Group living is often hailed as a survival strategy that confers significant advantages—enhanced defense capacity, cooperative vigilance, and collective strength against rivals. Yet, it is also burdened by internal competition for limited resources. The study, led by Professor Susan Perry and her collaborators at the Max Planck Institute of Animal Behavior (MPI-AB) and the University of Konstanz, shows how this delicate trade-off between the benefits of strength in numbers and the costs of sharing finite food supplies dynamically shifts in response to climatic cycles.

For over 33 years, Perry’s team meticulously observed 12 adjacent groups of capuchin monkeys inhabiting the dwindling patches of tropical dry forest in Costa Rica, collecting detailed behavioral data on 335 individuals. Satellite-derived environmental imagery complemented these observations, offering precise measurements of forest greenness and canopy coverage. This interdisciplinary approach unveiled how seasons and multi-year climate phenomena like El Niño and La Niña modulate resource distribution and consequently influence group interactions and territorial behavior.

Under typical environmental conditions, capuchin groups demonstrated a distinct pattern: individuals in larger parties consumed fruit at reduced rates due to intensified intragroup competition. This slower intake rate was, however, mitigated by the ability of larger groups to expand their ranges, often encroaching upon and displacing neighboring smaller groups. By monopolizing higher-quality foraging zones, these bigger coalitions effectively internalized the cost of feeding more mouths, thereby balancing the competitive disadvantages of size.

The tropical dry forest environment serves as the perfect natural laboratory for such studies. Unlike rainforests with relatively constant resources, this biome experiences stark seasonal fluctuations, particularly a pronounced dry season beginning around January. During this arid period, essential resources—water, fruit, and shade—become sparse and clumped along river corridors, intensifying competition as groups are forced into closer proximity. Intriguingly, during the dry season, capuchin territories showed reduced overlap, yet encounters between groups became more frequent and aggressive, signifying heightened territorial defense.

Crucially, the study documented how the influence of group size on space use and competition becomes destabilized when extreme climatic events disrupt usual seasonal patterns. El Niño-induced droughts and La Niña-related heavy rains both heightened foraging costs and undermined the dominance advantages larger groups typically enjoy. In extreme conditions, the spatial buffer that allowed large groups to secure better resources disappears, eroding the benefits of living in a sizeable coalition.

As Dr. Brendan Barrett, co-senior author and research leader at MPI-AB, explains, “While large groups usually benefit by outcompeting smaller neighbors for prime foraging grounds, these benefits reach a threshold under climatic extremes. Prolonged environmental stress may push individuals to leave or even cause entire groups to fragment, fundamentally reshaping the social landscape.” Such fragmentation could have cascading effects on population structure, social interactions, and ultimately species survival.

These findings question long-standing ecological assumptions about the stability and scalability of group living in primates and other social animals. The prevailing model suggested that bigger groups reliably compensate for their increased resource demands by simply enlarging their home ranges. Yet, the new research highlights a more nuanced reality: the efficacy of this strategy is contingent on the consistency of environmental conditions, making social structures vulnerable to intensified climate variability projected under global warming scenarios.

The study underscores the value of long-term data collections in teasing apart the complexities of ecological and social phenomena. Founded in 1990, the Lomas Barbudal Monkey Project led by Perry is among the longest-running field primate studies worldwide. Its unique combination of behavioral monitoring and satellite environmental data enabled unprecedented insights into how habitat quality fluctuates seasonally and across broader climate cycles, affecting the monkeys’ social and spatial strategies.

From a methodological standpoint, this work exemplifies the power of combining fine-scale behavioral ecology with remote sensing technology. Behavioral observations quantified energetic intake rates through fruit consumption metrics, while satellite imagery tracked vegetation health and resource distribution. This integrative approach allowed researchers to model the interactive effects of group size, neighborhood dynamics, and environmental variation on competition and territory use.

Still, the study measured energetic costs rather than directly linking these changes to survival or reproductive success—parameters that will be critical in predicting long-term population trajectories under shifting climates. Dr. Odd Jacobson, lead author, emphasizes the next frontier: “To forecast how climate change could destabilize social living, we must connect the dots from resource competition and space use to birth and death rates. Understanding this chain will reveal whether erratic environmental pressures permanently alter animal societies.”

In a broader context, this research reveals how climatic volatility, an aspect intensifying with anthropogenic climate change, might not only challenge individual species’ survival but also fundamentally transform the social infrastructures upon which many species rely. For social primates, whose evolutionary success intertwines with group living, such changes may alter mating systems, cooperative behaviors, and even genetic exchange over time.

As natural climate oscillations like El Niño and La Niña increase in frequency and severity in the twenty-first century, the primates of Costa Rica’s tropical dry forests serve as a poignant sentinel. Their adaptive responses and vulnerabilities offer crucial foresight about how entire ecosystems might restructure. The study leaves an open question: how will primate societies adapt if climatic extremes become the new normal, and what conservation strategies can support these complex social networks amid inevitable environmental upheaval?

Subject of Research: Animals

Article Title: Environmental fluctuations alter the competitive trade-offs of group size in a social primate

News Publication Date: 6-May-2026

Web References:
https://lbmp.anthro.ucla.edu/
DOI: 10.1038/s41559-026-03048-8

Image Credits: Susan Perry / University of California Los Angeles

Keywords: Ecology, Ethology, Climate Change, Social Primate, Group Dynamics, Tropical Dry Forest, Foraging Competition, El Niño, La Niña, Animal Behavior, Conservation

The study centers around the phenomenon of self-scratching, a common yet significant behavior seen in primates, specifically Japanese macaques. Self-scratching is widely regarded as a behavioral indicator of stress, signaling the internal emotional states of individuals. With this in mind, researchers aimed to establish a clearer understanding of the social factors influencing this behavior among female macaques in a natural setting. Through rigorous observation over extensive hours, the study broke new ground in elucidating the connection between proximity to other individuals, social dynamics, and stress levels.

The research took place on Kinkazan Island in Miyagi Prefecture, where a group of 11 adult female Japanese macaques was carefully monitored. Over 205 hours, the researchers meticulously recorded instances of self-scratching alongside factors like dominance ranks and the proximity of individuals nearby. This level of observation provided a robust dataset that allowed insightful analysis of the interplay between social interactions and stress.

One of the striking observations from this study was that during periods of rest, female macaques exhibited higher instances of self-scratching when they were alone compared to when companions were present. This behavioral pattern presents a paradox in the context of social animals. While companionship generally alleviates stress in many social species, it appeared that in this specific context, solitude could lead to greater levels of stress for these females.

In contrast, when the macaques were foraging, the presence of closely related individuals, such as mothers or sisters, had a significant impact on their self-scratching behavior. Interestingly, the frequency of this stress marker was actually elevated when females foraged in the presence of kin, deviating from the expectations set by previous research on social relationships in primates. This unexpected result raises intriguing questions about the nature of competition and stress—suggesting that even within close kinship ties, competition for food resources can spur stress instead of fostering comfort.

The study elucidates the nuanced layers of social interaction among Japanese macaques. Specifically, while related females are often seen as advantageous partners within a social structure, the dynamic changes when resource competition is introduced. This revelation invites further exploration into how closely-knit social bonds can sometimes become sources of competition, complicating the understanding of primate social behavior.

Upon analyzing the behavioral data, researchers emphasized that social affiliations in macaques result in a blend of positive and negative influences, depending on contextual factors. Social proximity does not inherently reduce stress but rather, depending on the situation—like foraging or resting—can induce varying levels of stress. This complexity challenges popular notions within animal behavior, that social ties uniformly lead to stress alleviation.

Moreover, the study’s findings contribute significantly to the discourse around animal welfare and their social needs. Recognizing that social interactions can both mitigate and exacerbate stress among social animals affirms the need for a nuanced approach in managing animal welfare in captivity and conservation efforts.

Understanding the conditions under which stress manifests in social species is particularly crucial for developing strategies to improve their living conditions. It encourages caregivers to create environments that acknowledge the complex social structures of animals and their varying emotional needs. The implications of this research extend beyond just Japanese macaques, shedding light on the behaviors of many other social species and their stress responses.

The research team, led by behavioral ecologists, emphasizes the importance of continued investigation into the relationship between environment, social structure, and stress. The study serves as a vital addition to the existing literature on primate behavior, opening avenues for future research into how these intricate dynamics play out across different species and contexts. By advancing the comprehension of how social environments impact stress levels, we can work towards fostering more conducive habitats for the welfare of animals, both in the wild and in managed care.

Japanese macaques are not only remarkable for their behavioral complexity but also serve as models for understanding broader ecological and sociological theories. The study of their self-scratching behavior reveals the profound impacts social structures have on individual well-being and stress management. As research in this area evolves, we may uncover even more concerning the delicate balance between companionship and competition in animal social systems.

In addressing these intricate dynamics, we also gain insights beneficial for conservation efforts aimed at protecting natural habitats. By fostering environments where social interactions can thrive in a manner that reduces stress and competition, we not only enhance the well-being of these incredible animals but also contribute to the health of ecosystems they inhabit. The findings from this study invite us to rethink the nature of social relationships among animals, establishing new frameworks through which we can understand and support their complex lives.

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Subject of Research: Self-scratching behavior in wild female Japanese macaques
Article Title: Influence of proximate individuals on self-scratching behaviour in wild Japanese macaques
News Publication Date: 22-Feb-2025
Web References: 10.1016/j.anbehav.2025.123111
References: Animal Behaviour
Image Credits: Maisa Sekizawa

Keywords: Japanese macaques, self-scratching, social behavior, stress, primatology, animal welfare, social dynamics, kinship, foraging behavior, competition.