Intergroup encounters in wild moor macaques (Macaca maurus)

Lethal combats in the forest among wild western gorillas

Lethal intergroup encounters occur in many species because of sexual selection. While documented in mountain gorillas, they are absent in western gorillas as, instead, it is predicted by their higher feeding (frugivory) and mate competition (single-vs. multi-male groups). We investigate whether the injuries on three dead silverbacks and one adult female from four groups of western gorillas in the Central African Republic, resulted from interactions with gorillas or leopards. We identified two distinct injury patterns caused by gorillas (isolated lacerations, round wounds) and leopards (punctures clustered on head/neck) by analyzing injuries caused by mountain gorillas and leopards to gorillas and non-gorilla species, respectively. The western gorilla injury pattern is similar to that of mountain gorillas suggesting that lethal encounters occur, albeit infrequently, as predicted by sexual selection in a one-male society. While sexual dimorphism and polygynous sociality favored the evolution of violent encounters, multiple males in groups may influence their frequency.

Male services during between-group conflict: the 'hired gun' hypothesis revisited

In many group-living mammals, philopatric females form the stable core of the group and defend food or shelter against other groups of females. Where males are larger, their participation could give their female group the edge. How can females secure the contribution of males that are neither the father of current infants, nor the dominant male expecting to sire the next generation of infants? It has been proposed that females recruit these males as 'hired guns', receiving social support and copulations in exchange for fighting, against the interests of the dominant male. We first develop the logic of this hypothesis in unprecedented detail by considering the potential pay-off consequences for females and males. We then provide empirical evidence for the existence of hired guns in this context in several primate species. The game-theoretical aspects of the phenomenon remain to be studied, as is the distribution across contexts (e.g. predation avoidance) and species of the hired gun phenomenon. This article is part of the theme issue 'Intergroup conflict across taxa'.

Roadside monkeys: anthropogenic effects on moor macaque (Macaca maura) ranging behavior in Bantimurung Bulusaraung National Park, Sulawesi, Indonesia

A growing body of research focuses on how anthropogenic factors affect the behavior and ecology of primates and their ecosystems. Infrastructural development, such as roads, is an increasingly pervasive anthropogenic impact that destroys primate habitat, affects the distribution and dispersal of primates, and facilitates human-primate interactions. At our field site in Bantimurung-Bulusaraung National Park, Sulawesi, Indonesia, a major road bisects the habitat of the endangered moor macaque (Macaca maura). Beginning in 2015, we observed a behavioral shift by our main study group: they began spending more time along the road foraging in trash pits and waiting for provisions from vehicles. Our objective in this study was to examine how access to anthropogenic foods has affected the group's ranging behavior by comparing ranging data collected before (2010-2011) and after the shift (2016-2017). In contrast to what we expected, home ranges were significantly larger and daily travel distance was significantly longer after the shift compared to before. As predicted, mean distance to the road decreased after the shift. These results likely reflect the irregular and spatially dispersed nature of provisioning at this site. The macaques appear to be attracted to the road because it presents opportunities to obtain palatable and energy-dense foods. Our results indicate that moor macaques are able to flexibly adjust their ranging behavior in response to anthropogenic impacts. However, given the risks of being in proximity to roads and humans, management of this emerging human-macaque interface is needed.

Activity budget, home range, and habitat use of moor macaques (Macaca maura) in the karst forest of South Sulawesi, Indonesia

Primate behavior can be responsive to the different ecological pressures associated with different habitats, as well as to the effects of direct and indirect anthropogenic disturbance. The karst forest ecosystem of South Sulawesi (Indonesia) represents one of the few intact forests available for residual populations of the moor macaque, but our understanding of its habitat use is limited. In the present study, this gap in knowledge was addressed by observing the activity and habitat use of two groups of moor macaques and by assessing the suitability of different habitats in the karst forest. Through a fine-scale vegetation analysis of 1 ha of forest in Bantimurung Bulusaraung National Park, we identified the presence of two distinct habitats that differ in terms of forest structure and composition. The karst plain forest (KPF) provided a greater abundance and diversity of food resources than the karst tower forest (KTF). In addition, anthropogenic disturbance was high in the KPF but low in the KTF. Behavioral data collected via group scans indicate that the macaques devoted more time to feeding activities when in the KPF, suggesting an ability to adjust their feeding behavior to meet their nutritional needs. However, the larger of the two groups used the food-rich KPF more than expected, implying that the KTF may represent a valuable refuge for the smaller group, as it is a less risky portion of its home range. The results of this study therefore provide novel information on the ecology of moor macaques and their habitats that can inform conservation planning for remnant populations.

The influence of phylogeny, social style, and sociodemographic factors on macaque social network structure

Among nonhuman primates, the evolutionary underpinnings of variation in social structure remain debated, with both ancestral relationships and adaptation to current conditions hypothesized to play determining roles. Here we assess whether interspecific variation in higher-order aspects of female macaque (genus: Macaca) dominance and grooming social structure show phylogenetic signals, that is, greater similarity among more closely-related species. We use a social network approach to describe higher-order characteristics of social structure, based on both direct interactions and secondary pathways that connect group members. We also ask whether network traits covary with each other, with species-typical social style grades, and/or with sociodemographic characteristics, specifically group size, sex-ratio, and current living condition (captive vs. free-living). We assembled 34-38 datasets of female-female dyadic aggression and allogrooming among captive and free-living macaques representing 10 species. We calculated dominance (transitivity, certainty), and grooming (centrality coefficient, Newman's modularity, clustering coefficient) network traits as aspects of social structure. Computations of K statistics and randomization tests on multiple phylogenies revealed moderate-strong phylogenetic signals in dominance traits, but moderate-weak signals in grooming traits. GLMMs showed that grooming traits did not covary with dominance traits and/or social style grade. Rather, modularity and clustering coefficient, but not centrality coefficient, were strongly predicted by group size and current living condition. Specifically, larger groups showed more modular networks with sparsely-connected clusters than smaller groups. Further, this effect was independent of variation in living condition, and/or sampling effort. In summary, our results reveal that female dominance networks were more phylogenetically conserved across macaque species than grooming networks, which were more labile to sociodemographic factors. Such findings narrow down the processes that influence interspecific variation in two core aspects of macaque social structure. Future directions should include using phylogeographic approaches, and addressing challenges in examining the effects of socioecological factors on primate social structure.

Intergroup encounters in Verreaux's sifakas (Propithecus verreauxi): who fights and why?

ABSTRACT

Individuals living in groups have to achieve collective action for successful territorial defense. Because conflicts between neighboring groups always involve risks and costs, individuals must base their decision to participate in a given conflict on an evaluation of the trade-off between potential costs and benefits. Since group members may differ in motivation to engage in group encounters, they exhibit different levels of participation in conflicts. In this study, we investigated factors influencing participation in intergroup encounters in Verreaux's sifakas (Propithecus verreauxi), a group-living primate from Madagascar. Over a period of 12 months, we studied eight adjacent sifaka groups in Kirindy Forest. We observed 71 encounters between known neighboring groups in which adult females and males participated equally as often. No individual participated in every encounter, and non-participation occurred more often in larger groups. Females participated less often in encounters when they had dependent infants, presumably to reduce the risk of infanticide. Male participation was influenced by social status: dominant males participated in most encounters, whereas males with fewer opportunities to reproduce participated less often, hence male participation is influenced by the incentive of maintaining access to females. The number of actively participating individuals in the opponent group positively influenced the participation in both sexes. Thus, sifakas seem to decide joining a given encounter opportunistically, most likely based on a combination of individual incentives and the actual circumstance of each encounter, suggesting that the complexity in intergroup relationships appears to be the product of decisions made by each individual group member.

SIGNIFICANCE STATEMENT

Cooperation among group-living animals is often challenged by collective action problems resulting from individual differences in interests in contributing to collective behaviors. Intergroup encounters involve distinguished costs and benefits for each individual despite being in the same social group. Therefore, encounters between groups offer a good opportunity to investigate individual participation in collective action. In this study, we investigate the influence of different incentives on individual participation in intergroup encounters in wild Malagasy primate, Verreaux's sifakas. We propose a novel approach that takes into account the variable circumstances of each conflict, such as the number of individuals fighting in both groups as a predictor for participation. We believe that our study not only provides novel data on wild sifakas, but it also offers new perspectives for the interpretation of intergroup relationships in other taxa.

Intergroup conflict: Ecological predictors of winning and consequences of defeat in a wild primate population

In many social species, competition between groups is a major factor proximately affecting group-level movement patterns and space use and ultimately shaping the evolution of group living and complex sociality. Here we evaluated the factors influencing group-level dominance among 5 social groups of wild baboons (Papio cynocephalus), in particular focusing on the spatial determinants of dominance and the consequences of defeat. When direct conflict occurred between conspecific baboon groups, the winning group was predicted by differences in the number of adult males in each group and/or groups that had used the areas surrounding the encounter location more intensively than their opponent in the preceding 9 or 12 months. Relative intensity of space use over shorter timescales examined (3 and 6 months) was a poor predictor of the interaction's outcome. Losing groups but not winning groups experienced clear short-term costs. Losing groups used the area surrounding the interaction less following an agonistic encounter (relative to their intensity of use of the area prior to the interaction). These findings offer insight into the influences and consequences of intergroup competition on group-level patterns of space use.