Changes in limb bone diaphyseal structure in chimpanzees during development.

OBJECTIVES: This study tests if femoral and humeral cross-sectional geometry (CSG) and cross-sectional properties (CSPs) in an ontogenetic series of wild-caught chimpanzees (Pan troglodytes ssp.) reflect locomotor behavior during development. The goal is to clarify the relationship between limb bone structure and locomotor behavior during ontogeny in Pan. MATERIALS AND METHODS: The latex cast method was used to reconstruct cross sections at the midshaft femur and mid-distal humerus. Second moments of area (SMAs) (Ix, Iy, Imax, Imin), which are proportional to bending rigidity about a specified axis, and the polar SMA (J), which is proportional to average bending rigidity, were calculated at section locations. Cross-sectional shape (CSS) was assessed from Ix/Iy and Imax/Imin ratios. Juvenile and adult subsamples were compared. RESULTS: Juveniles and adults have significantly greater femoral J compared to humeral J. Mean interlimb proportions of J are not significantly different between the groups. There is an overall decreasing trend in diaphyseal circularity between the juvenile phase of development and adulthood, although significant differences are only found in the humerus. DISCUSSION: Juvenile chimpanzee locomotion includes forelimb- and hindlimb-biased behaviors. Juveniles and adults preferentially load their hindlimbs relative to their forelimbs. This may indicate similar locomotor behavior, although other explanations including a diversity of hindlimb-biased locomotor behaviors in juveniles cannot be ruled out. Different ontogenetic trends in forelimb and hindlimb CSS are consistent with limb bone CSG reflecting functional adaptation, albeit the complex nature of bone functional adaptation requires cautious interpretations of skeletal functional morphology from biomechanical analyses.

Becoming uniquely human? Comparing chimpanzee to human infancy.

Does comparing behavioral development between chimpanzees and humans during infancy hold the key to understanding what is uniquely human? Recent work shows that while many behaviors emerge at similar ages in chimpanzees, human infants develop behavioral traits underpinning our prosocial and ultra-cooperative nature at a much accelerated rate.

Assessing the status of the KNM-ER 42700 fossil using Homo erectus neurocranial development.

Based on ontogenetic data of endocranial shape, it has been proposed that a younger than previously assumed developmental status of the 1.5-Myr-old KNM-ER 42700 calvaria could explain why the calvaria of this fossil does not conform to the shape of other Homo erectus individuals. Here, we investigate (ecto)neurocranial ontogeny in H. erectus and assess the proposed juvenile status of this fossil using recent Homo sapiens, chimpanzees (Pan troglodytes), and Neanderthals (Homo neanderthalensis) to model and discuss changes in neurocranial shape from the juvenile to adult stages. We show that all four species share common patterns of developmental shape change resulting in a relatively lower cranial vault and expanded supraorbital torus at later developmental stages. This finding suggests that ectoneurocranial data from extant hominids can be used to model the ontogenetic trajectory for H. erectus, for which only one well-preserved very young individual is known. However, our study also reveals differences in the magnitudes and, to a lesser extent, directions of the species-specific trajectories that add to the overall shared pattern of neurocranial shape changes. We demonstrate that the very young H. erectus juvenile from Mojokerto together with subadult and adult H. erectus individuals cannot be accommodated within the pattern of the postnatal neurocranial trajectory for humans. Instead, the chimpanzee pattern might be a better 'fit' for H. erectus despite their more distant phylogenetic relatedness. The data are also compatible with an ontogenetic shape trajectory that is in some regards intermediate between that of recent H. sapiens and chimpanzees, implying a unique trajectory for H. erectus that combines elements of both extant species. Based on this new knowledge, neurocranial shape supports the assessment that KNM-ER 42700 is a young juvenile H. erectus if H. erectus followed an ontogenetic shape trajectory that was more similar to chimpanzees than humans.

Human uniqueness? Life history diversity among small-scale societies and chimpanzees.

BACKGROUND: Humans life histories have been described as "slow", patterned by slow growth, delayed maturity, and long life span. While it is known that human life history diverged from that of a recent common chimpanzee-human ancestor some ~4-8 mya, it is unclear how selection pressures led to these distinct traits. To provide insight, we compare wild chimpanzees and human subsistence societies in order to identify the age-specific vital rates that best explain fitness variation, selection pressures and species divergence. METHODS: We employ Life Table Response Experiments to quantify vital rate contributions to population growth rate differences. Although widespread in ecology, these methods have not been applied to human populations or to inform differences between humans and chimpanzees. We also estimate correlations between vital rate elasticities and life history traits to investigate differences in selection pressures and test several predictions based on life history theory. RESULTS: Chimpanzees' earlier maturity and higher adult mortality drive species differences in population growth, whereas infant mortality and fertility variation explain differences between human populations. Human fitness is decoupled from longevity by postreproductive survival, while chimpanzees forfeit higher potential lifetime fertility due to adult mortality attrition. Infant survival is often lower among humans, but lost fitness is recouped via short birth spacing and high peak fertility, thereby reducing selection on infant survival. Lastly, longevity and delayed maturity reduce selection on child survival, but among humans, recruitment selection is unexpectedly highest in longer-lived populations, which are also faster-growing due to high fertility. CONCLUSION: Humans differ from chimpanzees more because of delayed maturity and lower adult mortality than from differences in juvenile mortality or fertility. In both species, high child mortality reflects bet-hedging costs of quality/quantity tradeoffs borne by offspring, with high and variable child mortality likely regulating human population growth over evolutionary history. Positive correlations between survival and fertility among human subsistence populations leads to selection pressures in human subsistence societies that differ from those in modern populations undergoing demographic transition.

Testing the Giles hypothesis using geometric morphometrics.

OBJECTIVES: The Giles hypothesis posits that differences in the cranial morphology of Pan troglodytes and Gorilla gorilla are largely the result of allometric scaling. However, previous support for the Giles hypothesis was based on bivariate plots of linear measurements. This investigation uses geometric morphometric methods to retest this hypothesis and its prediction that extending the ontogenetic trajectory of a chimpanzee would produce an adult gorilla-like cranial morphology. MATERIALS AND METHODS: Forty-three 3D cranial landmarks were collected from an ontogenetic series of 76 Pan troglodytes and 58 Gorilla gorilla specimens. Ontogenetic trajectories of cranial shape change were computed via multivariate regression of Procrustes aligned coordinates against LnCS (size vector) and molar eruption stage (developmental vector). These two vectors were then used in developmental simulations to extend the ontogenetic trajectories of adult chimpanzees. Allometric trajectories of chimpanzees and gorillas were also directly compared using Procrustes ANOVA. RESULTS: Pan and Gorilla significantly differ in their allometric trajectories, and none of the Pan developmental simulations resembled actual adult gorillas. Additionally, the more the Pan developmental vector was extended, the more morphologically distinct the simulations became from actual adult gorillas. DISCUSSION: Taken together, these results do not support the Giles hypothesis that allometric scaling is primarily responsible for observed morphological differences between chimpanzee and gorilla crania. This investigation demonstrates that neither "growing" a chimpanzee to the size of a gorilla, nor extending a chimpanzee's developmental shape trajectory will result in an adult gorilla-like cranial morphology as they differ in their patterns of allometry.

Object sorting into a two-dimensional array in humans and chimpanzees.

Object-sorting tasks have been used as a means of assessing the cognitive development of humans. In order to investigate cognitive development from a comparative perspective, an object-sorting task was conducted in a longitudinal face-to-face situation involving three juvenile/adolescent chimpanzees (7-9 years old) and 17 children (2-5 years old). The subjects were requested to place nine blocks of different categories (distinguished by three colors and three shapes) into the cells of a box arrayed in a three-by-three pattern. Chimpanzees showed complete or partial categorical sorting in 24-43% of pre-cued trials. The youngest children had difficulty in completing a trial by placing all nine blocks into the box. Humans older than 2 years succeeded in making a one-to-one correspondence by placing a block in each cell, while the end-state pattern remained random. The children gradually increased their rate of categorical sorting, where objects of one category were placed in the same row/column; this tendency peaked at 4 years of age. Above this age, the humans spontaneously shifted their sorting strategy to make a completely even configuration (resulting in a Latin square), which may be more cognitively demanding than categorical sorting. While chimpanzees and older children used both color and shape cues for categorical sorting, younger humans preferred to use shape cues. The results of the present study show fundamental similarities between humans and chimpanzees at the basic level of categorical sorting, which indicates that some autonomous rules are applied during object manipulation.

Patterns of urinary cortisol levels during ontogeny appear population specific rather than species specific in wild chimpanzees and bonobos.

Compared with most mammals, postnatal development in great apes is protracted, presenting both an extended period of phenotypic plasticity to environmental conditions and the potential for sustained mother-offspring and/or sibling conflict over resources. Comparisons of cortisol levels during ontogeny can reveal physiological plasticity to species or population specific socioecological factors and in turn how these factors might ameliorate or exaggerate mother-offspring and sibling conflict. Here, we examine developmental patterns of cortisol levels in two wild chimpanzee populations (Budongo and Taï), with two and three communities each, and one wild bonobo population (LuiKotale), with two communities. Both species have similar juvenile life histories. Nonetheless, we predicted that key differences in socioecological factors, such as feeding competition, would lead to interspecific variation in mother-offspring and sibling conflict and thus variation in ontogenetic cortisol patterns. We measured urinary cortisol levels in 1394 samples collected from 37 bonobos and 100 chimpanzees aged up to 12 years. The significant differences in age-related variation in cortisol levels appeared population specific rather than species specific. Both bonobos and Taï chimpanzees had comparatively stable and gradually increasing cortisol levels throughout development; Budongo chimpanzees experienced declining cortisol levels before increases in later ontogeny. These age-related population differences in cortisol patterns were not explained by mother-offspring or sibling conflict specifically; instead, the comparatively stable cortisol patterns of bonobos and Taï chimpanzees likely reflect a consistency in experience of competition and the social environment compared with Budongo chimpanzees, where mothers may adopt more variable strategies related to infanticide risk and resource availability. The clear population-level differences within chimpanzees highlight potential intraspecific flexibility in developmental processes in apes, suggesting the flexibility and diversity in rearing strategies seen in humans may have a deep evolutionary history.

Digitization of the Nissen-Riesen chimpanzee radiological growth series.

Longitudinal morphological growth data of apes are incredibly difficult to obtain. Long life histories, combined with practical and ethical issues of obtaining such long-term data have resulted in few longitudinal data sets in chimpanzees of known chronological ages. One classic, long-term growth study of chimpanzees was that of Drs Nissen and Riesen initiated at the Yale Laboratories of Primate Biology in 1939. Through that study, whole-body radiological images were taken on a regular basis from a "normative" group of chimpanzees from birth to adulthood. Here we have digitized the known remaining radiographs from that growth study, many of which are deteriorating, and uploaded the data set to the free, online database MorphoSource. The database comprises 3,568 X-ray images of 15 of the 16 chimpanzee subjects in the normative group and 1 individual from an experimental group. Herein, we briefly review the historical context of this study and specific details of the data set.

Wild chimpanzees exhibit humanlike aging of glucocorticoid regulation.

Cortisol, a key product of the stress response, has critical influences on degenerative aging in humans. In turn, cortisol production is affected by senescence of the hypothalamic-pituitary-adrenal (HPA) axis, leading to progressive dysregulation and increased cortisol exposure. These processes have been studied extensively in industrialized settings, but few comparative data are available from humans and closely related species living in natural environments, where stressors are very different. Here, we examine age-related changes in urinary cortisol in a 20-y longitudinal study of wild chimpanzees (n = 59 adults) in the Kanyawara community of Kibale National Park, Uganda. We tested for three key features of HPA aging identified in many human studies: increased average levels, a blunted diurnal rhythm, and enhanced response to stressors. Using linear mixed models, we found that aging was associated with a blunting of the diurnal rhythm and a significant linear increase in cortisol, even after controlling for changes in dominance rank. These effects did not differ by sex. Aging did not increase sensitivity to energetic stress or social status. Female chimpanzees experienced their highest levels of cortisol during cycling (versus lactation), and this effect increased with age. Male chimpanzees experienced their highest levels when exposed to sexually attractive females, but this effect was diminished by age. Our results indicate that chimpanzees share some key features of HPA aging with humans. These findings suggest that impairments of HPA regulation are intrinsic to the aging process in hominids and are side effects neither of extended human life span nor of atypical environments.

Gene expression analysis reveals the tipping points during infant brain development for human and chimpanzee.

BACKGROUND: Postpartum developmental delay has been proposed as an important phenotype of human evolution which contributes to many human-specific features including the increase in brain size and the advanced human-specific cognitive traits. However, the biological processes and molecular functions underlying early brain development still remain poorly understood, especially in human and primates. RESULTS: In this paper, we comparatively and extensively studied dorsolarteral prefrontal cortex expression data in human and chimpanzee to investigate the critical processes or biological events during early brain development at a molecular level. By using the dynamic network biomarker (DNB) model, we found that there are tipping points around 3 months and 1 month, which are crucial periods in infant human and chimpanzee brain development, respectively. In particular, we shown that the human postnatal development and the corresponding expression changes are delayed 3 times relative to chimpanzee, and we also revealed that many common biological processes are highly involved in those critical periods for both human and chimpanzee, e.g., physiological system development functions, nervous system development, organismal development and tissue morphology. These findings support that the maximal rates of brain growth will be in those two critical periods for respective human and primates. In addition, different from chimpanzee, our analytic results also showed that human can further develop a number of advanced behavior functions around this tipping point (around 3 months), such as the ability of learning and memory. CONCLUSION: This work not only provides biological insights into primate brain development at a molecular level but also opens a new way to study the criticality of nonlinear biological processes based on the observed omics data.

Wild bonobo and chimpanzee females exhibit broadly similar patterns of behavioral maturation but some evidence for divergence.

OBJECTIVES: Primates exhibit variation in rates of growth and development. Variation in female growth and development across ape species appears to be explained by the Ecological Risk Aversion Hypothesis (ERAH). Indeed, existing data on variation in somatic growth and reproductive maturation between humans' closest living ape relatives, bonobos and chimpanzees, appear to be consistent with this hypothesis. However, existing data on behavioral maturation between the two species appear to contradict this hypothesis. We present novel behavioral data on infant and juvenile females from wild populations of both species in order to further evaluate predictions of the ERAH as it relates to the speed of behavioral maturation. MATERIALS AND METHODS: We analyzed 3 years of behavioral data on 17 female bonobos (<8 years of age) from LuiKotale, Democratic Republic of the Congo and 40 years of behavioral data on 30 age-matched female chimpanzees from Gombe, Tanzania. We compared the timing of (a) the attainment of independence from mothers and (b) the development of social skills using the following proxies: proximity between females and their mothers and the time that females spent engaged in eating, suckling, social play, social grooming, and riding on their mothers. RESULTS: We did not find species differences in the proportion of time that females spent in contact with their mothers or engaged in eating, suckling, social play, or social grooming. Female bonobos spent more time riding on their mothers than did female chimpanzees. Female bonobos spent more time at distances greater than 5 m from their mothers during the ages of 3-8 years, but females did not differ during the ages of 0-3 years. DISCUSSION: Behavioral maturation is largely similar between females of the two species based on the ages and proxies considered herein. We propose alternative explanations for the differences that we found in proximity and riding that do not invoke differences in underlying rates of maturation.

Human-like adrenal development in wild chimpanzees: A longitudinal study of urinary dehydroepiandrosterone-sulfate and cortisol.

The development of the adrenal cortex varies considerably across primates, being most conspicuous in humans, where a functional zona reticularis-the site of dehydroepiandrosterone-sulfate (DHEA/S) production-does not develop until middle childhood (5-8 years). Prior reports suggest that a human-like adrenarche, associated with a sharp prepubertal increase in DHEA/S, may only occur in the genus Pan. However, the timing and variability in adrenarche in chimpanzees remain poorly described, owing to the lack of longitudinal data, or data from wild populations. Here, we use urine samples from East African chimpanzees (Pan troglodytes schweinfurthii) collected over 20 years at Kanyawara in Kibale National Park, Uganda, to trace the developmental trajectories of DHEAS (n = 1,385 samples, 53 individuals) and cortisol (n = 12,726 samples, 68 individuals). We used generalized additive models (GAM) to investigate the relationship between age, sex, and hormone levels. Adrenarche began earlier in chimpanzees (~2-3 years) compared with what has been reported in humans (6-8 years) and, unlike humans, male and female chimpanzees did not differ significantly in the timing of adrenarche nor in DHEAS concentrations overall. Similar to what has been reported in humans, cortisol production decreased through early life, reaching a nadir around puberty (8-11 years), and a sex difference emerged with males exhibiting higher urinary cortisol levels compared with females by early adulthood (15-16 years). Our study establishes that wild chimpanzees exhibit a human-like pattern of cortisol production during development and corroborates prior reports from captive chimpanzees of a human-like adrenarche, accompanied by significant developmental increases in DHEAS. While the role of these developmental hormone shifts are as yet unclear, they have been implicated in stages of rapid behavioral development once thought unique to humans, especially in regard to explaining the divergence of female and male social behavior before pubertal increases in gonadal hormones.

Ontogenetic insights into the significance of mandibular corpus shape variation in hominoids: Developmental covariation between M2 crypt formation and corpus shape.

OBJECTIVES: Here, we quantify and compare the cross-sectional shape of the mandibular corpus between M1 and M2 during growth in Pan paniscus, Pan troglodytes, and Pongo pygmaeus. The goal is to assess the hypothesis that the shape of the corpus is influenced by the development of permanent molars in their crypts, by examining ontogenetic changes in corpus shape and investigating covariation between corpus shape and M2 and M3 molar crypt forms. MATERIALS AND METHODS: Ontogenetic changes in mandibular corpus shape were assessed using landmarks and semilandmarks, and measurements of length, width, and height were used to quantify molar crypts (M2 and M3 ). Ontogenetic changes in corpus growth from the eruption of M1 to the eruption of M3 were evaluated for each species through generalized Procrustes analysis and principal components analysis in shape-space and form-space. The relationship between corpus shape and molar crypt form was investigated at three different developmental stages using two-block partial least squares (2B-PLS) analysis. RESULTS: The results show clear differences in growth patterns among all three species and provide evidence that species-level differences in mandibular corpus growth occur prior to the emergence of M1 . The results of the 2B-PLS analysis reveal that significant covariance between corpus shape and molar crypt form is limited to the developmental stage marked by the emergence of M1 , with covariance between corpus shape and M2 crypt width. Corpora that are relatively narrower in the inferior portion of the cross section covary with relatively narrower M2 crypts. CONCLUSIONS: These results have important implications for understanding the taxonomic and phylogenetic significance of mandibular corpus shape variation in the hominoid fossil record.

Basicranial ontogeny comparison in Pan troglodytes and Homo sapiens and its use for developmental stage definition of KNM-ER 42700.

OBJECTIVE: This study aims to develop a comparative basis for assessing the developmental stage of KNM-ER 42700 based on the ontogenetic pattern of the ectocranial surface of the basicranium in modern humans and chimpanzees. MATERIALS AND METHODS: A total of 33 landmarks were collected from an ontogenetic sample of modern humans (80), chimpanzees (51), and 12 individuals classified as Homo erectus s.l. Ontogenetic trajectories were analyzed, and common aspects were extracted for the purpose of discriminating age groups. A regression of size on the extracted shape variables was used to investigate common ontogenetic allometry. RESULTS: The basicranial development of chimpanzees and humans follows different trajectories; however, similarities are also present. The common shape component of development extracted can be used to define age groups in both chimpanzees and modern humans. The extracted shape component presents a similar ontogenetic and static-allometric pattern in these two species. The developmental stages of H. erectus s.l. specimens were attributed following these common traits. Our analysis correctly assigned developmental stages to those specimens of H. erectus for which developmental ages are known. DISCUSSION: The component used for assessing the developmental stage has an ontogenetic allometric component. However, this shape component can discriminate age group irrespective of size and is no longer related to size when static allometry is considered. Adult H. erectus s.l. specimens were attributed to the adult category. KNM-WT 15000 fell with the late juvenile age group, whereas D2700 plotted in the region of overlap between the juvenile and adult age groups and Mojokerto with the younger age groups, as predicted by their known developmental ages. KNM-ER 42700 fell within the adult variability despite its incompletely fused spheno-occipital synchondrosis.

Opportunistic feeding strategy in wild immature chimpanzees: Implications for children as active foragers in human evolution.

Modern human (Homo sapiens) children are generally considered to be dependent on older individuals for foods, even after weaning. However, recent studies of hunter-gatherer societies have reported that children can also acquire food by themselves, although the degree of self-provisioning by children differs among groups and is considered a facultative adaptation. To investigate the dependence of children on older individuals for food and the importance of self-provisioning in early hominins, I examined feeding behavior in wild, immature chimpanzees (Pan troglodytes schweinfurthii). I studied 19 mother-offspring chimpanzee pairs in the Mahale Mountains National Park, Tanzania for approximately 22 months. Feeding behavior and interactions between mothers and their offspring were recorded. The results supported these three predictions: (1) immature chimpanzees need to feed more frequently than mothers because of increased basal metabolic rate and immature stomach capacity; (2) mothers provide effective opportunities to feed on high-quality food items which are similar to those of the mothers'; and (3) when feeding independently of their mothers, immature chimpanzees consume highly accessible food including non-adult foods nearby mothers to avoid getting lost and physical burden as with self-provisioning of human children in hunter gatherer societies. During non-simultaneous feeding bouts, immature individuals frequently consumed pith and wood. They may be valuable food items for immature individuals during their growth stage because they can be consumed year round and contain relatively higher crude ash and protein amounts, which may enable immature chimpanzees to manage the confines of their immature bodies, preventing them from matching adult feeding rhythms. This opportunistic feeding strategy is similar to self-provisioning by human children in hunter-gatherer societies. These results suggested that early hominin children performed self-provisioning based on opportunistic feeding strategies, and contributed to their food consumption by snacking in accordance with their metabolic needs and physical confines.

Factors influencing wild chimpanzee (Pan troglodytes verus) relative abundance in an agriculture-swamp matrix outside protected areas.

Human population growth and anthropogenic activities are exacerbating pressures on biodiversity globally. Land conversion is aggravating habitat fragmentation and non-human primates are increasingly compelled to live in forest-agricultural mosaics. In Sierra Leone, more than half of the wild chimpanzee population (Pan troglodytes verus) occurs outside protected areas and competes for resources with farmers. Our study area, in the Moyamba district in south-western Sierra Leone, is practically devoid of forest and is dominated by cultivated and fallow fields, swamps and mangroves. In this region, traditional slash-and-burn agriculture modifies annually the landscape, sparing swamps and mangroves and semi-domesticated oil palms (Elaeis guineensis). This study aimed to explore ecological and anthropogenic factors influencing chimpanzee relative abundance across this highly degraded and human-impacted landscape. Between 2015 and 2016, we deployed 24 camera traps systematically across 27 1.25x1.25 km grid cells. Cameras were operational over a period of 8 months. We used binomial iCAR models to examine to what extent anthropogenic (roads, settlements, abandoned settlements and human presence) and habitat variables (swamps, farmland and mangroves) shape chimpanzee relative abundance. The best model explained 43.16% of the variation with distance to roads and swamps emerging as the best predictors of chimpanzee relative abundance. Our results suggest that chimpanzees avoid roads and prefer to maintain proximity to swamps. There was no significant effect of settlements, abandoned settlements, mangroves or human presence. It appears that chimpanzees do not avoid areas frequented by people; although, our findings suggest temporal avoidance between the two species. We highlight the importance of studying chimpanzee populations living in anthropogenic habitats like agricultural-swamp matrixes to better understand factors influencing their distribution and inform conservation planning outside protected areas.

Heterochrony in chimpanzee and bonobo spatial memory development.

OBJECTIVES: The emergence of human-unique cognitive abilities has been linked to our species' extended juvenile period. Comparisons of cognitive development across species can provide new insights into the evolutionary mechanisms shaping cognition. This study examined the development of different components of spatial memory, cognitive mechanisms that support complex foraging, by comparing two species with similar life history that vary in wild ecology: bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). MATERIALS AND METHODS: Spatial memory development was assessed using a cross-sectional experimental design comparing apes ranging from infancy to adulthood. Study 1 tested 73 sanctuary-living apes on a task examining recall of a single location after a 1-week delay, compared to an earlier session. Study 2 tested their ability to recall multiple locations within a complex environment. Study 3 examined a subset of individuals from Study 2 on a motivational control task. RESULTS: In Study 1, younger bonobos and chimpanzees of all ages exhibited improved performance in the test session compared to their initial learning experience. Older bonobos, in contrast, did not exhibit a memory boost in performance after the delay. In Study 2, older chimpanzees exhibited an improved ability to recall multiple locations, whereas bonobos did not exhibit any age-related differences. In Study 3, both species were similarly motivated to search for food in the absence of memory demands. DISCUSSION: These results indicate that closely related species with similar life history characteristics can exhibit divergent patterns of cognitive development, and suggests a role of socioecological niche in shaping patterns of cognition in Pan.

Root growth and dental eruption in modern human deciduous teeth with preliminary observations on great apes.

Recent studies of dental development have indicated that root growth rates are linked to the eruption of some permanent tooth types in modern humans and Pan troglodytes. Little is known about the potential links between these aspects of dental development in deciduous teeth of any primate species. This histology study calculates the rate at which roots extend in length for human deciduous maxillary teeth and a small sample of deciduous canines and premolars from P. troglodytes and Pongo pygmaeus. Links are sought between root extension rates and previously published data for deciduous tooth emergence in each of these species. Results reported here provide the first evidence that the roots of human deciduous incisors, canines, and premolars extend in length at an accelerated rate as these teeth emerge. Accelerated extension rates in a deciduous canine from Pan coincided with the age that this tooth type emerged in captive chimpanzees. High extension rates in a canine from Pongo preceded emergence age. Preliminary observations indicate that deciduous canine and premolar roots of Pan and Pongo extend in length rapidly when compared to these tooth types from modern human children. This study provides a starting point from which to investigate new links between the incremental development of deciduous roots and tooth emergence in primates.

Epiphyseal fusion and dental development in Pan paniscus with comparisons with Pan troglodytes.

OBJECTIVES: Compared with frequent studies of skeletal development in chimpanzees, relatively little is known about bonobo skeletal development. This study seeks to explore the relationship between skeletal and dental development in both species of Pan. New data are presented for fusion sites not previously observed in bonobos. MATERIALS AND METHODS: In a sample of 34 Pan paniscus and 168 Pan troglodytes subadults, state of fusion was recorded for 30 epiphyseal fusion sites using a three-stage system of unfused, midfusion, and complete fusion based on Wintheiser, Clauser, and Tappen. Stage of dental development for permanent mandibular dentition was assessed using the Demrijian, Goldstein, and Tanner method. These data allowed for comparisons of both species of Pan and the two subspecies of P. troglodytes. RESULTS: The sequence of fusion events was generally consistent between the two species, but some exceptions may exist for the knee and ankle. The number of fusion events that occurred after complete dental mineralization was similar in both species. No statistically significant differences were found in the fusion timing for the subspecies of P. troglodytes. DISCUSSION: Bolter and Zihlman suggested that fusion at the acetabulum occurs earlier in Pan paniscus, while fusion of epiphyses at the knee are delayed, compared with P. troglodytes. Our data do not indicate earlier fusion of the acetabulum, but fusion events at the knee may complete later relative to dental mineralization in Pan pansicus. Compared with Homo sapiens, both P. troglodytes and Pan paniscus demonstrate later completion of epiphyseal fusion relative to dental mineralization.

Brain size growth in wild and captive chimpanzees (Pan troglodytes).

Despite many studies of chimpanzee brain size growth, intraspecific variation is under-explored. Brain size data from chimpanzees of the Taï Forest and the Yerkes Primate Research Center enable a unique glimpse into brain growth variation as age at death is known for individuals, allowing cross-sectional growth curves to be estimated. Because Taï chimpanzees are from the wild but Yerkes apes are captive, potential environmental effects on neural development can also be explored. Previous research has revealed differences in growth and health between wild and captive primates, but such habitat effects have yet to be investigated for brain growth. Here, I use an iterative curve fitting procedure to estimate brain growth and regression parameters for each population, statistically comparing growth models using bootstrapped confidence intervals. Yerkes and Taï brain sizes overlap at all ages, although the sole Taï newborn is at the low end of captive neonatal variation. Growth rate and duration are statistically indistinguishable between the two populations. Resampling the Yerkes sample to match the Taï sample size and age group composition shows that ontogenetic variation in the two groups are remarkably similar despite the latter's limited size. Best fit growth curves for each sample indicate cessation of brain size growth at around 2 years, earlier than has previously been reported. The overall similarity between wild and captive chimpanzees points to the canalization of brain growth in this species.