing groups from four LPs for 4 days, in 10 randomly selected sample areas within the sanctuary. The point transect method gave an average density of 3.13 groups km-2 , higher than the estimates of group density within fixed-radius areas without correction for detectability. A new method of analysis of singing probability per day (p[1]) gave an estimate of 0.547. Htamanthi WS is an important conservation area containing an estimated 7000 (95% confidence interval: 5000-10,000) hoolock groups. Surveys at Htamanthi WS and locations in the Hukaung Valley suggest that the extensive evergreen forests in northern Myanmar have the capacity to support 2-4 (average about 3) groups of hoolock gibbons per km2 , but most forests in its range have yet to be surveyed.

Monitoring occurrence, extinction, and colonization probabilities for gibbon populations.

All gibbon species (Family: Hylobatidae) are considered threatened with extinction and recognized on the International Union for Conservation of Nature Red List of Threatened Species. Because gibbons are one of the most threatened families of primates, monitoring their status is now critically important. Long-term monitoring programs applying occupancy approaches, in addition to assessing occurrence probability, improves understanding of other population parameters such as site extinction or colonization probabilities, which elucidate temporal and spatial changes and are therefore important for guiding conservation efforts. In this study, we used multiple season occupancy models to monitor occurrence, extinction, and colonization probabilities for northern yellow-cheeked crested gibbon Nomascus annamensis in three adjacent protected areas in the Central Annamites mountain range, Vietnam. We collected data at 30 listening posts in 2012, 2014, and 2016 using the auditory point count method. Occurrence probabilities were highest in 2012 (0.74, confidence interval [CI]: 0.56-0.87) but slightly lower in 2014 (0.66, CI: 0.51-0.79) and 2016 (0.67, CI: 0.49-0.81). Extinction probabilities during the 2012-2014 and 2014-2016 intervals were 0.26 (0.14-0.44) and 0.25 (0.12-0.44), respectively. Colonization probabilities during 2012-2014 were 0.44 (0.19-0.73) and between 2014 and 2016 was 0.51 (0.26-0.75). Although local site extinctions have occurred, high recolonization probability helped to replenish the unoccupied sites and kept the occurrence probability stable. Long-term monitoring programs which use occurrence probability alone might not fully reveal the true dynamics of gibbon populations. We strongly recommend including multiple season occupancy models to monitor occurrence, extinction, and colonization probabilities in long-term gibbon monitoring programs.

Fur color change and hormonal development in captive females of northern white-cheeked (Nomascus leucogenys) and buff-cheeked (Nomascus gabriellae) gibbons.

Gibbons of the genus Nomascus exhibit strong sexual dichromatism in fur color. Change of fur color in sub-adult wild Nomascus females is associated with the onset of puberty and the time of their dispersal. The variability in fur change may be influenced by social factors. In this study, we determined whether in captive females of crested gibbons begin reproductive maturity prior to dispersing and with association to their fur color. We collected 287 fecal extracts to analyze pregnandiol -3- glucuronide and 17ß estradiol profiles of 4 sub-adult females (Nomascus leucogenys and Nomascus gabriellae) and 183 samples from their mothers, using enzyme immunoassays. The sub-adult females were monitored from 4 years of age. Their hormone profiles showed the onset of ovulatory cycling between 4.6 and 5.8 years. Based on the information about the estrogen influence to the secondary sex characteristic (fur color of female) the positive link between estrogen concentration and age of the sub-adult females was found. However, the amount of the estrogen can apparently be influenced by the presence of mother. If the mother was presented, the level of estrogen was higher than if the mother was missing. Our findings suggest that the probability of changing to beige fur color by the sub-adult females increased with increased age and if they were without mother. This initial study presents the maternal influence as a possible social factor affecting the fur color change of female offspring.

Infant loss during and after male replacement in gibbons.

According to the sexual selection hypothesis, infanticide during resident male replacement is an adaptive strategy that has evolved because the killing of unweaned offspring sired by previous males shortens the inter-birth intervals of the mothers whose infants are targeted and thereby increases the reproductive fitness of the perpetrator. To test this hypothesis, we describe previously unreported cases of primary male replacement for two gibbon species (Hylobates lar and Nomascus nasutus), and review all other reported cases of primary male replacement in gibbons. Overall, infants were present in nearly half of all cases (16/33, 48%) and of the 18 infants present during replacement, 50% (N = 9) disappeared within 2 months of the event. In four of the five cases where there was sufficient demographic information to identify the likely sire of the subsequent offspring of females that lost infants, the new male was believed to be the sire. Infants were also less likely to die or disappear if the new male and original resident male were possible kin. However, there was no significant difference in the age of infants between those that died or disappeared following replacement and those that survived to weaning (p = .630). Our review of takeover-related infant loss in gibbons confirms that periods of male instability are risky for unweaned infants and that replacing males benefit from infant loss. Nevertheless, variability in the context of infant loss and difficulties related to data collection in the field make it difficult to test competing hypotheses concerning the mechanisms and functions of infanticide in the small apes.

Effects of cold weather on the sleeping behavior of Skywalker hoolock gibbons (Hoolock tianxing) in seasonal montane forest.

Considering the high energetic costs of maintaining constant body temperature, mammals must adjust their thermoregulatory behaviors in response to cold temperatures. Although primate daytime thermoregulation is relatively well studied, there is limited research in relation to nighttime strategies. To investigate how Skywalker hoolock gibbons (Hoolock tianxing) cope with the low temperatures found in montane forests, we collected sleep-related behavior data from one group (NA) and a single female (NB) at Nankang (characterized by extensive tsaoko plantations) between July 2010 and September 2011, and one group (BB) at Banchang (relatively well-managed reserve forest) between May 2013 and May 2015 in Mt. Gaoligong, Yunnan, China. The annual mean temperature was 13.3°C at Nankang (October 2010 to September 2011) and 13.0°C at Banchang (June 2013 to May 2015) with temperatures dropping below -2.0°C at both sites, making them the coldest known gibbon habitats. The lowest temperatures at both sites remained below 5.0°C from November to March, which we, therefore, defined as the "cold season". The hoolock gibbons remained in their sleeping trees for longer periods during the cold season compared to the warm season. Sleeping trees found at lower elevations and closer to potential feeding trees were favored during cold seasons at both sites. In addition, the gibbons were more likely to huddle together during cold seasons. Our results suggest that cold temperatures have a significant effect on the sleeping behavior of the Skywalker hoolock gibbon, highlighting the adaptability of this threatened species in response to cold climates.

Influence of food availability and climate on behavior patterns of western black crested gibbons (Nomascus concolor) at Mt. Wuliang, Yunnan, China.

Food abundance and climatic factors can significantly affect the behavior of animals and constrain their activity budgets. The population of western black crested gibbons (Nomascus concolor) in Mt. Wuliang lives in montane forest and is close to the northern extreme of the distribution for gibbons (Hylobatidae). Their habitats show remarkable seasonal variation in terms of food availability, temperature, and rainfall. To understand behavioral adaptations of western black crested gibbons to different sets of ecological conditions, we examined relationships among food availability, mean temperature, rainfall, and behavior patterns by observing two groups for 1 year each. Our results revealed that activity budget was affected by food availability and mean temperature. The gibbons spent more time eating flowers when that resource was more available and spent less time moving when fruit was more available. The gibbons spent less time feeding and more time resting, and spent less feeding time on fruit and leaves when the mean temperature was lower. These results suggest that the gibbons displayed a pronounced preference for flowers as a food resource and adopted a time minimizer strategy when high-nutrient food items (i.e., fruit) were more available. In addition, the gibbons adopted an energy-conserving strategy during periods of low temperature. The flexibility of behavioral patterns in responding to food availability and temperature may potentially improve the gibbons' prospects of surviving and reproducing in a northern montane forest.

Space Use by Eastern Hoolock Gibbons (Hoolock leuconedys) in Nankang, Mt. Gaoligong, Southwest Yunnan, China.

To understand the differences in space use by eastern hoolock gibbons (Hoolock leuconedys) for meeting different behavioural needs and adapting to a disturbed environment, an eastern hoolock gibbon family in Nankang, Mt. Gaoligong, China, was observed, and data on their behaviour, spatial position and substrate use were collected through instantaneous scanning from October 2, 2010, to September 15, 2011. Our results suggested that connectivity was a primary factor influencing the movements of eastern hoolock gibbons. However, there were differences between spatial location and substrate use with different behaviours. Sitting and hanging were the main resting behaviours, but spatial position and substrate use for these two postures were different. Because tsaoko cultivation in this area has made understory plants rare, gibbons primarily used long, thin, horizontal branches in the first and second layers of the tree canopy for their movement. Although gibbons can selectively use the habitat and alter their spatial utilisation pattern accordingly, they are facing increased difficulty in movement and risk falling because of increased human disturbance and spread of tsaoko plantations. Therefore, it is important to limit the scope of tsaoko plantation to maintain a suitable habitat for Nankang gibbons.

The gibbon's Achilles tendon revisited: consequences for the evolution of the great apes?

The well-developed Achilles tendon in humans is generally interpreted as an adaptation for mechanical energy storage and reuse during cyclic locomotion. All other extant great apes have a short tendon and long-fibred triceps surae, which is thought to be beneficial for locomotion in a complex arboreal habitat as this morphology enables a large range of motion. Surprisingly, highly arboreal gibbons show a more human-like triceps surae with a long Achilles tendon. Evidence for a spring-like function similar to humans is not conclusive. We revisit and integrate our anatomical and biomechanical data to calculate the energy that can be recovered from the recoiling Achilles tendon during ankle plantar flexion in bipedal gibbons. Only 7.5% of the required external positive work in a stride can come from tendon recoil, yet it is delivered at an instant when the whole-body energy level drops. Consequently, an additional similar amount of mechanical energy must simultaneously dissipate elsewhere in the system. Altogether, this challenges the concept of an energy-saving function in the gibbon's Achilles tendon. Cercopithecids, sister group of the apes, also have a human-like triceps surae. Therefore, a well-developed Achilles tendon, present in the last common 'Cercopithecoidea-Hominoidea' ancestor, seems plausible. If so, the gibbon's anatomy represents an evolutionary relict (no harm-no benefit), and the large Achilles tendon is not the premised key adaptation in humans (although the spring-like function may have further improved during evolution). Moreover, the triceps surae anatomy of extant non-human great apes must be a convergence, related to muscle control and range of motion. This perspective accords with the suggestions put forward in the literature that the last common hominoid ancestor was not necessarily great ape-like, but might have been more similar to the small-bodied catarrhines.

Body mass estimation in hominoids: Age and locomotor effects.

While there are a number of methods available for estimation of body mass in adult nonhuman primates, very few are available for juveniles, despite the potential utility of such estimations in both analyses of fossils and in museum collection based research. Furthermore, because of possible scaling differences, adult based body mass estimation equations may not be appropriate for non-adults. In this study, we present new body mass estimation equations for both adult and immature nonhuman hominoids based on joint and metaphyseal dimensions. Articular breadths of the proximal and distal femur, distal humerus and tibial plateau, and metaphyseal breadths of the distal femur and humerus were collected on a reference sample of 159 wild Pan, Gorilla, Pongo, Hylobates, and Symphalangus specimens of known body mass from museum and research collections. Scaling of dimensions with body weight was assessed in both the adult and the ontogenetic sample at several taxonomic levels using reduced major axis regression, followed by regression of each dimension against body mass to generate body mass estimation equations. Joint dimensions were found to be good predictors of body mass in both adult and immature hominoids, with percent prediction errors of 10-20%. However, subtle scaling differences between taxa impacted body mass estimation, suggesting that phylogeny and locomotor effects should be considered when selecting reference samples. Unlike patterns of joint growth in humans, there was little conclusive evidence for consistently larger joints relative to body mass in the non-adult sample. Metaphyseal breadths were strong predictors of body mass and, with some exceptions, gave more precise body mass estimates for non-adults than epiphyseal breadths.

Look in the trees: Hylobatids as evolutionary models for extinct hominins.

Studying extant apes is of central importance to paleoanthropology. This approach is informative in inferring how hominin skeletal morphology reflects phylogeny, behavior, development, and ecological context. Traditionally, great apes have dominated the paleoanthropological literature as extant analogs for extinct hominins, to the exclusion of their phylogenetic sister group, the hylobatids. Phylogenetic proximity, large body size, and high encephalization quotients may have contributed to decisions to use great apes as models for hominins. However, if we reexamine hylobatids as extant models for extinct hominins-using modern phylogenetic, behavioral, and ecological data-this clade is uniquely poised to inform future frameworks in paleoanthropology. The following features make hylobatids strong analogs for extinct hominins: taxonomic diversity, the timing of diversification, hybridization between species, small body size, and reduced sexual dimorphism. Based on these shared features, hylobatids offer future opportunities to paleoanthropology, and provide a much richer extant analog than is currently recognized.

A comparison of paternity data and relative testes size as measures of level of sperm competition in the Hominoidea.

OBJECTIVES: The phrase "level of sperm competition" is used only vaguely in the primate literature. There is also little distinction between the important elements of frequency and intensity of sperm competition, largely because the two current forms of measurement (socio-sexual system and relative testes size) are both proxies which allow neither precision nor fine distinctions. Both measures have critics, socio-sexual system in particular being branded subjective, misleading, and changeable. Testes size is considered the more reliable despite its validation resting on correlations with the other, less reliable, proxy. Recently, genetic paternity studies have been mooted to provide a potentially superior third measure of sperm competition but so far lack a formal interpretive framework. Here we use the published and relatively comprehensive genetic field studies of the Hominoidea to develop such a framework. MATERIALS AND METHODS: Formulae are derived to convert paternity data into a direct measure of the frequency, intensity, and overall level of sperm competition. We then compare these measures with relative testes size at the study, species, and phylogenetic levels. RESULTS: A significant correlation between level of sperm competition and relative testes size was obtained at each level. These correlations provide independent support for the continuing use of testes size as a proxy measure when such a measure is sufficient. However, they also suggest that paternity data and our formulae yield a viable alternative measure. DISCUSSION: This alternative measure based on paternity data has a number of advantages. Not only is it a potentially direct measure of the level of sperm competition but it also allows the roles of frequency and intensity to be studied separately when of interest.

A distance sampling approach to estimate density and abundance of gibbon groups.

All 18 species of gibbons are considered threatened with extinction and listed on the IUCN Red List of Threatened Species. Because gibbons (Hylobatidae) are one of the most threatened primate families, a great need exists to determine and monitor their status effectively. To meet this need, we employed distance sampling methods to estimate the density and number of gibbon groups. We focused on southern yellow-cheeked crested gibbon in the Nam Cat Tien sector of Cat Tien National Park, Vietnam, from July to October, 2016. We used an auditory point count method at 48 listening posts to detect gibbon groups. We estimated our detection probability of calling groups of gibbons within 1,200 m of a listening post was 0.63 (95% CI: 0.54-0.74). In addition, we estimated the daily calling probability of a gibbon group to be 0.46 (95% CI: 0.33-0.59) and interpreted this as the probability that a group was available to be detected. We adjusted our group density and abundance estimates by both detection and availability probabilities. Ultimately we estimated 325 (95% CI: 232-455) gibbon groups in the Nam Cat Tien sector. Our results suggest that Cat Tien National Park contains one of the largest populations of southern yellow-cheeked crested gibbon in Vietnam. Our methods are one way of avoiding underestimation of gibbon group density and abundance by incorporating uncertainty in gibbon group availability and detection.

Dispersal and female philopatry in a long-term, stable, polygynous gibbon population: Evidence from 16 years field observation and genetics.

Gibbons are generally reported to live in small socially monogamous family groups in which both sexes disperse when they reach maturity. For the first time, we documented the dispersal pattern in a population of gibbons living in stable polygynous groups (Nomascus concolor) integrating 16 years' field observation and genetic information from fecal DNA. All subadult males except for one dispersed at 9.8 ± 1.4 years of age (range: 8-12, N = 10). The last male remained in his natal group and obtained the breeding position at age 11 by evicting the original dominant male. Females reached sexual maturity (as evidenced by the change in body color from black to yellow) at 8 years (N = 4). Three of them dispersed and one obtained a position as a breeding female and bred in her natal group. We also observed one female returning to her natal group with her infant after her presumed father was taken over by a neighboring male. We identified only three mtDNA haplotypes from 22 individuals at Dazhaizi. Individuals in one group shared the same haplotype, with only one exception. Genetic results showed that the two breeding females were mother-daughter pairs in all three study groups at the end of this study, implying some degree of female philopatry. We argue that in the case of black crested gibbons, dispersal decisions appear to represent highly opportunistic events in response to reproductive opportunities in their natal and neighboring groups.

Historical data as a baseline for conservation: reconstructing long-term faunal extinction dynamics in Late Imperial-modern China.

Extinction events typically represent extended processes of decline that cannot be reconstructed using short-term studies. Long-term archives are necessary to determine past baselines and the extent of human-caused biodiversity change, but the capacity of historical datasets to provide predictive power for conservation must be assessed within a robust analytical framework. Local Chinese gazetteers represent a more than 400-year country-level dataset containing abundant information on past environmental conditions and include extensive records of gibbons, which have a restricted present-day distribution but formerly occurred across much of China. Gibbons show pre-twentieth century range contraction, with significant fragmentation by the mid-eighteenth century and population loss escalating in the late nineteenth century. Isolated gibbon populations persisted for about 40 years before local extinction. Populations persisted for longer at higher elevations, and disappeared earlier from northern and eastern regions, with the biogeography of population loss consistent with the contagion model of range collapse in response to human demographic expansion spreading directionally across China. The long-term Chinese historical record can track extinction events and human interactions with the environment across much longer timescales than are usually addressed in ecology, contributing novel baselines for conservation and an increased understanding of extinction dynamics and species vulnerability or resilience to human pressures.

Giant pandas (Carnivora: Ailuropoda melanoleuca) and living hominoids converge on lumbar vertebral adaptations to orthograde trunk posture.

Living hominoids share a common body plan characterized by a gradient of derived postcranial features that distinguish them from their closest living relatives, cercopithecoid monkeys. However, the evolutionary scenario(s) that led to the derived postcranial features of hominoids are uncertain. Explanations are complicated by the fact that living hominoids vary considerably in positional behaviors, and some Miocene hominoids are morphologically, and therefore probably behaviorally, distinct from modern hominoids. Comparative studies that aim to identify morphologies associated with specific components of positional behavioral repertoires are an important avenue of research that can improve our understanding of the evolution and adaptive significance of the hominoid postcranium. Here, we employ a comparative approach to offer additional insight into the evolution of the hominoid lumbar vertebral column. Specifically, we tested whether giant pandas (Carnivora: Ailuropoda melanoleuca) converge with living hominoids on lumbar vertebral adaptations to the single component of their respective positional behavioral repertoires that they share--orthograde (i.e., upright) trunk posture. We compare lumbar vertebral morphologies of Ailuropoda to those of other living ursids and caniform outgroups (northern raccoons and gray wolves). Mirroring known differences between living hominoids and cercopithecoids, Ailuropoda generally exhibits fewer, craniocaudally shorter lumbar vertebrae with more dorsally positioned transverse processes that are more dorsally oriented and laterally directed, and taller, more caudally directed spinous processes than other caniforms in the sample. Our comparative evidence lends support to a potential evolutionary scenario in which the acquisition of hominoid-like lumbar vertebral morphologies may have evolved for generalized orthograde behaviors and could have been exapted for suspensory behavior in crown hominoids and for other locomotor specializations (e.g., brachiation) in extant lineages.

Micro-finite element (µFE) modeling of the siamang (Symphalangus syndactylus) third proximal phalanx: the functional role of curvature and the flexor sheath ridge.

Phalangeal curvature is a commonly used morphological feature for the interpretation of extant and fossil primate locomotor behaviour. Here, we build on a recent biomechanical study (Richmond, 2007) in two ways: first, we use a 3D micro-FE model, which models the real internal microstructure (i.e., cortical thickness and trabecular bone structure) and, second, we model four siamang third proximal phalanges. We test identical 2D homogenized FE models and two 3D micro-FE phalanx models that are mathematically straightened to isolate the biomechanical significance of curvature. We further investigate how varying the loading configuration (e.g., boundary constraints) and modeling (e.g., 2D versus 3D) affects the biomechanical behaviour of the phalanx. Finally, we examine how intraspecific variation in external and internal bony morphology affects the biomechanical behaviour of the phalanx. Simulation results demonstrate that the general pattern of strain and displacement is similar between the 3D micro-FE and 2D homogenized FE models but the absolute values differ substantially. The biomechanical behaviour of the 3D FE models more closely match the relative strain patterns from the validation experiment than the 2D homogenized FE models, indicating the 3D microstructure model is preferable. Varying the loading configuration can have dramatic effects on the biomechanical behaviour of the phalanx depending on individual morphology, but overall a cantilevered beam model is an equally valid, if not better, configuration for modeling the phalanx as other previously-proposed models. Variation in flexor ridge morphology has a substantial effect on phalanx strain; the taller the ridge, the less strain incurred by other regions of the palmar shaft. Finally, phalangeal curvature reduces overall strain experienced by the phalanx, but does not necessarily reduce bending or increase the compression-to-tension ratio. These results confirm the adaptive role of phalangeal curvature during flexed-finger grasping postures and demonstrate that modeling variation in cortical thickness and flexor ridge morphology improves the behaviour of the FE model, which has important implications for the functional interpretation of phalanx form.

A comparison of facial expression properties in five hylobatid species.

Little is known about facial communication of lesser apes (family Hylobatidae) and how their facial expressions (and use of) relate to social organization. We investigated facial expressions (defined as combinations of facial movements) in social interactions of mated pairs in five different hylobatid species belonging to three different genera using a recently developed objective coding system, the Facial Action Coding System for hylobatid species (GibbonFACS). We described three important properties of their facial expressions and compared them between genera. First, we compared the rate of facial expressions, which was defined as the number of facial expressions per units of time. Second, we compared their repertoire size, defined as the number of different types of facial expressions used, independent of their frequency. Third, we compared the diversity of expression, defined as the repertoire weighted by the rate of use for each type of facial expression. We observed a higher rate and diversity of facial expression, but no larger repertoire, in Symphalangus (siamangs) compared to Hylobates and Nomascus species. In line with previous research, these results suggest siamangs differ from other hylobatids in certain aspects of their social behavior. To investigate whether differences in facial expressions are linked to hylobatid socio-ecology, we used a Phylogenetic General Least Square (PGLS) regression analysis to correlate those properties with two social factors: group-size and level of monogamy. No relationship between the properties of facial expressions and these socio-ecological factors was found. One explanation could be that facial expressions in hylobatid species are subject to phylogenetic inertia and do not differ sufficiently between species to reveal correlations with factors such as group size and monogamy level.

Partitioning variance in a signaling trade-off under sexual selection reveals among-individual covariance in trait allocation.

Understanding the evolution of traits subject to trade-offs is challenging because phenotypes can (co)vary at both the among- and within-individual levels. Among-individual covariation indicates consistent, possibly genetic, differences in how individuals resolve the trade-off, while within-individual covariation indicates trait plasticity. There is also the potential for consistent among-individual differences in behavioral plasticity, although this has rarely been investigated. We studied the sources of (co)variance in two characteristics of an acoustic advertisement signal that trade-off with one another and are under sexual selection in the gray treefrog, Hyla chrysoscelis: call duration and call rate. We recorded males on multiple nights calling spontaneously and in response to playbacks simulating different competition levels. Call duration, call rate, and their product, call effort, were all repeatable both within and across social contexts. Call duration and call rate covaried negatively, and the largest covariance was at the among-individual level. There was extensive plasticity in calling with changes in social competition, and we found some evidence for among-individual variance in call rate plasticity. The significant negative among-individual covariance in trait values is perpendicular to the primary direction of sexual selection in this species, indicating potential limits on the response to selection.