What does variation in primate behavior mean?

Variation in monthly and seasonal elevation use impacts behavioral and dietary flexibility in Rhinopithecus bieti

Black-and-white snub-nosed monkeys (Rhinopithecus bieti) rely on behavioral and dietary flexibility to survive in temperate latitudes at high-elevation habitats characterized by climate and resource seasonality. However, little is known about how elevation influences their behavioral and dietary flexibility at monthly or seasonal scales. We studied an isolated R. bieti population at Mt. Lasha in the Yunling Provincial Nature Reserve, Yunnan, China, between May 2008 and August 2016 to assess the impacts of elevation on feeding behavior and diet. Across our sample, R. bieti occupied elevations between 3031 and 3637 m above mean sea level (amsl), with a 315.1 m amsl range across months and a 247.3 m amsl range across seasons. Contrary to expectations, individuals spent less time feeding when ranging across higher elevations. Lichen consumption correlated with elevation use across months and seasons, with individuals spending more time feeding on this important resource at higher elevations. Leaf consumption only correlated with elevation use during the spring. Our results suggest that R. bieti do not maximize their food intake at higher elevations and that monthly and seasonal changes in lichen and leaf consumption largely explain variation in elevation use. These findings shed light on the responses of R. bieti to environmental change and offer insight into strategies for conserving their habitats in the face of anthropogenic disturbance.

Is resilience a unifying concept for the biological sciences?

There is increasing interest in applying resilience concepts at different scales of biological organization to address major interdisciplinary challenges from cancer to climate change. It is unclear, however, whether resilience can be a unifying concept consistently applied across the breadth of the biological sciences, or whether there is limited capacity for integration. In this review, we draw on literature from molecular biology to community ecology to ascertain commonalities and shortcomings in how resilience is measured and interpreted. Resilience is studied at all levels of biological organization, although the term is often not used. There is a suite of resilience mechanisms conserved across biological scales, and there are tradeoffs that affect resilience. Resilience is conceptually useful to help diverse researchers think about how biological systems respond to perturbations, but we need a richer lexicon to describe the diversity of perturbations, and we lack widely applicable metrics of resilience.

Rushing for "burned" food: Why and how does a group of patas monkeys (Erythrocebus patas) reach freshly burned areas?

Recently, considerable attention has been paid to animal adaptations to anthropogenic environments, such as foraging in burned areas where plants are promoted to regenerate by anthropogenic burning. However, among primates, reports on the utilization of resources that are available immediately after burning have been limited to a few primate species. In this study, we investigated and compared the activity budgets and food categories of a group of patas monkeys (Erythrocebus patas) in freshly burned areas by comparing them with those in previously burned areas and unburned areas. We also assessed the proportion of time spent in the freshly burned area before and after the fire: GPS collars were fitted to five of the six adults in the group, and their patterns when they traveled toward freshly burned and unburned feeding areas were compared. Patas monkeys spent more time in freshly burned areas after the fire, and they visited such areas mostly for feeding, particularly on roasted seeds of Cissus populnea. Furthermore, patas monkeys traveled faster and in a more synchronized way toward freshly burned areas. This "apparent goal-directed" travel began at least 1 h before arriving. Results indicate that the group recognized freshly burned areas as valuable, and the monkeys were able to travel in a goal-directed manner to them despite their variable locations. We suggest that smoke from freshly burned areas provides a visual cue with which to orient to the burned areas. Our results also support the notion that some primates are flexible enough to adapt to and benefit from anthropogenic environmental changes.

Socioecological factors influencing intraspecific variation in ranging dynamics of western lowland gorillas (Gorilla gorilla gorilla) in Ndoki Forest

Ranging dynamics are physical and behavioral representations of how different socioecological factors affect an organism's spatial decisions and space use strategies. Western lowland gorillas (Gorilla gorilla gorilla) are a model species to investigate the drivers of spatial dynamics based on both the natural variation in socioecological factors within the species and compared with their mountain gorilla counterparts. In this study, we evaluate the influences of resource seasonality and social dynamics on variation in home range size, utilization, and intergroup overlap among multiple gorilla groups over an 8-year study period in the northern Republic of Congo. This study shows that western lowland gorillas can have small home ranges comparable to mountain gorillas, rather than universally larger home ranges as previously supposed, and that home ranges are stable through time. The largest source of variation in space use was the degree of intergroup home range overlap. The study groups did not demonstrate intraspecific variation in range size nor changes in intergroup overlap with respect to seasonality of fruit resources, but all groups demonstrated expansion of monthly range and core area with group size, matching predictions of intragroup feeding competition. These findings highlight the potential impact of intergroup relationships on space use and prompt further research on the role of social dynamics in ranging strategies. In this study, we reveal a greater degree of variability and flexibility in gorilla ranging behavior than previously realized which is relevant to improving comparative studies and informing conservation strategies on behalf of these endangered primates.

Group-specific expressions of co-feeding tolerance in bonobos and chimpanzees preclude dichotomous species generalizations

Bonobos are typically portrayed as more socially tolerant than chimpanzees, yet the current evidence supporting such a species-level categorization is equivocal. Here, we used validated group-level co-feeding assays to systematically test expressions of social tolerance in sixteen groups of zoo- and sanctuary-housed bonobos and chimpanzees. We found that co-feeding tolerance substantially overlaps between the species, thus precluding categorical inference at the species level. Instead, marked differences were observed between groups, with some bonobo communities exhibiting higher social tolerance than chimpanzee communities, and vice versa. Moreover, considerable intergroup variation was found within species living in the same environment, which attests to Pan's behavioral flexibility. Lastly, chimpanzees showed more tolerance in male-skewed communities, whereas bonobos responded less pronounced to sex-ratio variation. We conclude that the pervasive dichotomy between the tolerant bonobo and the belligerent chimpanzee requires quantitative nuance, and that accurate phylogenetic tracing of (human) social behavior warrants estimations of intraspecific group variation.

Vigilance has mainly a social function in a wild group of spider monkeys (Ateles geoffroyi)

Vigilance is a widespread behavior that allows individuals to socially acquire information and/or effectively detect potential risks posed by predators and conspecifics. In this study, we aimed to investigate how social factors (i.e., subgroup size, number of males and immatures in the subgroup, presence of fission and fusion events, subgroup activity), individual characteristics (i.e., sex, presence of dependent offspring) and possible vulnerability to predation (i.e., being in smaller subgroups or lower in the canopy) explain variation in vigilance behavior in a wild group of spider monkeys (Ateles geoffroyi). We collected vigilance data during focal samples of all adults, subadults and juveniles of the group (N = 38), and ran generalized linear mixed models with a Bayesian approach. We found that the proportion of time both sexes spent in vigilance increased with subgroup size and during fusion events. Individuals also spent more time in vigilance when the subgroup was resting or socializing compared to other activities. Moreover, the proportion of time spent in vigilance increased in subgroups with more immatures. An alternative model with similar fit suggested that the proportion of time spent in vigilance increased in females when subgroups included more adult and subadult males. Overall, these results suggest that our study group mainly directed vigilance toward conspecifics (i.e., social vigilance), probably as a result of the relatively low predation pressure experienced, and contribute to the understanding of the multiple social factors affecting vigilance in group-living primates.

Social networks and tolerance differences associated to grouping patterns in Ateles geoffroyi

Each species displays a certain level of social tolerance. However, within a species, individuals show differences in their ability to accept the presence of conspecifics around resources of mutual interest. Social structure research allows studying how social relationships arise within a group, how they survive or end through time, and how they are influenced by various factors such group composition, mating system, and habitat quality variations. Furthermore, it can help elucidate the tolerance of individuals to other group members and how changes in it may lead to changes in social stability. Social Network Analysis (SNA) is a tool that allows the assessment of social dynamics and interactions. Our goal was to evaluate and compare social tolerance in captive spider monkeys (Ateles geoffroyi) by examining social networks formed in social groups that varied in size and composition. We compared social networks of captive groups varying in sex ratio, group size, and age ratio. Results showed relationship between networks with similar size and composition. Smaller groups showed greater social distancing, while larger ones displayed shorter inter-individual distances and more affiliation. Agonism was infrequent in unisexual groups compared to groups formed by both sexes. Finally, groups with more males showed greater social distancing and greater agonism. Overall, largest groups (five members or more) and similar sex ratio (or unisexual) are better connected. Largest groups showed higher proximity levels and a more tolerance to conspecifics than small groups.

Factors influencing terrestriality in primates of the Americas and Madagascar

Among mammals, the order Primates is exceptional in having a high taxonomic richness in which the taxa are arboreal, semiterrestrial, or terrestrial. Although habitual terrestriality is pervasive among the apes and African and Asian monkeys (catarrhines), it is largely absent among monkeys of the Americas (platyrrhines), as well as galagos, lemurs, and lorises (strepsirrhines), which are mostly arboreal. Numerous ecological drivers and species-specific factors are suggested to set the conditions for an evolutionary shift from arboreality to terrestriality, and current environmental conditions may provide analogous scenarios to those transitional periods. Therefore, we investigated predominantly arboreal, diurnal primate genera from the Americas and Madagascar that lack fully terrestrial taxa, to determine whether ecological drivers (habitat canopy cover, predation risk, maximum temperature, precipitation, primate species richness, human population density, and distance to roads) or species-specific traits (body mass, group size, and degree of frugivory) associate with increased terrestriality. We collated 150,961 observation hours across 2,227 months from 47 species at 20 sites in Madagascar and 48 sites in the Americas. Multiple factors were associated with ground use in these otherwise arboreal species, including increased temperature, a decrease in canopy cover, a dietary shift away from frugivory, and larger group size. These factors mostly explain intraspecific differences in terrestriality. As humanity modifies habitats and causes climate change, our results suggest that species already inhabiting hot, sparsely canopied sites, and exhibiting more generalized diets, are more likely to shift toward greater ground use.

Geographic, climatic, and phylogenetic drivers of variation in colobine activity budgets

Folivorous primates are typically considered time minimizers because the constraints of their unique digestive systems require them to spend a large portion of their activity budgets resting. However, inter- and intraspecific behavioral variation in their activity budgets may be influenced by local geographic and climatic conditions and evolutionary history. We compiled 48 studies representing ten genera, 31 species, and 50 populations to assess geographic (elevation, latitude), climatic (precipitation, temperature), and phylogenetic correlates of colobine activity budgets. Time spent resting negatively correlated with time spent feeding, moving, and socializing. Except for time spent socializing, activity budgets were independent of phylogeny, with more time spent feeding and less time resting in higher-elevation habitats and at lower temperatures. Among the four most common genera in our sample, only in Rhinopithecus did time spent feeding increase with higher elevations (range, 950-3950 m above mean sea level) and lower temperatures (range, 0.9-25 ℃). Only in Trachypithecus did time spent resting decrease with lower temperatures (range, 19-25 ℃). Our findings suggest that there are no apparent effects of geographic or climatic gradients on colobine activity budgets except for Rhinopithecus and Trachypithecus, whose activities are biased in favor of energy maximization rather than time minimization. Compared with other colobines, the ability of Rhinopithecus to adapt their activity budget at high elevations may make them less vulnerable to climate change, while the greater sensitivity of Trachypithecus to heat may make them more vulnerable to climate change.

Social complexity as a driving force of gut microbiota exchange among conspecific hosts in non-human primates

The emergent concept of the social microbiome implies a view of a highly connected biological world, in which microbial interchange across organisms may be influenced by social and ecological connections occurring at different levels of biological organization. We explore this idea reviewing evidence of whether increasing social complexity in primate societies is associated with both higher diversity and greater similarity in the composition of the gut microbiota. By proposing a series of predictions regarding such relationship, we evaluate the existence of a link between gut microbiota and primate social behavior. Overall, we find that enough empirical evidence already supports these predictions. Nonetheless, we conclude that studies with the necessary, sufficient, explicit, and available evidence are still scarce. Therefore, we reflect on the benefit of founding future analyses on the utility of social complexity as a theoretical framework.

Dietary variability of western gorillas (Gorilla gorilla gorilla)

Spatial and temporal variability in the availability of food resources will lead to variation in a species’ diet, which can then influence patterns of space use, sociality, and life history characteristics. Despite such potential impacts, little information is available about dietary variability for some species with large geographical ranges. Here we quantify the diet and nutritional content of plants consumed by western lowland gorillas (Gorilla gorilla gorilla) in Loango National Park, Gabon over a 2.6 year period and make comparisons with two study sites located 800 km away. The major foods consumed by the Loango gorillas differed greatly from the other two study sites, but gorillas at all three locations spent a similar proportion of feeding time consuming herbaceous vegetation and tree leaves (~ 50%) and fruit (35%). The Loango gorillas spent approximately 10% of feeding time eating nuts, which were not consumed at the other two study sites. Gorillas at those sites spent about 5% of feeding time eating insects, which were not consumed by Loango gorillas. Even though the species composition of the diet differed among the three sites, the nutritional composition of the major food items differed very little, suggesting that western gorillas consume foods of similar nutritional values to meet their dietary needs. This study shows the flexibility of diet of a species with a wide geographic distribution, which has implications for understanding variation in life history characteristics and can be useful for conservation management plans.

Not just females: the socio-ecology of social interactions between spider monkey males

Male-male relationships are mostly characterized by competition. However, males also cooperate with one another if socio-ecological conditions are suitable. Due to their male philopatry, the need for cooperation in home range defence and high degree of fission-fusion dynamics, spider monkeys provide an opportunity to investigate how male-male interactions are associated with socio-ecological factors, such as the presence of potentially receptive females, the degree of food availability and the likelihood of home range defence. We tested predictions about changes in social interactions between wild spider monkey males in relation to these factors. First, males did not change their interaction patterns when potentially receptive females were in the subgroup compared to when they were absent. Second, males tended to be less tolerant of one another when feeding, but spent more time grooming, in contact and proximity with one another when food availability was lower than when it was higher. Third, males exchanged fewer embraces, spent less time grooming, in proximity and in contact with one another, and spent more time vigilant at the home range boundary area than at other locations. Our findings contribute to the understanding of social flexibility and the importance of considering males in socio-ecological models of any group-living species.

Feeding postural behaviors and food geometric and material properties in bearded capuchin monkeys ( Sapajus libidinosus )

Objectives

Foods that are geometrically and mechanically challenging to eat have been associated with specializations in feeding behavior and craniodental morphology across primates, and many of these foods are embedded, requiring a variety of positional behaviors during feeding. However, variation in positional behaviors in response to food properties is not well understood. Here, we examine differences in feeding postural behaviors across feeding events in relation to substrate and food geometric and material properties in a species of extractive foragers, bearded capuchins (Sapajus libidinosus).

Methods and materials

We coded over 1400 co‐occurring postural and feeding behaviors, their durations, and relative sizes of substrate and food from videos recorded at Fazenda Boa Vista in Gilbués, Piauí, Brazil. Food material properties were measured from foods collected at the time of the video recordings.

Results

Our results suggest that bearded capuchin feeding postures significantly differ across the feeding sequence, with substrate size, and between foods of high and low toughness and elastic modulus. Feeding postures were less variable for highly mechanically challenging foods. Food size also had a significant effect on postural behaviors. Large foods were more likely to be associated with suspended postures and small foods with sitting and squatting. Feeding postural behaviors were best explained by a combination of substrate and food variables.

Conclusions

Our results indicate that food geometric and mechanical properties have a significant influence on feeding postural behaviors in bearded capuchins. We posit that feeding postural behaviors reflect a combination of substrate variables and food properties, and large, mechanically challenging foods have a limiting effect on postural variation.

Intra-specific Variation in the Social Behavior of Barbary macaques (Macaca sylvanus)

Non-human primates show an impressive behavioral diversity, both across and within species. However, the factors explaining intra-specific behavioral variation across groups and individuals are yet understudied. Here, we aimed to assess how group size and living conditions (i.e., captive, semi-free-ranging, wild) are linked to behavioral variation in 5 groups of Barbary macaques (N=137 individuals). In each group, we collected observational data on the time individuals spent in social interactions and on the group dominance style, along with experimental data on social tolerance over food and neophobia. Our results showed that differences in group size predicted differences in the time spent in social interactions, with smaller groups spending a higher proportion of time in close spatial proximity, but a lower proportion of time grooming. Moreover, group size predicted variation in dominance style, with smaller groups being more despotic. Social tolerance was affected by both group size and living conditions, being higher in smaller groups and in groups living in less natural conditions. Finally, individual characteristics also explained variation in social tolerance and neophobia, with socially integrated individuals having higher access to food sources, and higher-ranking ones being more neophobic. Overall, our results support the view that intra-specific variation is a crucial aspect in primate social behavior and call for more comparative studies to better understand the sources of within-species variation.

The influence of seasonality, anthropogenic disturbances, and cyclonic activity on the behavior of northern sportive lemurs (Lepilemur septentrionalis) at Montagne des Français, Madagascar

Anthropogenic tropical deforestation and degradation imminently threaten primates today. Primates living in these disturbed habitats may also be subjected to increasingly severe tropical storms such as cyclones or hurricanes. These disturbances pose an immediate risk to their livelihood and can dramatically alter their habitats, in turn potentially shifting behavioral patterns. We had the unique opportunity to study the effects of seasonality, anthropogenic disturbances, and the immediate effects of a cyclone on the behavior of the critically endangered northern sportive lemur (NSL) in an anthropogenically disturbed forest in Madagascar. Cyclone Enawo made landfall on March 7, 2017 in northeast Madagascar with sustained wind speeds of 230 km/h. We collected behavioral data on nine individual NSLs during both wet and dry seasons, before and after Cyclone Enawo, and in areas of differing human activity, using scan sampling at 5-min intervals. We ran generalized linear mixed models to test the effects of seasonality and disturbances on behavior. We found that NSLs spent more time feeding in dry months compared with wet (Z = -4.21, p < 0.001). More specifically, they spent more time-consuming leaves and vine species in the dry season (Z = -2.26, p = 0.02; Z = -2.3; p = 0.02). We also found that NSLs were observed at lower heights in trees after the cyclone (Z = -2.45; p = 0.016) and spent more time in the interior portions of trees (Z = 3.44; p < 0.001), perhaps due to extensive limb damage of emergent trees documented after the cyclone. Our analyses show that seasonality affected most aspects of NSL behavior, followed by the effects of Cyclone Enawo, with anthropogenic disturbances having little effect. Our data suggest that the behavioral flexibility of NSLs in response to predictable seasonality may enable them to respond similarly to stochastic climatic disturbances. However, their generalist diet may allow them to persist in anthropogenically disturbed landscapes without the need to greatly alter their behavior.

Science, Policy, and Conservation Management for a Critically Endangered Primate in the Atlantic Forest of Brazil

Long-standing concerns about the status of the world's endangered primates have stimulated significant international efforts, such as the primate action plans published by the Primate Specialist Group of the International Union for Conservation of Nature's Species Survival Commission. However, national-level action plans that bring together diverse scientific experts, non-governmental organizations, and governmental agencies to focus on improving the status of endangered species are generally rare. Here, we highlight one such plan published a decade ago, the Brazilian National Action Plan for the Conservation of Muriquis, which promoted the integration of scientific findings about the behavioral ecology, demography, and genetics of northern muriquis with conservation measures supported by the Brazilian government. This plan provided a holistic framework for the development of an effective national strategy that has contributed to significant advances in research and management applied to the conservation of this Critically Endangered species. We hope that this model for muriquis will stimulate conservationists around the world to pursue integrative national-level sponsorship of action plans on behalf of other endangered species.

Comparative cognition in three understudied ungulate species: European bison, forest buffalos and giraffes

Background

Comparative cognition has historically focused on a few taxa such as primates, birds or rodents. However, a broader perspective is essential to understand how different selective pressures affect cognition in different taxa, as more recently shown in several studies. Here we present the same battery of cognitive tasks to two understudied ungulate species with different socio-ecological characteristics, European bison (Bison bonasus) and forest buffalos (Syncerus caffer nanus), and we compare their performance to previous findings in giraffes (Giraffa camelopardalis). We presented subjects with an Object permanence task, Memory tasks with 30 and 60 s delays, two inference tasks based on acoustic cues (i.e. Acoustic inference tasks) and a control task to check for the use of olfactory cues (i.e. Olfactory task).

Results

Overall, giraffes outperformed bison and buffalos, and bison outperformed buffalos (that performed at chance level). All species performed better in the Object permanence task than in the Memory tasks and one of the Acoustic inference tasks (which they likely solved by relying on stimulus enhancement). Giraffes performed better than buffalos in the Shake full Acoustic inference task, but worse than bison and buffalos in the Shake empty Acoustic inference task.

Conclusions

In sum, our results are in line with the hypothesis that specific socio-ecological characteristics played a crucial role in the evolution of cognition, and that higher fission-fusion levels and larger dietary breadth are linked to higher cognitive skills. This study shows that ungulates may be an excellent model to test evolutionary hypotheses on the emergence of cognition.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00417-w.

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.

Who’s there? Third parties affect social interactions between spider monkey males

Spider monkeys provide an intriguing opportunity to examine behavioural flexibility in relation to their social environment given their high degree of fission–fusion dynamics and the nature of male–male relationships. These characteristics allow us to examine how flexibility in social interactions is modulated by the perception of risk and uncertainty related to other group members. We investigated whether male–male interactions vary according to partner identity and presence of third parties in wild Geoffroy’s spider monkey (Ateles geoffroyi). We used proportion of approaches followed by an embrace, an indicator of risk and uncertainty, or by grooming, an affiliative behaviour. To confirm the monkeys’ perception of risk or uncertainty we used aggression rates and time spent vigilant. We collected data on eight adult spider monkey males: three of them belonged to one clique and the other five to another clique based on distinct patterns of residence. We found higher proportions of approaches followed by embraces and lower proportions of approaches followed by grooming between males of different cliques than between males of the same clique. In addition, we found higher aggression rates between males from different cliques. The proportions of approaches followed by embraces in the five-male clique were higher when the three-male clique was no longer in the group. The five males were more vigilant when the other three males were present in the group, indicating the monkeys perceived higher risk or uncertainty under these circumstances. We found lower proportions of approaches followed by grooming between two males when there was at least one other male in the subgroup than when there were only the two males. Our results provide evidence for behavioural flexibility in the interactions between spider monkey males as an example of how animals can cope with social challenges by adjusting their behaviour.

White-handed gibbons (Hylobates lar) alter ranging patterns in response to habitat type

Small apes are often characterized as inhabiting small home ranges and being dependent on evergreen forest due to their dietary specialization on ripe fruits. Yet few primate studies, particularly those with gibbons, have considered intraspecific variations in ranging behaviors in response to local ecological conditions. This study examines Endangered white-handed gibbon (Hylobates lar) ranging patterns in a heterogeneous landscape. We conducted 13 months of behavioral observations on four white-handed gibbon groups living in Huai Kha Khaeng Wildlife Sanctuary in western Thailand, and combine these data with group location and transect-based productivity data. We compare home range area, site fidelity, and microhabitat preferences. Home range (HR) area varied considerably among the four groups (17-61 hectares). Site fidelity was higher in one of the groups with more evergreen forest in the HR (0.72 ± 0.1) than one of the groups with very little evergreen forest in the habitat (0.47 ± 0.07). While groups with more evergreen forest in the HR preferred evergreen forest areas, groups with very little evergreen forest within the HR demonstrated less preference for evergreen forest areas. We conclude that gibbons at this site exhibit a considerable degree of behavioral variation in response to local ecological conditions. These findings suggest that while gibbons exhibit significant ecological flexibility, this flexibility may be limited by habitat type and key food resources.

The evolution of pair-living, sexual monogamy, and cooperative infant care: Insights from research on wild owl monkeys, titis, sakis, and tamarins

"Monogamy" and pair bonding have long been of interest to anthropologists and primatologists. Their study contributes to our knowledge of human evolutionary biology and social evolution without the cultural trappings associated with studying human societies directly. Here, we first provide an overview of theoretical considerations, followed by an evaluation of recent comparative studies of the evolution of "social monogamy"; we are left with serious doubts about the conclusions of these studies that stem from the often poor quality of the data used and an overreliance on secondary sources without vetting the data therein. We then describe our field research program on four "monogamous" platyrrhines (owl monkeys, titis, sakis, and tamarins), evaluate how well our data support various hypotheses proposed to explain "monogamy," and compare our data to those reported on the same genera in comparative studies. Overall, we found a distressing lack of agreement between the data used in comparative studies and data from the literature for the taxa that we work with. In the final section, we propose areas of research that deserve more attention. We stress the need for more high-quality natural history data, and we urge researchers to be cautious about the uncritical use of variables of uncertain internal validity. Overall, it is imperative that biological anthropologists establish and follow clear criteria for comparing and combining results from published studies and that researchers, reviewers, and editors alike comply with these standards to improve the transparency, reproducibility, and interpretability of causal inferences made in comparative studies.

Trait variation and trait stability in common marmosets (Callithrix jacchus) inhabiting ecologically distinct habitats in northeastern Brazil

Understanding the set of factors that promote and constrain a species' ability to exploit ecologically distinct habitats is central for addressing questions of intraspecific variability in behavior and morphology. In this study, we compared newly collected data with published data on body measurements, group size and composition, daily path length, home range, and reproductive output in wild common marmosets naturally inhabiting two contrasting environments in northeastern Brazil: the Atlantic Forest (AF), which is characterized by high biodiversity and reduced seasonality in food availability and the Caatinga (CAT), which is characterized by a severe hot and dry season lasting from 5 to 11 months, drought-resistant plant species, and reduced primary productivity. Despite marked differences in ecological conditions, CAT marmosets and AF marmosets differed minimally in daily path length, home range, reproductive output, and infant survivorship. CAT marmosets were found to live in smaller groups containing fewer adult females than AF marmosets, and also were characterized by a greater surface area to body mass ratio, a trait that may represent an adaptation to the hot and dry conditions of the Caatinga. We propose that in conjunction with body mass reduction, minor adjustments in behavior, the exploitation of cacti as a source of water and nutrients, and access to exudates as a dependable year-round food resource, common marmosets successfully used the same adaptive pattern to maintain high reproductive output and infant survivorship in exploiting these two ecologically distinct environments.

Effects of Environmental Stress on Primate Populations

Environmental stress on primate populations can take many forms. Abiotic factors, such as temperature and precipitation, may directly influence the behavior of primates owing to physiological demands of thermoregulation or through indirect influences on vegetation that primates rely on for food. These effects can also scale up to the macro scale, impacting primate distributions and evolution. Primates also encounter stress during interactions within and between species (i.e., biotic interactions). For example, selective pressure from male-perpetrated infanticide can drive the development of female counterstrategies and can impact life-history traits. Predation on primates can modify group size, ranging behavior, and habitat use. Finally, humans have influenced primate populations for millennia. More recently, hunting, habitat disturbance, disease, and climate change have increased in frequency and severity with detrimental impacts on primate populations worldwide. These effects and recent evidence from camera traps emphasize the importance of maintaining protected areas for conserving primate populations.

Variation in the social organization of gorillas: Life history and socioecological perspectives

A focus of socioecological research is to understand how ecological, social, and life history factors influence the variability of social organization within and between species. The genus Gorilla exhibits variability in social organization with western gorilla groups being almost exclusively one-male, yet approximately 40% of mountain gorilla groups are multimale. We review five ultimate causes for the variability in social organization within and among gorilla populations: human disturbance, ecological constraints on group size, risk of infanticide, life history patterns, and population density. We find the most evidence for the ecological constraints and life history hypotheses, but an over-riding explanation remains elusive. The variability may hinge on variation in female dispersal patterns, as females seek a group of optimal size and with a good protector male. Our review illustrates the challenges of understanding why the social organization of closely related species may deviate from predictions based on socioecological and life history theory.

Seasonal and sex differences in the fission-fusion dynamics of spider monkeys (Ateles geoffroyi yucatanensis) in Belize

Spider monkeys (Ateles sp.) are characterized by high fission-fusion dynamics, meaning their social grouping pattern is fluid and consists of subgroups that vary in size, composition, and spatial cohesion over time. In this study, we quantify the fission-fusion dynamics of a group of spider monkeys at Runaway Creek Nature Reserve in Belize by measuring subgroup size, spatial cohesion, and stability using data spanning 5 years. We then test whether variation in these three subgroup measures differ according to season, subgroup sex composition, and the reproductive status of female subgroup members. Our results show that subgroups were larger in size and less stable in membership during the wet season compared to the dry season. All-female subgroups were less spatially cohesive but more stable in membership than all-male subgroups. Finally, we report that subgroups with one or more non-lactating females (i.e., without nursing young) were smaller on average than subgroups containing lactating females with nursing young. These data contribute to a growing body of research documenting the ecological and social dimensions along which grouping patterns might vary.

Nutrient intake and balancing among female Colobus angolensis palliatus inhabiting structurally distinct forest areas: Effects of group, season, and reproductive state

Understanding intraspecific behavioral and dietary variation is critical for assessing primate populations' abilities to persist in habitats characterized by increasing anthropogenic disturbances. While it is evident that some species exhibit considerable dietary flexibility (in terms of species-specific plant parts) in relation to habitat disturbance, it is unclear if primates are characterized by similar variation and flexibility regarding nutrient intake. This study examined the effects of group, season, and reproductive state on nutrient intake and balancing in adult female Colobus angolensis palliatus in the Diani Forest, Kenya. During July 2014 to December 2015, estimates of nutrient intake were recorded for eight females from three groups inhabiting structurally and ecologically distinct forest areas differing in tree species composition and density. There were differences in metabolizable energy (ME) and macronutrient intakes among groups, seasons, and reproductive states. Most notably, females inhabiting one of the more disturbed forest areas consumed less ME and macronutrients compared to females in the more intact forest area. Contrary to prediction, females in early lactation consumed significantly less ME and macronutrients compared to non-lactating and late lactation females. Despite differences in macronutrient intake, the relative contribution of macronutrients to ME were generally more conservative among groups, seasons, and reproductive states. Average daily intake ratios of non-protein energy to available protein ranged from approximately 3.5:1-4.3:1 among groups. These results indicate that female C. a. palliatus demonstrate a consistent nutrient balancing strategy despite significant intergroup differences in consumption of species-specific plant parts. Data from additional colobine species inhabiting different forest types are required to assess the extent to which nutrient balancing is constrained by phylogeny or is more flexible to local ecological conditions.

Behavioural Variability in Macaques and Langurs of the Western Ghats, India

Studies that compare differences in the behavioural variability across species and genera are rare among south Asian primates. Such studies are important for understanding within-group feeding competition in primates as interindividual difference in frequency of behaviour is a good indicator of feeding competition. We compared the variability in individual activities of lion-tailed macaques, bonnet macaques, Nilgiri langurs, and black-footed grey langurs. Both macaque species showed variability in their activities, with bonnet macaques showing higher variability than lion-tailed macaques. This indicated higher within-group competition in bonnet macaques than in lion-tailed macaques. Folivorous Nilgiri langurs and black-footed grey langurs did not show variability in any behaviour, indicating weak or negligible within-group competition. Except in the bonnet macaque, the interindividual differences in activities in the other species were potentially due to the differences between lactating and non-lactating females. We observed that langurs were less variable than macaques, indicating higher within-group competition in macaques than in langurs. We also observed a higher frequency of aggressive interactions during feeding among macaques than langurs, substantiating higher within-group competition in macaques than in langurs. We further discuss the different possibilities of within-group contest and scramble competition in these species.