Adult Chimpanzees Research Articles

Age-related change in adult chimpanzee social network integration.

BackgroundSocial isolation is a key risk factor for the onset and progression of age-related disease and mortality in humans. Nevertheless, older people commonly have narrowing social networks, with influences from both cultural factors and the constraints of senescence. We evaluate evolutionarily grounded models by studying social aging in wild chimpanzees, a system where such influences are more easily separated than in humans, and where individuals are long-lived and decline physically with age.MethodologyWe applied social network analysis to examine age-related changes in social integration in a 7+ year mixed-longitudinal dataset on 38 wild adult chimpanzees (22 females, 16 males). Metrics of social integration included social attractivity and overt effort (directed degree and strength), social roles (betweenness and local transitivity) and embeddedness (eigenvector centrality) in grooming networks.ResultsBoth sexes reduced the strength of direct ties with age (males in-strength, females out-strength). However, males increased embeddedness with age, alongside cliquishness. These changes were independent of age-related changes in social and reproductive status. Both sexes maintained highly repeatable inter-individual differences in integration, particularly in mixed-sex networks.Conclusions and implicationsAs in humans, chimpanzees appear to experience senescence-related declines in social engagement. However, male social embeddedness and overall sex differences were patterned more similarly to humans in non-industrialized versus industrialized societies. Such comparisons suggest common evolutionary roots to ape social aging and that social isolation in older humans may hinge on novel cultural factors of many industrialized societies. Lastly, individual and sex differences are potentially important mediators of successful social aging in chimpanzees, as in humans. Lay summary: Few biological models explain why humans so commonly have narrowing social networks with age, despite the risk factor of social isolation that small networks pose. We use wild chimpanzees as a comparative system to evaluate models grounded in an evolutionary perspective, using social network analysis to examine changes in integration with age. Like humans in industrialized populations, chimpanzees had lower direct engagement with social partners as they aged. However, sex differences in integration and older males’ central positions within the community network were more like patterns of sociality in several non-industrialized human populations. Our results suggest common evolutionary roots to human and chimpanzee social aging, and that the risk of social isolation with age in industrialized populations stems from novel cultural factors.

Comparative analysis of sperm motility in liquid and seminal coagulum portions between Bornean orangutan (Pongo pygmaeus) and chimpanzee (Pan troglodytes).

Coagulum in the semen of some primates plays different roles, depending on the species. In the present study, we examined sperm motility in the coagulum and liquid portions of semen collected from captive individuals from two great ape species: two adult Bornean orangutans (Pongo pygmaeus) (n = 27) and three adult chimpanzees (Pan troglodytes) (n = 14). The results revealed that orangutan sperm remained motile for significantly longer in the coagulum than in the liquid portion (> 18h). By contrast, chimpanzee sperm motility did not differ significantly over time between the two portions of the semen, although motility was slightly higher in the liquid portion than in the coagulum. The evolution of the seminal coagulum is thought to be related to postcopulatory sperm competition; however, functions of seminal coagulum have not been completely elucidated. Our data from the orangutan semen suggest that in this species, seminal coagulum may strengthen own-sperm survival. This report is the first to provide evidence for this distinctive function of the seminal coagulum. This unique property of orangutan seminal coagulum might be attributable to their reproductive traits, e.g., difficulty in predicting ovulation due to a lack of genital swelling during estrus. The orangutan is a Critically Endangered species, and captive breeding, including artificial insemination (AI), is expected. However, worldwide, only one case of orangutan AI has been successful. Our findings may contribute to an understanding of their basic semen characteristics and help improve the AI method.

The development of affiliative and coercive reproductive tactics in male chimpanzees.

Like many animals, adult male chimpanzees often compete for a limited number of mates. They fight other males as they strive for status that confers reproductive benefits and use aggression to coerce females to mate with them. Nevertheless, small-bodied, socially immature adolescent male chimpanzees, who cannot compete with older males for status nor intimidate females, father offspring. We investigated how they do so through a study of adolescent and young adult males at Ngogo in Kibale National Park, Uganda. Adolescent males mated with nulliparous females and reproduced primarily with these first-time mothers, who are not preferred as mating partners by older males. Two other factors, affiliation and aggression, also influenced mating success. Specifically, the strength of affiliative bonds that males formed with females and the amount of aggression males directed toward females predicted male mating success. The effect of male aggression toward females on mating success increased as males aged, especially when they directed it toward females with whom they shared affiliative bonds. These results mirror sexual coercion in humans, which occurs most often between males and females involved in close, affiliative relationships.

Testing the Giles hypothesis using geometric morphometrics.

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. 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. 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. 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.

Fellatio among male sanctuary-living chimpanzees during a period of social tension

Abstract Same-sex sexual behaviour has been documented across the animal kingdom, and is thought to reflect and enhance dyadic cooperation and tolerance. For instance, same-sex fellatio — the reception of a partner’s penis into another’s mouth — has been reported in several mammalian species other than humans. Although same-sex sexual behaviour is observed in our close relatives, the chimpanzees, fellatio appears to be very rare — as yet there are no published reports clearly documenting its occurrence. At Chimfunshi Wildlife Orphanage in Zambia, we observed an instance of fellatio occurring during a post-conflict period between two adult male chimpanzees (born and mother-reared at the sanctuary) where one of the males was the victim. We discuss this event with respect to the putative functions of homosexual behaviour in great apes. Given its rarity in chimpanzees, this fellatio between adult males also highlights the apparent behavioural flexibility present in our close relatives.

Social Network Predicts Exposure to Respiratory Infection in a Wild Chimpanzee Group.

Respiratory pathogens are expected to spread through social contacts, but outbreaks often occur quickly and unpredictably, making it challenging to simultaneously record social contact and disease incidence data, especially in wildlife. Thus, the role of social contacts in the spread of infectious disease is often treated as an assumption in disease simulation studies, and few studies have empirically demonstrated how pathogens spread through social networks. In July–August 2015, an outbreak of respiratory disease was observed in a wild chimpanzee community in Kibale National Park, Uganda, during an ongoing behavioral study of male chimpanzees, offering a rare opportunity to evaluate how social behavior affects individual exposure to socially transmissible diseases. From May to August 2015, we identified adult and adolescent male chimpanzees displaying coughs and rhinorrhea and recorded 5-m proximity data on males (N = 40). Using the network k-test, we found significant relationships between male network connectivity and the distribution of cases within the network, supporting the importance of short-distance contacts for the spread of the respiratory outbreak. Additionally, chimpanzees central to the network were more likely to display clinical signs than those with fewer connections. Although our analyses were limited to male chimpanzees, these findings underscore the value of social connectivity data in predicting disease outcomes and elucidate a potential evolutionary cost of being social.Supplementary informationThe online version of this article (10.1007/s10393-020-01507-7) contains supplementary material, which is available to authorized users.

Dexterity and technique in termite fishing by chimpanzees (Pan troglodytes troglodytes) in the Goualougo Triangle, Republic of Congo

Although the phenomenon of termite fishing by chimpanzees (Pan troglodytes) has historical and theoretical importance for primatology, we still have a limited understanding of how chimpanzees accomplish this activity, and in particular, about details of skilled actions and the nature of individual variation in fishing techniques. We examined movements, hand positions, grips, and other details from remote video footage of seven adult and subadult female chimpanzees using plant probes to extract Macrotermes muelleri termites from epigeal nests. Six chimpanzees used exclusively one hand (left or right) to grip the probe during termite fishing. All chimpanzees used the same repertoire of actions to insert, adjust, and withdraw the probe but differed in the frequency of use of particular actions. Chimpanzees have been described as eating termites in two ways—directly from the probe or by sweeping them from the probe with one hand. We describe a third technique: sliding the probe between the digits of one stationary hand as the probe is extracted from the nest. The sliding technique requires complementary bimanual coordination (extracting with one hand and grasping lightly with the other, at the same time). We highlight the importance of actions with two hands—one gripping, one assisting—in termite fishing and discuss how probing techniques are correlated with performance. Additional research on digital function and on environmental, organismic, and task constraints will further reveal manual dexterity in termite fishing.

Unpacking chimpanzee (Pan troglodytes) patch use: Do individuals respond to food patches as predicted by the marginal value theorem?

The marginal value theorem is an optimal foraging model that predicts how efficient foragers should respond to both their ecological and social environments when foraging in food patches, and it has strongly influenced hypotheses for primate behavior. Nevertheless, experimental tests of the marginal value theorem have been rare in primates and observational studies have provided conflicting support. As a step towards filling this gap, we test whether the foraging decisions of captive chimpanzees (Pan troglodytes) adhere to the assumptions and qualitative predictions of the marginal value theorem. We presented 12 adult chimpanzees with a two-patch foraging environment consisting of both low-quality (i.e., low-food density) and high-quality (i.e., high-food density) patches and examined the effect of patch quality on their search behavior, foraging duration, marginal capture rate, and its proxy measures: giving-up density and giving-up time. Chimpanzees foraged longer in high-quality patches, as predicted. In contrast to predictions, they did not depress high-quality patches as thoroughly as low-quality patches. Furthermore, since chimpanzees searched in a manner that fell between systematic and random, their intake rates did not decline at a steady rate over time, especially in high-quality patches, violating an assumption of the marginal value theorem. Our study provides evidence that chimpanzees are sensitive to their rate of energy intake and that their foraging durations correlate with patch quality, supporting many assumptions underlying primate foraging and social behavior. However, our results question whether the marginal value theorem is a constructive model of chimpanzee foraging behavior, and we suggest a Bayesian foraging framework (i.e., combining past foraging experiences with current patch sampling information) as a potential alternative. More work is needed to build an understanding of the proximate mechanisms underlying primate foraging decisions, especially in more complex socioecological environments.

Social relationships among adult male chimpanzees (Pan troglodytes schweinfurthii): variation in the strength and quality of social bonds

Socioecological environments that promote frequent interaction and social tolerance have favored the evolution of strong and equitable social bonds, which facilitate cooperation and confer fitness benefits. In most species, kinship is the primary predictor of bond strength and quality, but it does not adequately explain partner choice among adult male chimpanzees. Instead, most bonds and cooperative interactions occur among unrelated individuals, likely because of a lack of brothers among available partners. To identify what factors drive partner choice when kinship does not, we investigated the strength and quality of social bonds among adult male chimpanzees (N = 26) at Gombe National Park, Tanzania. Maternal brothers, when present, formed stronger bonds than other dyads, and maternal brothers who were close in age or rank formed the strongest bonds. Among dyads that were not maternal brothers, however, the strength of social bonds was not associated with either age difference or rank difference. Additionally, dyads with stronger bonds groomed more equitably than other dyads, as did maternal brothers and dyads close in rank. Bonds were stable, community-wide, for 2 years on average, while lasting up to 13 years. Overall, there remains no clear-cut explanation for partner choice among male chimpanzees. Demographic constraints limit the impact of kinship, and the effects of age and rank difference are small or nonexistent, suggesting that bond strength results from a more complex process than a simple accounting of basic characteristics. Instead, dyads may possess some distinct quality that engenders strength and stability, such as compatible personalities. Strong social ties are associated with increased fitness in several species, including humans, baboons, feral horses, and dolphins. Although close kin typically form the strongest bonds, kinship explains only a limited number of bonds (and associated cooperation) among adult male chimpanzees. To identify what factors influence partner choice when kinship does not, we examined social bonds among adult male chimpanzees at Gombe National Park, Tanzania. Males formed strong, stable bonds and groomed more equitably with strong bond partners. Nevertheless, kinship, age difference, and rank difference did not fully explain male partner choice. These results contrast with findings in other species, where stronger and more equitable bonds are typically formed by kin as well as those that are close in age and rank. Instead, male chimpanzee dyads may possess some distinct quality that promotes strength and stability, such as compatible personalities.

Adolescent and young adult male chimpanzees form affiliative, yet aggressive, relationships with females

Primates frequently form affiliative relationships that have important fitness consequences. Affiliative relationships between unrelated males and females are ubiquitous in humans but are not widely reported in humans' closest living relatives, chimpanzees (Pan troglodytes). Instead, adult male chimpanzees are extremely aggressive to females, using the aggression to coerce females to mate with them. Adolescent male chimpanzees are physically and socially immature and unable to use aggression toward females in the same way as adult males. Instead, adolescent males might build affiliative relationships with females as an alternative tactic to increase their chances of mating and reproducing. To investigate this possibility, we recorded social interactions between 20 adolescent and 10 young adult males and 78 adult female chimpanzees over 2 years at Ngogo in Kibale National Park, Uganda. Analyses using grooming and proximity as assays revealed that adolescent and young adult males formed differentiated, affiliative relationships with females. These relationships were as strong as the bonds young males formed with maternal kin and unrelated males and increased in strength and number as males aged and started to dominate females. Male-female relationships extended outside the immediate context of mating. Although males affiliated slightly more often with females when they were cycling, they also did so when females were pregnant and nursing young infants. Males and females who formed bonds reassured each other, looked back and waited for each other while traveling, and groomed more equitably than other male-female pairs, even after the time they spent together in association and the female's reproductive state were taken into account. Despite the affiliative nature of these relationships, adolescent and young adult males selectively targeted their female partners for aggression. These findings provide new insights into the evolution of social bonds between human females and males, which can involve both affiliation and coercive violence.

The role of early social rearing, neurological, and genetic factors on individual differences in mutual eye gaze among captive chimpanzees

Mutual eye gaze plays an important role in primate social development and communication. In the current study, we examined the underlying experiential, genetic, and neuroanatomical basis of mutual eye gaze variation in adult captive chimpanzees. A multivariate analysis of variance revealed a significant rearing effect on bout length, with human-reared chimpanzees engaging in longer bouts of mutual gaze compared to mother-reared and wild-born individuals. Next, we utilized source-based morphometry (SBM) to examine gray matter covariation in magnetic resonance imaging scans and determine the relationship between the resulting gray matter covariation components and mutual eye gaze. One SBM component was negatively correlated with gaze duration (nucleus accumbens and anterior insular cortex), while two components were positively correlated with bout length (posterior cingulate cortex, inferior occipital cortex, middle temporal cortex, hippocampus, and the precentral sulcus). Finally, heritability analyses revealed mutual eye gaze to be modestly heritable and significant genetic correlations between bout length and two gray matter covariation components. This study reveals that non-genetic factors, and to a lesser extent, genetic factors appear to influence mutual eye gaze in adult chimpanzees, and is the first to report neuroanatomical correlates of mutual eye gaze variation in chimpanzees.

The Activity Budget of Adult Chimpanzees (Pan troglodytes troglodytes) and Environmental Conditions in Mefou Primate Sanctuary, Centre Region, Cameroon

Time is a limited parameter of various activities for most wild animals. The study of time budget allotment among various diurnal activities such as feeding, resting, grooming, travelling is essential for the characterization of living and working manner of a primate species, which lay the foundation for interrelating ecology and the behaviour of the species. The survey was focused on determining the activity budget of chimpanzees and their environmental needs in Mefou primate sanctuary. The research data collection was initiated after a brief pilot study of the study area and the animal group. A 4-month-data collection from April – July was carried out and was done the first 10 days of each month, from 8:00am – 6:00pm. The activities of the focal animal observed were recorded alongside the scan data obtained in a 15-minute-interval period. Additionally, observations were recorded on environmental changes. The research data was analyzed by the use of chi-square statistical model. The focal animal revealed a significant relationship on activity budget, X2 = 23.818 df=6, p=0.001. Resting (30%), movement (28%) and feeding (22%) dominated the activity budget profile, while grooming (9%), aggression (6%), play (3%) and climbing (2%) were the least activities observed. The Adult chimpanzees’ behavior showed an association on weather type, X2 = 15.801 df=3, p=0.001 and X2 = 28.975 df=18, p=0.049, respectively. Seasonality witnessed a weak association on the adult animals, X2 = 2.492 df=6, p<0.05. Also, a weak significance was recorded on social activity and seasonality, X2 = 1.379 df=6, p<0.05. This survey discovered that the activities of the adult chimpanzees witnessed alterations due to the changes of environmental conditions.

A chomped chimp: New evidence of tooth marks on an adult chimpanzee (Pan troglodytes verus).

To describe and interpret previously unreported marks on the dry cranium of an adult chimpanzee (Pan troglodytes verus) from Côte d'Ivoire at the Smithsonian's National Museum of Natural History (USNM 450071). All marks on the cranium were documented and assessed through physical examination of the specimen, photography, micro-computed tomography (micro-CT), and 3D laser scanning. Pits and punctures were measured with digital calipers for comparison with published carnivore tooth mark measurements. The cranium shows perimortem or postmortem damage to the temporal, occipital, and maxillary regions that is not recent. Size and color variation in the marks suggest two damage events, possibly involving chewing by different animals, at least one of which was a large-bodied mammal. The 22 tooth pits and punctures (0.89-8.75 mm in maximum length and 0.88-6.63 mm in breadth) overlap in size with those inflicted by wild leopards, the most significant predators of common chimpanzees due to their largely overlapping ecological distributions. Based on qualitative and quantitative evidence, we conclude that leopards are the most likely cause of the most prominent marks on the cranium. However, we cannot rule out the additional possibility of other chimpanzees, although there are no published studies of chimpanzee tooth marks for direct comparison. This study is the most extensive documentation to date of a modern adult chimpanzee skull exhibiting tooth marks by a large mammal, thus providing new evidence to help identify and interpret other events of predation and scavenging of large-bodied apes in the modern and fossil records.

Adolescent male chimpanzees (Pan troglodytes) form social bonds with their brothers and others during the transition to adulthood.

Social relationships play an important role in animal behavior. Bonds with kin provide indirect fitness benefits, and those with nonkin may furnish direct benefits. Adult male chimpanzees (Pan troglodytes) exhibit social bonds with maternal brothers as well as unrelated adult males, facilitating cooperative behavior, but it is unclear when these bonds develop. Prior studies suggest that social bonds emerge during adolescence. Alternatively, bonds may develop during adulthood when male chimpanzees can gain fitness benefits through alliances used to compete for dominance status. To investigate these possibilities and to determine who formed bonds, we studied the social relationships of adolescent and young adult male chimpanzees (N = 18) at Ngogo in Kibale National Park, Uganda. Adolescent male chimpanzees displayed social bonds with other males, and they did so as often as did young adult males. Adolescent and young adult males frequently joined subgroups with old males. They spent time in proximity to and grooming with old males, although they also did so with their age peers. Controlling for age and age difference, males formed strong association and proximity relationships with their maternal brothers and grooming relationships with their fathers. Grooming bonds between chimpanzee fathers and their adolescent and young adult sons have not been documented before and are unexpected because female chimpanzees mate with multiple males. How fathers recognize their sons and vice versa remains unclear but may be due to familiarity created by relationships earlier in development.

Dominance rank and the presence of sexually receptive females predict feces-measured body temperature in male chimpanzees.

Quantifying the costs of mating is key for understanding life-history trade-offs. As a reflection of metabolic rate, body temperature is one metric for assaying these costs. However, conventional methods for measuring body temperature are invasive and unsuitable for the study of free-living populations of endangered species, including great apes. A promising proxy for body temperature is fecal temperature, the internal temperature of fecal deposits shortly following defecation. We validated this method with humans, finding that maximum fecal temperature is a reliable proxy for rectal temperature. We then applied this method to wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda. We collected and analyzed 101 fecal temperature measurements from 43 adult chimpanzees (male: N = 28; female: N = 15). Chimpanzee fecal temperature ranged from 33.4 to 38.9 °C, with a mean of 35.8 °C. Although fecal temperature was not predicted by sex, age, or ambient temperature, male fecal temperature was 1.1 °C higher on days when sexually receptive females were present and was positively correlated with male dominance rank. Post hoc analyses showed that overall copulation rates, but not aggression rates, were positively correlated with fecal temperature, suggesting that sexual physiology and behavior best explain mating-related temperature variation. Together, these results indicate fecal temperature is a reliable proxy for core body temperature in large-bodied mammals, captures metabolic costs associated with male mating behavior, and represents a valuable noninvasive tool for biological field research.

Spontaneous categorization of tools based on observation in children and chimpanzees

The acquisition of the concept of ‘tool’ remains intriguing from both developmental and comparative perspectives. Our current model of tool use development in children is based on humans’ supposedly unique ability to adopt a teleological stance: the understanding of a demonstrator’s goal-based intentions when using a tool. It is however unclear how children and chimpanzees, our closest relatives, combine their knowledge of different objects whose function is to act on other parts of the environment, and assign them to a single category of ‘tools’. Here, we used a function-based approach to address this question. We exposed 7 to 11-year-old children and adult chimpanzees to a Matching-to-Function (MTF) task to explore whether they would sort tools and non-tools separately after demonstration of their function by an experimenter. MTF is a variant of Matching-to-Sample where the sample and the target are from the same category/kind rather than identical. Around 40% of children paired objects according to their function in the MTF task, with only one child younger than 8 years doing so. Moreover, when verbally questioned, these children offered a function-based answer to explain their choices. One of six chimpanzees also successfully paired objects according to function. Children and at least one chimpanzee can thus spontaneously sort tools into functional categories based on observing a demonstrator. The success of a single chimpanzee in our task suggests that teleological reasoning might already have been present in our last common ancestor but also shows that human children more readily conceptualize tools in a spontaneous fashion.

URINARY MARKERS OF OXIDATIVE STRESS CORRESPOND TO INFECTION AND AGING IN WILD CHIMPANZEES

Oxidative stress (OS) plays a central role in aging and results from a variety of stressors, making it a powerful measure of health and a way to examine phylogenetic variation in life history. However, few urinary OS markers have been examined under field conditions, particularly in primates, and their utility to non-invasively monitor acute vs. chronic conditions is poorly understood. In this study, we examined variation in 5 urinary markers of oxidative damage and protection under 5 validation paradigms in 37 wild, adult chimpanzees living in the Kibale National Park, Uganda. We used 925 urine samples to conduct both cross-sectional and within-individual analyses of responses to acute infection and variation with age. Markers of damage (8-OHdG, F-isoprostanes, MDA-TBARS, and neopterin) and total antioxidant capacity were generally positively correlated with one another. Within individuals, all markers responded to at least one if not both types of acute infection. Markers of damage also varied with age, particularly in individuals near death. Unlike in human and rodent tissues, DNA damage in urine decreased with age, both across and within individuals near death, suggesting a potential decline in DNA repair and/or metabolic rate during senescence. Our results suggest that OS can be measured using field-collected urine and may be useful for both short- and long-term indicators of health. Our results further confirm that using multiple markers and longitudinal sampling within individuals is the most productive approach for studies that seek to determine the role of OS in health and lifespan in long-lived organisms.

Exploration of potential triggers for self-directed behaviours and regurgitation and reingestion in zoo-housed chimpanzees

The unique challenges faced by animals living in zoos can lead to the production of anxiety-related behaviours. In this study we aimed to understand what specific factors may cause chimpanzees to display these behaviours. In non-human primates, displacement behaviours, such as self-scratching and yawning, are considered markers of anxiety and stress, and Regurgitation and Reingestion (R/R) is considered an abnormal behaviour with negative consequences for physical health. We examined the possible triggers of R/R, scratching, and yawning in a group of zoo-housed chimpanzees and followed this up with an analysis of long-term data to examine further aspects of R/R behaviour. In the first study we conducted focal observations on 18 adult chimpanzees at Edinburgh Zoo, UK, in addition to all occurrence sampling of visitors using flash photography, screaming and banging on the glass in the exhibit. 158 h of data were analysed and Generalised Linear Mixed Models revealed that yawning was significantly more likely if there was a long period of time since the last feed and when there were moderate numbers of visitors in the zoo. There were trends that yawning was more likely to occur if children screamed and that scratching was more likely to occur if visitors used flash photography. R/R occurred most often within 40 min of a feed, but was not affected by the inter-feed interval preceding that feed, positive or negative social interactions, or visitor numbers or behaviour. As there was no obvious daily trigger for R/R, an analysis of long-term data (2009 to 2015) was conducted to investigate if social or dietary factors affected rates of R/R over a larger timescale. It was found that R/R rates in the months before a significant diet change were not different from R/R rates in the months after, but it was found that R/R rates decreased over the five-year period. Lastly, we found no evidence that the introduction of individuals engaging in R/R lead to resident chimpanzees habitually adopting the behaviour, despite considerable opportunities to observe it. These findings have implications for welfare interventions aimed to reduce R/R and/or anxiety behaviours in captive populations and for the translocation of individuals that are known to engage in R/R between groups.

Short and long-term temporal consistency of hand preference in sanctuary chimpanzees (Pan troglodytes) for unimanual and bimanual coordinated tasks

Longitudinal research on manual preferences in humans and non-human primates has mainly been conducted from a developmental perspective, with only a few studies exploring long-term stability of this trait during adulthood. Therefore, we investigated short-term (1 year) and long-term (10 and 11 years) consistency of hand preference in a naturalistic group of 19 juvenile and adult chimpanzees (Pan troglodytes) by using two experimental tasks, one unimanual (simple reaching) and one bimanual (tube task). The experimental sessions were conducted in 2007, 2008 and 2018. We found that the direction of hand preference (right vs. left) in the tube task remained stable after both short-time and long-time periods. Conversely, hand preference direction for simple reaching was not consistent after the longest period (11 years), but the handedness indices (HI) between 2007 and 2008 (1-year period) and between 2008 and 2018 (10-year period) were positively correlated. The comparison between tasks confirmed that all the chimpanzees were more strongly lateralised for the tube task. Interestingly, however, the strength of hand preference in the tube task showed an increasing trend in the long term. We hypothesize that this could be a consequence of practice and experience with a particular motor action.

Comparative morphology and ontogeny of the thoracolumbar transition in great apes, humans, and fossil hominins

Variation among extant hominoid taxa in the anatomy of the thoracolumbar vertebral transition is well-established and constitutes an important framework for making inferences about posture and locomotion in fossil hominins. However, little is known about the developmental bases of these differences, posing a challenge when interpreting the morphology of juvenile hominins. In this study, we investigated ontogenetic variation in the thoracolumbar transition of juvenile and adult great apes, humans, and fossils attributed to Australopithecus and early Pleistocene Homo erectus. For each vertebra involved in the transition, we quantified functionally relevant aspects of zygapophyseal form: facet curvature in the transverse plane, facet orientation relative to midline, and the shift in these variables across the thoracolumbar transition, from the antepenultimate rib-bearing thoracic to the first lumbar vertebra (L1). Among extant hominids, adult individuals of Pan and Homo exhibit a greater shift in facet morphology across the thoracolumbar transition in comparison to Gorilla and Pongo. This pattern is driven by interspecific differences in the L1 facets, with those of chimpanzees and humans being more curved and more sagittally oriented. Chimpanzees and humans also experience more change in facet morphology during development relative to gorillas and orangutans. Humans differ from chimpanzees in achieving their adultlike configuration much earlier in development. The fossil specimens indicate that early hominins had adult morphologies that were similar to those of extant Homo and Pan, and that they achieved their adult morphologies early in development, like extant humans. Although it is unclear why adult chimpanzees and hominins share an adult morphology, we speculate that the early acquisition of adultlike L1 zygapophyseal morphology in hominins is an evolutionary novelty related to conferring stability to a relatively long lumbar spine as young individuals are learning to walk bipedally.