Facial asymmetry tracks genetic diversity among Gorilla subspecies.

Mountain gorillas are particularly inbred compared to other gorillas and even the most inbred human populations. As mountain gorilla skeletal material accumulated during the 1970s, researchers noted their pronounced facial asymmetry and hypothesized that it reflects a population-wide chewing side preference. However, asymmetry has also been linked to environmental and genetic stress in experimental models. Here, we examine facial asymmetry in 114 crania from three Gorilla subspecies using 3D geometric morphometrics. We measure fluctuating asymmetry (FA), defined as random deviations from perfect symmetry, and population-specific patterns of directional asymmetry (DA). Mountain gorillas, with a current population size of about 1000 individuals, have the highest degree of facial FA (explaining 17% of total facial shape variation), followed by Grauer gorillas (9%) and western lowland gorillas (6%), despite the latter experiencing the greatest ecological and dietary variability. DA, while significant in all three taxa, explains relatively less shape variation than FA does. Facial asymmetry correlates neither with tooth wear asymmetry nor increases with age in a mountain gorilla subsample, undermining the hypothesis that facial asymmetry is driven by chewing side preference. An examination of temporal trends shows that stress-induced developmental instability has increased over the last 100 years in these endangered apes.

Muscle attachment sites and behavioral reconstruction: An experimental test of muscle-bone structural response to habitual activity.

OBJECTIVE: Behavioral reconstruction from muscle attachment sites (entheses) is a common practice in anthropology. However, experimental evidence provides mixed support for the assumed association between enthesis size and shape with changes in habitual activity. In this study, a laboratory mouse model was used to experimentally test whether activity level and type alters muscle architecture and the underlying bone cross-sectional geometry of entheses in order to assess the underlying assumption that behavioral changes lead to quantifiable differences in both muscle and enthesis morphology. MATERIALS AND METHODS: Female wild-type mice were separated into one control group and two experimentally increased activity groups (running, climbing) over an 11-week study period. At the start of the experiment, half of the mice were 4 weeks and half were 7 weeks of age. The postmortem deltoideus and biceps brachii muscles were measured for potential force production (physiological cross-sectional area) and potential muscle excursion (fiber length). Bone cross-sectional geometry variables were measured from microCT scans of the humerus and radius at the enthesis and non-enthesis regions of interest across activity groups. RESULTS: Activity level and type altered potential force production and potential muscle excursion of both muscles in the younger cohort. We observed differences in cortical bone geometry in both the humerus enthesis and radius non-enthesis region driven exclusively among the younger wheel-running mice. DISCUSSION: These results indicate that in addition to muscle architectural changes, bone structural properties at the enthesis do show an adaptive response to increased activity, such as running but only during earlier development. However, further research is required in order to apply these findings to the reconstruction of living behavior from anthropological specimens.

Body proportions and environmental adaptation in gorillas.

OBJECTIVES: Limb length and trunk proportions are determined in a large, taxonomically and environmentally diverse sample of gorillas and related to variation in locomotion, climate, altitude, and diet. MATERIALS AND METHODS: The sample includes 299 gorilla skeletons, 115 of which are infants and juveniles, distributed between western lowland (G. gorilla gorilla), low and high elevation grauer (G. beringei graueri), and Virunga mountain gorillas (G. b. beringei). Limb bone and vertebral column lengths scaled to body mass are compared between subgroups by age group. RESULTS: All G. beringei have relatively short 3rd metapodials and manual proximal phalanges compared to G. gorilla, and this difference is apparent in infancy. All G. beringei also have shortened total limb lengths relative to either body mass or vertebral column length, although patterns of variation in individual skeletal elements are more complex, and infants do not display the same patterns as adults. Mountain gorillas have relatively long clavicles, present in infancy, and a relatively long thoracic (but not lumbosacral) vertebral column. DISCUSSION: A variety of environmental factors likely contributed to observed patterns of morphological variation among extant gorillas. We interpret the short hand and foot bones of all G. beringei as genetic adaptations to greater terrestriality in the last common ancestor of G. beringei; variation in other limb lengths to climatic adaptation, both genetic and developmental; and the larger thorax of G. b. beringei to adaptation to reduced oxygen pressure at high altitudes, again as a product of both genetic differences and environmental influences during development.

In vivo deciduous dental eruption in LuiKotale bonobos and Gombe chimpanzees.

OBJECTIVES: Existing data on bonobo and chimpanzee dental eruption timing are derived predominantly from captive individuals or deceased wild individuals. However, recent advances in noninvasive photographic monitoring of living, wild apes have enabled researchers to characterize dental eruption in relatively healthy individuals under naturalistic conditions. At present, such data are available for only one population of wild chimpanzees. We report data for an additional population of wild chimpanzees and the first dental eruption data for wild bonobos. MATERIALS AND METHODS: We collected photographs and video footage of teeth from the open mouths of wild bonobos and East African chimpanzees of known age from LuiKotale, Democratic Republic of the Congo, and Gombe National Park, Tanzania, respectively. We scored the presence and absence of deciduous teeth from photographs and video footage to characterize deciduous dental eruption timing in these two populations. RESULTS: Deciduous dental eruption ages in our sample fall within the range of variation previously documented for captive chimpanzees, but eruption ages are later in wild than in captive contexts. We found substantial variation in deciduous canine eruption timing, particularly among bonobos. One bonobo had a deciduous canine present by 227 days old while another did not have a deciduous canine present at 477 days old. DISCUSSION: Our data indicate that deciduous teeth erupt later in wild individuals than in captive individuals. We also found that deciduous dental eruption timing varies considerably between individuals within our study populations, a pattern that is consistent with previous studies. Future studies should consider sources of variation in deciduous canine eruption timing and relationships with other aspects of life history as additional data become available.

Chest beats as an honest signal of body size in male mountain gorillas (Gorilla beringei beringei).

Acoustic signals that reliably indicate body size, which usually determines competitive ability, are of particular interest for understanding how animals assess rivals and choose mates. Whereas body size tends to be negatively associated with formant dispersion in animal vocalizations, non-vocal signals have received little attention. Among the most emblematic sounds in the animal kingdom is the chest beat of gorillas, a non-vocal signal that is thought to be important in intra and inter-sexual competition, yet it is unclear whether it reliably indicates body size. We examined the relationship among body size (back breadth), peak frequency, and three temporal characteristics of the chest beat: duration, number of beats and beat rate from sound recordings of wild adult male mountain gorillas. Using linear mixed models, we found that larger males had significantly lower peak frequencies than smaller ones, but we found no consistent relationship between body size and the temporal characteristics measured. Taken together with earlier findings of positive correlations among male body size, dominance rank and reproductive success, we conclude that the gorilla chest beat is an honest signal of competitive ability. These results emphasize the potential of non-vocal signals to convey important information in mammal communication.

3D enamel profilometry reveals faster growth but similar stress severity in Neanderthal versus Homo sapiens teeth.

Early life stress disrupts growth and creates horizontal grooves on the tooth surface in humans and other mammals, yet there is no consensus for their quantitative analysis. Linear defects are considered to be nonspecific stress indicators, but evidence suggests that intermittent, severe stressors create deeper defects than chronic, low-level stressors. However, species-specific growth patterns also influence defect morphology, with faster-growing teeth having shallower defects at the population level. Here we describe a method to measure the depth of linear enamel defects and normal growth increments (i.e., perikymata) from high-resolution 3D topographies using confocal profilometry and apply it to a diverse sample of Homo neanderthalensis and H. sapiens anterior teeth. Debate surrounds whether Neanderthals exhibited modern human-like growth patterns in their teeth and other systems, with some researchers suggesting that they experienced more severe childhood stress. Our results suggest that Neanderthals have shallower features than H. sapiens from the Upper Paleolithic, Neolithic, and medieval eras, mirroring the faster growth rates in Neanderthal anterior teeth. However, when defect depth is scaled by perikymata depth to assess their severity, Neolithic humans have less severe defects, while Neanderthals and the other H. sapiens groups show evidence of more severe early life growth disruptions.

Skeletal ageing in Virunga mountain gorillas.

Bone loss and heightened fracture risk are common conditions associated with ageing in modern human populations and have been attributed to both hormonal and other metabolic and behavioural changes. To what extent these age-related trends are specific to modern humans or generally characteristic of natural populations of other taxa is not clear. In this study, we use computed tomography to examine age changes in long bone and vertebral structural properties of 34 wild-adult Virunga mountain gorillas (Gorilla beringei beringei) whose skeletons were recovered from natural accumulations. Chronological ages were known or estimated from sample-specific dental wear formulae and ranged between 11 and 43 years. Gorillas show some of the same characteristics of skeletal ageing as modern humans, including endosteal and some periosteal expansion. However, unlike in humans, there is no decline in cortical or trabecular bone density, or in combined geometric-density measures of strength, nor do females show accelerated bone loss later in life. We attribute these differences to the lack of an extended post-reproductive period in gorillas, which provides protection against bone resorption. Increases in age-related fractures (osteoporosis) in modern humans may be a combined effect of an extended lifespan and lower activity levels earlier in life. This article is part of the theme issue 'Evolution of the primate ageing process'.

Dominance rank but not body size influences female reproductive success in mountain gorillas.

According to life history theory, natural selection has shaped trade-offs for allocating energy among growth, reproduction and maintenance to maximize individual fitness. In social mammals body size and dominance rank are two key variables believed to influence female reproductive success. However, few studies have examined these variables together, particularly in long-lived species. Previous studies found that female dominance rank correlates with reproductive success in mountain gorillas (Gorilla beringei beringei), which is surprising given they have weak dominance relationships and experience seemingly low levels of feeding competition. It is not currently known whether this relationship is primarily driven by a positive correlation between rank and body size. We used the non-invasive parallel laser method to measure two body size variables (back breadth and body length) of 34 wild adult female mountain gorillas, together with long-term dominance and demography data to investigate the interrelationships among body size, dominance rank and two measures of female reproductive success (inter-birth interval N = 29 and infant mortality N = 64). Using linear mixed models, we found no support for body size to be significantly correlated with dominance rank or female reproductive success. Higher-ranking females had significantly shorter inter-birth intervals than lower-ranking ones, but dominance rank was not significantly correlated with infant mortality. Our results suggest that female dominance rank is primarily determined by factors other than linear body dimensions and that high rank provides benefits even in species with weak dominance relationships and abundant year-round food resources. Future studies should focus on the mechanisms behind heterogeneity in female body size in relation to trade-offs in allocating energy to growth, maintenance and lifetime reproductive success.

Pelvic shape variation among gorilla subspecies: Phylogenetic and ecological signals.

Gorillas occupy habitats that range in elevation from 0 to 3850 m. Populations at higher elevations tend to be less arboreal than lowland populations. Variation in habitat-specific behaviors among closely related populations makes gorillas a unique model to study the relationship between locomotion and morphology. The pelvis reflects differences in locomotion in other primates, and thus may also reflect locomotor differences among gorillas. We tested the hypothesis that pelvic morphology exhibits clinal variation across elevation within Gorilla. Using 3D geometric morphometrics and principal components analysis (PCA), we characterized pelvic shape in three gorilla subspecies representing 14 localities across gorillas' full elevation range: western lowland gorillas (Gorilla gorilla gorilla), mountain gorillas (Gorilla beringei beringei), and Grauer's gorillas (Gorilla beringei graueri). We found that the first principal component (PC1) usually reflects differences between western and eastern gorillas in the lateral margin of the ilium and, in males, the obturator foramen. When sexes are considered together, the second principal component (PC2) indicates some separation between G. b. beringei and G. b. graueri, albeit with considerable overlap, corresponding to the shape of the iliac crest. When sexes were analyzed separately, there was no distinction. Phylogenetic generalized least squares regression was used to evaluate the relationship between elevation and pelvic shape under varying phylogenetic assumptions. Models were compared to assess how phylogenetic adjustment affects model fit. Neither of the first two PCs nor overall shape yielded a significant relationship with elevation in any of the pooled-sex and individual-sex samples. This suggests that covariation between pelvic morphology and elevation is sex-specific and dependent on phylogenetic assumptions. Our results find complex interactions between sex, phylogeny, elevation, and pelvic morphology, suggesting that there is not one ecomorphological pattern that characterizes all gorillas.

Faster growth corresponds with shallower linear hypoplastic defects in great ape canines.

Deeper or more 'severe' linear enamel hypoplasia (LEH) defects are hypothesized to reflect more severe stress during development, but it is not yet clear how depth is influenced by intrinsic enamel growth patterns. Recent work documented inter- and intraspecific differences in LEH defect depth in extant great apes, with mountain gorillas having shallower defects than other taxa, and females having deeper defects than males. Here, we assess the correspondence of inter- and intraspecific defect depth and intrinsic aspects of enamel growth: enamel extension rates, outer enamel striae of Retzius angles, and linear enamel thickness. Thin sections of great ape canines (n = 40) from Gorilla beringei beringei, Gorilla gorilla gorilla, Pan troglodytes, and Pongo spp. were analyzed. Enamel extension rates were calculated within deciles of enamel-dentine junction length. Linear enamel thickness and the angle of intersection between striae of Retzius and the outer enamel surface were measured in the imbricational enamel. Mountain gorillas have faster enamel extension rates and shallower striae angles than the other taxa examined. Mountain gorillas have thinner imbricational enamel than western lowland gorillas and orangutans, but not chimpanzees. In the combined-taxon sample, females exhibit larger striae angles and thicker imbricational enamel than males. Enamel extension rates are highly negatively correlated with striae angles and LEH defect depth. Enamel growth variation corresponds with documented inter- and intraspecific differences in LEH defect depth in great ape canines. Mountain gorillas have shallower striae angles and faster extension rates than other taxa, which might explain their shallow LEH defect morphology and the underestimation of their LEH prevalence in previous studies. These results suggest that stressors of similar magnitude and timing might produce defects of different depths in one species or sex vs. another, which has implications for interpretations of stress histories in hominins with variable enamel growth patterns.

Incisor tooth wear and age determination in mountain gorillas from Volcanoes National Park, Rwanda.

OBJECTIVES: Ecological factors, but also tooth-to-tooth contact over time, have a dramatic effect on tooth wear in primates. The aim of this study is to test whether incisor tooth wear changes predictably with age and can thus be used as an age estimation method in a wild population of mountain gorillas (Gorilla beringei beringei) from Volcanoes National Park, Rwanda. MATERIALS AND METHODS: In mountain gorillas of confidently known chronological age (N = 24), we measured the crown height of all permanent maxillary and mandibular incisors (I1 , I1 , I2 , I2 ) as a proxy for incisal macrowear. Linear and quadratic regressions for each incisor were used to test whether age can be predicted by crown height. Using these models, we then predicted age at death of two individual mountain gorillas of probable identifications, based on their incisor crown height. RESULTS: Age decreased significantly with incisor height for all teeth, but the upper first incisors (I1 ) provided the best results, with the lowest Akaike's Information Criterion corrected for small sample size (AICc) and lowest Standard Error of the Estimate (SEE). When the best age equations for each sex were applied to gorillas with probable identifications, the predicted ages differed 1.58 and 3.33 years from the probable ages of these individuals. CONCLUSIONS: Our findings corroborate that incisor crown height, a proxy for incisal wear, varies predictably with age. This relationship can be used to estimate age at death of unknown gorillas in the skeletal collection, and in some cases, to corroborate the identity of individual gorillas recovered from the forest postmortem at an advanced state of decomposition. Such identifications help fill gaps in the demographic database and support research that requires individual-level data.

Phylogenetic and environmental effects on limb bone structure in gorillas.

OBJECTIVES: The effects of phylogeny and locomotor behavior on long bone structural proportions are assessed through comparisons between adult and ontogenetic samples of extant gorillas. MATERIALS AND METHODS: A total of 281 wild-collected individuals were included in the study, divided into four groups that vary taxonomically and ecologically: western lowland gorillas (G. g. gorilla), lowland and highland grauer gorillas (G. b. graueri), and Virunga mountain gorillas (G. b. beringei). Lengths and articular breadths of the major long bones (except the fibula) were measured, and diaphyseal cross-sectional geometric properties determined using computed tomography. Ages of immature specimens (n = 145) were known or estimated from dental development. Differences between groups in hind limb to forelimb proportions were assessed in both adults and during development. RESULTS: Diaphyseal strength proportions among adults vary in parallel with behavioral/ecological differences, and not phylogeny. The more arboreal western lowland and lowland grauer gorillas have relatively stronger forelimbs than the more terrestrial Virunga mountain gorillas, while the behaviorally intermediate highland grauer gorillas have intermediate proportions. Diaphyseal strength proportions are similar in young infants but diverge after 2 years of age in western lowland and mountain gorillas, at the same time that changes in locomotor behavior occur. There are no differences between groups in length or articular proportions among either adults or immature individuals. CONCLUSION: Long bone diaphyseal strength proportions in gorillas are developmentally plastic, reflecting behavior, while length and articular proportions are much more genetically canalized. These findings have implications for interpreting morphological variation among fossil taxa.

Quantifying linear enamel hypoplasia in Virunga Mountain gorillas and other great apes.

OBJECTIVE: Linear enamel hypoplasia (LEH) is a condition marked by localized reductions in enamel thickness, resulting from growth disruptions during dental development. We use quantitative criteria to characterize the depth of LEH defects and "normal" perikymata in great apes. We test the hypothesis that mountain gorillas have shallow defects compared to other taxa, which may have led to their underestimation in previous studies. MATERIALS AND METHODS: Previous attempts to characterize LEH morphology quantitatively have been limited in sample size and scope. We generated digital elevation models using optical profilometry (Sensofar PLu Neox) and extracted 2D coordinates using ImageJ to quantify depths in canines from three great ape genera (N = 75 perikymata; 255 defects). RESULTS: All defect depths fall outside the distribution of perikymata depths. Mountain gorilla defects are significantly shallower than those of other great ape taxa examined, including western lowland gorillas. Females have significantly deeper defects than males in all taxa. The deepest defect belongs to a wild-captured zoo gorilla. Virunga mountain gorilla specimens collected by Dian Fossey exhibit deeper defects than those collected recently. DISCUSSION: Shallow defect morphology in mountain gorillas may have led to an underestimation of LEH prevalence in past studies. Defect depth is used as a proxy for insult severity, but depth might be influenced by inter- and intra-specific variation in enamel growth. Future studies should test whether severe insults are associated with deeper defects, as might be the case with Haloko, a wild-captured gorilla. Ongoing histologic studies incorporating associated behavioral records will test possible factors that underlie differences in defect morphology.

Unexpected terrestrial hand posture diversity in wild mountain gorillas.

OBJECTIVES: Gorillas, along with chimpanzees and bonobos, are ubiquitously described as 'knuckle-walkers.' Consequently, knuckle-walking (KW) has been featured pre-eminently in hypotheses of the pre-bipedal locomotor behavior of hominins and in the evolution of locomotor behavior in apes. However, anecdotal and behavioral accounts suggest that mountain gorillas may utilize a more complex repertoire of hand postures, which could alter current interpretations of African ape locomotion and its role in the emergence of human bipedalism. Here we documented hand postures during terrestrial locomotion in wild mountain gorillas to investigate the frequency with which KW and other hand postures are utilized in the wild. MATERIALS AND METHODS: Multiple high-speed cameras were used to record bouts of terrestrial locomotion of 77 habituated mountain gorillas at Bwindi Impenetrable National Park (Uganda) and Volcanoes National Park (Rwanda). RESULTS: We captured high-speed video of hand contacts in 8% of the world's population of mountain gorillas. Our results reveal that nearly 40% of these gorillas used "non-KW" hand postures, and these hand postures constituted 15% of all hand contacts. Some of these "non-KW" hand postures have never been documented in gorillas, yet match hand postures previously identified in orangutans. DISCUSSION: These results highlight a previously unrecognized level of hand postural diversity in gorillas, and perhaps great apes generally. Although present at lower frequencies than KW, we suggest that the possession of multiple, versatile hand postures present in wild mountain gorillas may represent a shared feature of the African ape and human clade (or even great ape clade) rather than KW per se.

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.

Body growth and life history in wild mountain gorillas (Gorilla beringei beringei) from Volcanoes National Park, Rwanda.

OBJECTIVES: Great apes show considerable diversity in socioecology and life history, but knowledge of their physical growth in natural settings is scarce. We characterized linear body size growth in wild mountain gorillas from Volcanoes National Park, Rwanda, a population distinguished by its extreme folivory and accelerated life histories. METHODS: In 131 individuals (0.09-35.26 years), we used non-invasive parallel laser photogrammetry to measure body length, back width, arm length and two head dimensions. Nonparametric LOESS regression was used to characterize cross-sectional distance and velocity growth curves for males and females, and consider links with key life history milestones. RESULTS: Sex differences became evident between 8.5 and 10.0 years of age. Thereafter, female growth velocities declined, while males showed increased growth velocities until 10.0-14.5 years across dimensions. Body dimensions varied in growth; females and males reached 98% of maximum body length at 11.7 and 13.1 years, respectively. Females attained 95.3% of maximum body length by mean age at first birth. Neonates were 31% of maternal size, and doubled in size by mean weaning age. Males reached maximum body and arm length and back width before emigration, but experienced continued growth in head dimensions. CONCLUSIONS: While comparable data are scarce, our findings provide preliminary support for the prediction that mountain gorillas reach maximum body size at earlier ages compared to more frugivorous western gorillas. Data from other wild populations are needed to better understand comparative great ape development, and investigate links between trajectories of physical, behavioral, and reproductive maturation.

Toughness of the Virunga mountain gorilla (Gorilla beringei beringei) diet across an altitudinal gradient.

The robust masticatory system of mountain gorillas is thought to have evolved for the comminution of tough vegetation, yet, compared to other primates, the toughness of the mountain gorilla diet is unremarkable. This may be a result of low plant toughness in the mountain gorilla environment or of mountain gorillas feeding selectively on low-toughness foods. The goal of this paper is to determine how the toughness of the mountain gorilla diet varies across their habitat, which spans a large altitudinal range, and whether there is a relationship between toughness and food selection by mountain gorillas. We collected data on the following variables to determine whether, and if so how, they change with altitude: leaf toughness of two plant species consumed by mountain gorillas, at every 100 m increase in altitude (2,600-3,700 m); toughness of consumed foods comprising over 85% of the gorilla diet across five vegetation zones; and toughness of unconsumed/infrequently consumed plant parts of those foods. Although leaf toughness increased with altitude, the toughness of the gorilla diet remained similar. There was a negative relationship between toughness and consumption frequency, and toughness was a better predictor of consumption frequency than plant frequency, biomass, and density. Consumed plant parts were less tough than unconsumed/infrequently consumed parts and toughness of the latter increased with altitude. Although it is unclear whether gorillas select food based on toughness or use toughness as a sensory cue to impart other plant properties (e.g., macronutrients, chemicals), our results that gorillas maintain a consistent low-toughness dietary profile across altitude, despite toughness increasing with altitude, suggest that the robust gorilla masticatory apparatus evolved for repetitive mastication of foods that are not high in toughness.

A radiographic study of permanent molar development in wild Virunga mountain gorillas of known chronological age from Rwanda.

OBJECTIVES: While dental development is important to life history investigations, data from wild known-aged great apes are scarce. We report on the first radiographic examination of dental development in wild Virunga mountain gorillas, using known-age skeletal samples recovered in Rwanda. MATERIALS AND METHODS: In 43 individuals (0.0-14.94 years), we collected radiographs of mandibular molars, and where possible, cone beam CT scans. Molar crown and root calcification status was assessed using two established staging systems, and age prediction equations generated using polynomial regression. Results were compared to available data from known-age captive and wild chimpanzees. RESULTS: Mountain gorillas generally fell within reported captive chimpanzee distributions or exceeded them, exhibiting older ages at equivalent radiographic stages of development. Differences reflect delayed initiation and/or an extended duration of second molar crown development, and extended first and second molar root development, in mountain gorillas compared to captive chimpanzees. However, differences in the duration of molar root development were less evident compared to wild chimpanzees. DISCUSSION: Despite sample limitations, our findings extend the known range of variation in radiographic estimates of molar formation timing in great apes, and provide a new age prediction technique based on wild specimens. However, mountain gorillas do not appear accelerated in radiographic assessment of molar formation compared to chimpanzees, as they are for other life history traits. Future studies should aim to resolve the influence of species differences, wild versus captive environments, and/or sampling phenomena on patterns observed here, and more generally, how they relate to variation in tooth size, eruption timing, and developmental life history.

Ontogenetic scaling of fore limb and hind limb joint posture and limb bone cross-sectional geometry in vervets and baboons.

OBJECTIVES: Previous studies suggest that the postures habitually adopted by an animal influence the mechanical loading of its long bones. Relatively extended limb postures in larger animals should preferentially reduce anteroposterior (A-P) relative to mediolateral (M-L) bending of the limb bones and therefore decrease A-P/M-L rigidity. We test this hypothesis by examining growth-related changes in limb bone structure in two primate taxa that differ in ontogenetic patterns of joint posture. MATERIALS AND METHODS: Knee and elbow angles of adult and immature vervets (Chlorocebus aethiops, n = 16) were compared to published data for baboons (Papio hamadryas ursinus, n = 33, Patel et al., ). Ontogenetic changes in ratios of A-P/M-L bending rigidity in the femur and humerus were compared in skeletal samples (C. aethiops, n = 28; P. cynocephalus, n = 39). Size changes were assessed with linear regression, and age group differences tested with ANOVA. RESULTS: Only the knee of baboons shows significant postural change, becoming more extended with age and mass. A-P/M-L bending rigidity of the femur decreases during ontogeny in immature and adult female baboons only. Trends in the humerus are less marked. Adult male baboons have higher A-P/M-L bending rigidity of the femur than females. CONCLUSIONS: The hypothesized relationship between more extended joints and reduced A-P/M-L bending rigidity is supported by our results for immature and adult female baboon hind limbs, and the lack of significant age changes in either parameter in forelimbs and vervets. Adult males of both species depart from general ontogenetic trends, possibly due to socially mediated behavioral differences between sexes. Am J Phys Anthropol 161:72-83, 2016. © 2016 Wiley Periodicals, Inc.

Primary bone microanatomy records developmental aspects of life history in catarrhine primates.

A central challenge in human origins research is to understand how evolution has shaped modern human life history. As fossilized remains of our ancestors provide the only direct evidence for life history evolution, efforts to reconstruct life history in paleontological contexts have focused on hard tissues, particularly on dental development. However, among investigators of other vertebrate groups, there is a long tradition of examining primary bone microstructure to decipher growth rates and maturational timing, based on an empirical relationship between the microanatomy of primary bone and the rate at which it is deposited. We examined ontogenetic variation in primary bone microstructure at the midshaft femur of Chlorocebus aethiops, Hylobates lar, and Pan troglodytes to test whether tissue type proportions vary in accordance with predictions based on body mass growth patterns described previously. In all taxa, younger age classes were characterized by significantly higher percent areas of fibro-lamellar and/or parallel-fibered tissues, while older age classes showed significantly higher proportions of lamellar bone. In prior experimental studies, fibro-lamellar and parallel-fibered tissue types have been associated with faster depositional rates than lamellar bone. Principal components analysis revealed differences among taxa in the timing of this transition, and in the particular tissue types observed among individuals of similar dental emergence status. Among M1 and M2 age classes, higher proportions of parallel-fibered and fibro-lamellar tissues were observed in those taxa characterized by reportedly faster body mass growth rates. Further, persistence of fibro-lamellar tissue throughout DECID, M1 and M2 age classes in chimpanzees contrasts with the pattern reported previously for modern humans. Despite the necessary limitations of our cross-sectional study design and the secondary remodeling of bone in primates, large areas of primary bone remain intact and represent a valuable and independent source of information about the evolution of growth and development in the fossil record.