Primate Responses to Changing Environments in the Anthropocene

Zoonotic pathogens in wild Asian primates: a systematic review highlighting research gaps

Introduction

Ongoing global changes, including natural land conversion for agriculture and urbanization, modify the dynamics of human-primate contacts, resulting in increased zoonotic risks. Although Asia shelters high primate diversity and experiences rapid expansion of human-primate contact zones, there remains little documentation regarding zoonotic surveillance in the primates of this region.

Methods

Using the PRISMA guidelines, we conducted a systematic review to compile an inventory of zoonotic pathogens detected in wild Asian primates, while highlighting the coverage of primate species, countries, and pathogen groups surveyed, as well as the diagnostic methods used across the studies. Moreover, we compared the species richness of pathogens harbored by primates across diverse types of habitats classified according to their degree of anthropization (i.e., urban vs. rural vs. forest habitats).

Results and discussion

Searches of Scopus, PubMed, and the Global Mammal Parasite Database yielded 152 articles on 39 primate species. We inventoried 183 pathogens, including 63 helminthic gastrointestinal parasites, two blood-borne parasites, 42 protozoa, 45 viruses, 30 bacteria, and one fungus. Considering each study as a sample, species accumulation curves revealed no significant differences in specific richness between habitat types for any of the pathogen groups analyzed. This is likely due to the insufficient sampling effort (i.e., a limited number of studies), which prevents drawing conclusive findings. This systematic review identified several publication biases, particularly the uneven representation of host species and pathogen groups studied, as well as a lack of use of generic diagnostic methods. Addressing these gaps necessitates a multidisciplinary strategy framed in a One Health approach, which may facilitate a broader inventory of pathogens and ultimately limit the risk of cross-species transmission at the human-primate interface. Strengthening the zoonotic surveillance in primates of this region could be realized notably through the application of more comprehensive diagnostic techniques such as broad-spectrum analyses without a priori selection.

Ecological disturbance alters the adaptive benefits of social ties

Extreme weather events radically alter ecosystems. When ecological damage persists, selective pressures on individuals can change, leading to phenotypic adjustments. For group-living animals, social relationships may be a mechanism enabling adaptation to ecosystem disturbance. Yet whether such events alter selection on sociality and whether group-living animals can, as a result, adaptively change their social relationships remain untested. We leveraged 10 years of data collected on rhesus macaques before and after a category 4 hurricane caused persistent deforestation, exacerbating monkeys' exposure to intense heat. In response, macaques demonstrated persistently increased tolerance and decreased aggression toward other monkeys, facilitating access to scarce shade critical for thermoregulation. Social tolerance predicted individual survival after the hurricane, but not before it, revealing a shift in the adaptive function of sociality.

Behavioral thermoregulation in primates: A review of literature and future avenues

Primates face severe challenges from climate change, with warming expected to increase animals' thermoregulatory demands. Primates have limited long-term options to cope with climate change, but possess a remarkable capacity for behavioral plasticity. This creates an urgency to better understand the behavioral mechanisms primates use to thermoregulate. While considerable information exists on primate behavioral thermoregulation, it is often scattered in the literature in a manner that is difficult to integrate. This review evaluates the status of the available literature on primate behavioral thermoregulation to facilitate future research. We surveyed peer-reviewed publications on primate thermoregulation for N = 17 behaviors across four thermoregulatory categories: activity budgeting, microhabitat use, body positioning, and evaporative cooling. We recorded data on the primate taxa evaluated, support for a thermoregulatory function, thermal variable assessed, and naturalistic/manipulative study conditions. Behavioral thermoregulation was pervasive across primates, with N = 721 cases of thermoregulatory behaviors identified across N = 284 published studies. Most genera were known to utilize multiple behaviors (x ¯  = 4.5 ± 3.1 behaviors/genera). Activity budgeting behaviors were the most commonly encountered category in the literature (54.5% of cases), while evaporative cooling behaviors were the least represented (6.9% of cases). Behavioral thermoregulation studies were underrepresented for certain taxonomic groups, including lemurs, lorises, galagos, and Central/South American primates, and there were large within-taxa disparities in representation of genera. Support for a thermoregulatory function was consistently high across all behaviors, spanning both hot- and cold-avoidance strategies. This review reveals asymmetries in the current literature and avenues for future research. Increased knowledge of the impact thermoregulatory behaviors have on biologically relevant outcomes is needed to better assess primate responses to warming environments and develop early indicators of thermal stress.

Interindividual differences in crop foraging behavior of chimpanzees (Pan troglodytes schweinfurthii) at a forest–agriculture interface

The expansion of agriculture in equatorial areas is fragmenting and reducing wildlife habitats. For primates, it also increases opportunities to consume crops as high-energy resources, exacerbates conflicts with farmers, and increases exposure to diseases and agrochemicals at the edge of protected areas. In species with sex differences in ranging behavior, individual exposure to such opportunities and threats may vary by sex. Chimpanzees show a great feeding flexibility and are territorial species with varied ranging patterns according to site, sex, or individuals. Within a community whose territory is crossed by a high-traffic road and partially bordered by maize gardens, we tested hypotheses of interindividual differences in access to crops based on age, sex, ranging behavior, and kinship. By analyzing the presence of Sebitoli chimpanzees (Pan troglodytes schweinfurthii, Kibale National Park, Uganda) on video clips from 16 camera traps, we estimated the individual dispersion range across the community’s territory and the crop foraging frequency along maize gardens over 16 months. While all age and sex classes were represented at the forest–garden interface, large intrasex differences were observed: some mature males and females were not observed to participate. The crop foraging frequency of adult females in maize gardens was significantly correlated with the location of their ranging areas. Related individuals revealed similar range patterns within the forest territory without sharing crop foraging habits. However, social learning and energy and risks–benefits trade-offs as potential drivers of crop consumption are not excluded.

Drivers and outcomes of between-group conflict in vervet monkeys

Neighbouring groups compete over access to resources and territories in between-group encounters, which can escalate into between-group conflicts (BGCs). Both the ecological characteristics of a territory and the rival's fighting ability shape the occurrence and outcome of such contests. What remains poorly understood, however, is how seasonal variability in the ecological value of a territory together with fighting ability related to the likelihood of between-group encounters and the extent to which these escalate into conflicts. To test this, we observed and followed four vervet monkey groups in the wild, and recorded the group structure (i.e. size, composition), the locations and the outcomes of 515 BGCs. We then assessed key ecological measures at these locations, such as vegetation availability (estimated from Copernicus Sentinel 2 satellite images) and the intensity of usage of these locations. We tested to what extent these factors together influenced the occurrence and outcomes of BGCs. We found that the occurrence of BGCs increased at locations with higher vegetation availability relative to the annual vegetation availability within the group's home territory. Also, groups engaging in a BGC at locations far away from their home territory were less likely to win a BGC. Regarding group structure, we found that smaller groups systematically won BGCs against larger groups, which can be explained by potentially higher rates of individual free-riding occurring in larger groups. This study sheds light on how the ecology of encounter locations in combination with a group's social characteristics can critically impact the dynamics of BGCs in a non-human primate species.

This article is part of the theme issue ‘Intergroup conflict across taxa’.

All-You-Can-Eat: Influence of Proximity to Maize Gardens on the Wild Diet and the Forest Activities of the Sebitoli Chimpanzee Community in Kibale National Park

Simple Summary

Understanding the resilience of primate populations to the threat of agricultural expansion is critical for effective conservation. Based on individual monitoring from morning to evening of wild chimpanzees in and around a protected area, we showed that the availability of maize at the forest edge had little effect on their activity budget by less resting and no impact on their wild diet and energy expenditure. In this area, large, caloric wild fruits are available year-round, and we observed no behavioral or dietary changes regarding wild resource availability either. Thus, the chimpanzees consume maize opportunistically as a bonus treat in their diet, and the presence of this nutritious resource does not seem to affect their role in seed dispersal and forest regeneration.

Abstract

Frugivorous primates have developed several strategies to deal with wild fruit scarcity, such as modifying their activity budget or enlarging their diet. Agricultural expansion threatens primate habitats and populations (e.g., disease transmission, agrochemical exposure), but it also increases crop feeding opportunities. We aimed at understanding whether maize presence close to the natural habitat of chimpanzees, a threatened species, would lead to significant behavioral modifications. We monitored 20 chimpanzees over 37 months in Kibale National Park, Uganda, with maize gardens at the forest edge. Based on focal nest-to-nest data, we analyzed their diet, activity budget, and energy balance depending on wild fruit and maize availability. We found that the Sebitoli area is a highly nutritive habitat for chimpanzees, with large and caloric wild fruits available all year long. The chimpanzees opportunistically consume maize and exploit it by resting less during maize season. However, no significant variation was found in daily paths and energy expenditures according to maize availability. No behavioral or energy modification was observed regarding wild resources either. Despite the availability of nutritious domestic resources, chimpanzees still exploit wild fruits and do not limit their movements. Thus, their contribution to seed dispersal and forest regeneration in this area is not affected.

Observer effects in a remote population of large-headed capuchins, Sapajus macrocephalus

Human observers often are present when researchers record animal behavior, which can create observer effects. These effects are rarely explicitly investigated, often due to the assumption that the study animal is habituated to or unaffected by a human’s presence. We investigated the effect of human pressure gradients on a remote population of large-headed capuchins, Sapajus macrocephalus, looking specifically at the effects of number of observers, distance to observers, and distance to the research base. We conducted this study over 4 months in the Pacaya-Samiria Nature Reserve, Peru, and collected 199 two-minute focal samples of capuchin behavior. We found that capuchin monkeys fed less when human observers were closer to the focal individual, when more observers were present, and when capuchins were closer to the research base. We found no other consistent differences in capuchin monkey behavior across the measured human pressure gradients, although capuchins directed a high proportion of their vigilance toward humans (29% in adults and 47% in infants). Our results support the hypothesis that human pressure gradients influence animal behavior. Given the proportion of human directed vigilance, we recommend that all studies that use human observers to record animal behavior consider human-directed vigilance, record the number of observers, as well as the observer-focal animal distance, to check for these effects.

Chimpanzees (Pan troglodytes) adapt their nesting behavior after large-scale forest clearance and community decline

Chimpanzees (Pan troglodytes) build nests at night for sleeping and occasionally during daytime for resting. Over the course of seven years, forest fragments in Bulindi, Uganda, were reduced in size by about 80% when landowners converted forest to agricultural land. However, unlike other studies on nesting behavior in response to habitat disturbance, chimpanzees at Bulindi had no opportunity to retreat into nearby undisturbed forest. To understand behavioral adaptations to forest clearance, we compared Bulindi chimpanzees' nesting characteristics before and after this period of major deforestation. After deforestation, chimpanzees built nests at lower heights in shorter trees, and reused a larger proportion of their nests. Additionally, average nest group size increased after deforestation, even though community size declined by approximately 20% over the same period. The substantial decrease in available forest habitat may have caused the chimpanzees to aggregate for nesting. However, more cohesive nesting may also have been influenced by dietary shifts (increased reliance on agricultural crops) and a need for enhanced safety with increased human encroachment. Conversely, the chimpanzees selected similar tree species for nesting after deforestation, apparently reflecting a strong preference for particular species, nested less often in exotic species, and built integrated nests (constructed using multiple trees) at a similar frequency as before fragment clearance. Chimpanzees living in unprotected habitat in Uganda, as at Bulindi, face mounting anthropogenic pressures that threaten their survival. Nevertheless, our study shows that chimpanzees can adjust their nesting behavior flexibly in response to rapid, extensive habitat change. While behavioral flexibility may enable them to cope with deforestation, at least to a certain point, the long-term survival of chimpanzees in fast-changing human-modified landscapes requires intensive conservation efforts.

Primate conservation: Lessons learned in the last 20 years can guide future efforts

Twenty years ago, we published an assessment of the threats facing primates and with the passing of two decades, we re-evaluate identified threats, consider emerging pressures, identify exciting new avenues of research, and tackle how to change the system to rapidly advance primate and primate habitat conservation. Habitat destruction and hunting have increased, the danger of looming climate change is clearer, and there are emerging threats such as the sublethal effects of microplastics and pesticides. Despite these negative developments, protected areas are increasing, exciting new tools are now available, and the number of studies has grown exponentially. Many of the changes that need to occur to make rapid progress in primate conservation are in our purview to modify. We identify several dimensions indicating the time is right to make large advances; however, the question that remains is do we have the will to prevent widespread primate annihilation and extinction?

A meta-analysis of anthropogenic impacts on physiological stress in wild primates

As humanity continues to alter the environment extensively, comprehending the effect of anthropogenic disturbances on the health, survival, and fitness of wildlife is a crucial question for conservation science. Many primate populations occupy suboptimal habitats prone to diverse anthropogenic disturbances that may be sources of acute and chronic stress. Quantification of glucocorticoid (GC) concentrations has repeatedly been used to explore the impact of disturbances on physiological stress. Although it is still debated, prolonged elevation of GC levels may impair reproduction, growth, and immune system activity of individuals. We quantified the effect of anthropogenic disturbances on physiological stress in primates with a global meta-analysis based on data from 26 articles, covering 24 distinct species in 13 different countries. Anthropogenic disturbances were classified into 6 distinct categories: habitat loss, habitat degradation, ongoing logging, hunting, tourism, and other human activities. We calculated effect sizes (Hedges' g) with the standardized mean difference in GC concentrations between primates affected by human activity and their undisturbed conspecifics. We ran random-effects models and subgroup analyses to estimate the overall effect as well as a cumulative effect size for each disturbance category. Overall, primates inhabiting sites subject to anthropogenic disturbances exhibited significantly higher GC levels (g = 0.60; 95% CI: 0.28-0.93). Habitat loss and hunting were overall associated with increased GC concentrations, whereas the cumulative effects of the other disturbances were not statistically significant. Biologically, high GC levels may increase fitness by enabling individuals to overcome the challenges linked to anthropogenic disturbances. However, primates in disturbed environments may have sustained elevated GC levels. To strengthen future research, it is necessary to control confounding factors systematically (e.g., diet, reproductive status, predatory pressure, and resource availability) and improve understanding of the link between GC levels and the health, fitness, and survival of animals.

Use of an embedded fruit by Nicobar Long-tailed Macaque Macaca fascicularis umbrosus: II. Demographic influences on choices of coconuts Cocos nucifera and pattern of forays to palm plantations

Adaptive pressures of human-induced rapid environmental changes and insular ecological conditions have led to behavioral innovations among behaviorally flexible nonhuman primates.  Documenting long-term responses of threatened populations is vital for our understanding of species and location-specific adaptive capacities under fluctuating equilibrium.  The Nicobar Long-tailed Macaque  Macaca fascicularis umbrosus, an insular sub-species uses coconuts Cocos nucifera, an embedded cultivar as a food resource and is speculated to have enhanced its dependence as a result of anthropogenic and environmental alterations.  We explored demographic patterns of use and abandonment of different phenophases of fresh coconuts.  To study crop foraging strategies, we recorded daily entry and duration of forays into coconut plantations.  We divided age-classes into early juvenile (13–36 months), late juvenile (37–72 months), and adults (>72 months) and classified phenophase of coconuts into six types.  Consistent with the theory of life history strategies, late juveniles were found to use a greater number of coconuts, which was considerably higher in an urban troop but marginally higher in a forest-plantation dwelling group.  Except in late juveniles, males consumed a higher number of coconuts than females in the remaining age-classes.  Owing to developmental constraints, juveniles of both types used higher proportion of immature coconuts though adults showed equitable distribution across phenophases.  Pattern of entries to plantations and duration of forays were uniform through the day in the urban troop but modulatory in the forest-plantation group, perhaps due to frequent and hostile human interferences.  Observations corroborating adaptations to anthropogenic disturbances are described. 

Differential impact of severe drought on infant mortality in two sympatric neotropical primates

Extreme climate events can have important consequences for the dynamics of natural populations, and severe droughts are predicted to become more common and intense due to climate change. We analysed infant mortality in relation to drought in two primate species (white-faced capuchins, Cebus capucinus imitator, and Geoffroy's spider monkeys, Ateles geoffroyi) in a tropical dry forest in northwestern Costa Rica. Our survival analyses combine several rare and valuable long-term datasets, including long-term primate life-history, landscape-scale fruit abundance, food-tree mortality, and climate conditions. Infant capuchins showed a threshold mortality response to drought, with exceptionally high mortality during a period of intense drought, but not during periods of moderate water shortage. By contrast, spider monkey females stopped reproducing during severe drought, and the mortality of infant spider monkeys peaked later during a period of low fruit abundance and high food-tree mortality linked to the drought. These divergent patterns implicate differing physiology, behaviour or associated factors in shaping species-specific drought responses. Our findings link predictions about the Earth's changing climate to environmental influences on primate mortality risk and thereby improve our understanding of how the increasing severity and frequency of droughts will affect the dynamics and conservation of wild primates.

Forest fragments become farmland: Dietary Response of wild chimpanzees (Pan troglodytes) to fast-changing anthropogenic landscapes

Behavioral flexibility, including an ability to modify feeding behavior, is a key trait enabling primates to survive in forest fragments. In human-dominated landscapes, unprotected forest fragments can become progressively degraded, and may be cleared entirely, challenging the capacity of primates to adjust to the changes. We examined responses of wild chimpanzees (Pan troglodytes schweinfurthii) to major habitat change: that is, clearance of forest fragments for agriculture. Over 7 years, fragments in Bulindi, Uganda, were reduced in size by 80%. We compared the chimpanzees' diet at the start and end of this period of rapid deforestation, using data derived mainly from fecal analysis. Similar to other long-term study populations, chimpanzees in Bulindi have a diverse diet comprising over 169 plant foods. However, extensive deforestation seemed to impact their feeding ecology. Dietary changes after fragment clearance included reduced overall frugivory, reduced intake of figs (Ficus spp.; formerly a dietary "staple" for these chimpanzees), and reduced variety of fruits in fecal samples. Nevertheless, the magnitude of most changes was remarkably minor given the extent of forest loss. Agricultural fruits increased in dietary importance, with crops accounting for a greater proportion of fruits in fecal samples after deforestation. In particular, cultivated jackfruit (Artocarpus heterophyllus) became a "staple" food for the chimpanzees but was scarcely eaten before fragment clearance. Crops offer some nutritional benefits for primates, being high in carbohydrate energy and low in hard-to-digest fiber. Thus, crop feeding may have offset foraging costs associated with loss of wild foods and reduced overall frugivory for the chimpanzees. The adaptability of many primates offers hope for their conservation in fragmented, rural landscapes. However, long-term data are needed to establish whether potential benefits (i.e. energetic, reproductive) of foraging in agricultural matrix habitats outweigh fitness costs from anthropogenic mortality risk for chimpanzees and other adaptable primates.