As climate change intensifies, scientists are becoming increasingly concerned about how animals will cope with a more unpredictable world. One way to gain insight is by studying how animals have already responded to natural climate fluctuations. But for long-lived, social animals like humans and other primates, gathering this kind of evidence takes time.
"It takes decades to see how climatic cycles interact with the complex social dynamics of animal societies," says Professor Meg Crofoot, a Director at the Max Planck Institute of Animal Behavior (MPI-AB) who leads the Department for the Ecology of Animal Societies.
Now, a study that followed wild monkeys for three decades has offered clues: climate extremes impact societies by changing the costs and benefits of living together.
Animals benefit from living together, and bigger groups can yield bigger benefits: increased protection, more eyes scanning for predators, and stronger muscle to push rivals aside. But big groups also come with a cost: more mouths to feed, and so more competition among group members. The new study reveals how a population of neighboring primate groups balances this fundamental trade-off, and how climate extremes can tip the scales.
For over 33 years, a team led by Prof. Susan Perry tracked 12 neighboring groups of white-faced capuchin monkeys in Costa Rica, combining field observations with satellite-derived environmental data of the forest. Their findings show that the balance between competition within and between groups is not fixed, but shifts with climate cycles.
"The textbook view is that bigger groups need more space and need to travel further every day to find enough food for everyone," says lead author Dr. Odd Jacobson, a postdoctoral researcher at the Max Planck Institute of Animal Behavior. "What we found is more nuanced. How groups use space depends not just on their size, but also on the size of their neighbors and the climate conditions they all experience."
The study, led by researchers at the Max Planck Institute of Animal Behavior (MPI-AB), the University of Konstanz, and UCLA, was published in Nature Ecology & Evolution.
Strength in numbers
The capuchins in the study live in one of Costa Rica's last remaining fragments of tropical dry forest. They feed on a wide range of fruits and insects, and occasionally small vertebrates. For most of the year, food is abundant and widely distributed.
By following capuchins and recording how much fruit they ate, the scientists found that, under typical conditions, capuchins living in larger groups consumed fruit at a slower rate. "This was a clear sign that the group members were competing with each other, which is what we expected for large groups," says Jacobson.
Capuchins in large groups, however, had a solution. By expanding their range and claiming areas from smaller groups, they gained access to more foraging options and less-depleted food patches. "In doing so, large groups could offset the costs of internal competition," says Jacobson.
But then the season changed, and the competitive landscape changed too.
Dry season: when the stakes rise
Tropical dry forests experience far more extreme seasonal swings than a typical rainforest. Around January, the harsh dry season begins. Over the following months, the researchers observed that critical resources such as water, food, and shade became concentrated along rivers, forcing groups into closer contact.
The researchers also observed a shift among the capuchins: groups overlapped less in space but encountered each other more frequently, suggesting that they were competing more intensely with their neighbors and actively defending the scarce remaining resources. Larger groups dominated the highest-quality areas, while smaller groups were pushed into less productive parts of the forest.
In both seasons, larger groups found ways to offset the costs of their size by leveraging their dominance over smaller groups. But this depended on typical seasonal patterns, and the study revealed what happens when conditions deviate from the norm.
Climate extremes may disrupt social structures
El Niño events brought severe drought, while La Niña events brought unusually heavy rainfall. Both extremes increased the costs of foraging for large groups, intensifying competition for food and eroding the advantages of numerical superiority.
"Life in a big group has costs, and normally these can be buffered by out-competing other groups for the better foraging spots," says co-senior author Dr. Brendan Barrett, a research group leader at MPI-AB. "But under climatic extremes, that buffer may reach its limits, gradually eroding the advantages of group living. Under sustained environmental pressure, individuals may leave, and in some cases entire groups can split apart, fragmenting the social landscape. If climate change makes these extremes more frequent, the makeup of entire animal populations could look very different."
El Niño and La Niña are natural climate cycles, not a direct consequence of climate change. However, climate change is expected to make such extremes more frequent and intense, making it all the more important to understand how animal societies respond to them.
Rethinking the rules of group living under extreme climate
The findings challenge a long-standing idea that larger groups simply expand their range to subsidize their food needs. Instead, the study shows that this strategy is sensitive to climate variability—raising new questions about how ongoing climate change will influence the size and stability of animal societies in the wild.
The study was built on data from the Lomas Barbudal Monkey Project , which was founded by Prof. Susan Perry and remains one of the longest-running primate field studies in the world. The research combines detailed behavioral observations of 335 capuchin individuals with decades of satellite imagery measuring the greenness and tree cover of the surrounding forest, allowing the team to track how habitat conditions shifted across seasons and climate cycles.
"This approach allowed us to disentangle how group size, neighbor interactions, and climate variability jointly shape space use and competition," says co-senior author Perry, of UCLA.
"When I started this project with a single capuchin group in 1990, I never imagined it would grow to 12 neighboring groups, or that we'd one day use those early observations to study how movement patterns shift with climate," adds Perry. "That's the power of long-term data."
The study measured energetic costs rather than direct survival or reproduction, and the authors point out that this should be the focus of future work. "If we are to truly forecast whether increasingly erratic climates could permanently destabilize the costs and benefits of group living, we will need to connect birth and death rates to what we now understand about competition and climate," Jacobson adds.
