How do climate extremes alter the behaviors of animal societies

Many animals benefit from living together, and bigger groups can yield bigger benefits, including more allies to help them defeat predators and compete with rival groups for resources. Big groups, though, come with a cost: more mouths to feed, and more competition among members of the same social group. White-faced capuchin monkeys are constantly balancing the costs and benefits of group size, and may find this balancing act more challenging as they encounter climate conditions more extreme than those they have typically experienced in their evolutionary history.

How a population of white-faced capuchin monkeys has tackled these challenges over time forms the basis for the new study, published in Nature Ecology and Evolution and co-authored by UCLA anthropologist Susan Perry, who leads the Lomas Barbudal Monkey Project (LBMP) in Costa Rica.

The group of researchers, including Perry and co-authors from the Max Planck Institute of Animal Behavior (MPI-AB), found lower feeding rates among larger groups, especially during times of extreme climate conditions. They also discovered that home range size expands as group size increases. While those findings were expected, what surprised the team was that the length of the group's daily travel did not change as the group's size increased.

"It seems that larger groups compensate for the larger number of mouths to feed not by traveling further each day, but by having a larger variety of resources they can visit, which allows them to visit less depleted food patches," Perry said.

White-faced capuchin monkeys are small, highly social primates native to Central America. They live in multi-male, multi-female groups of 5-40 individuals. Females typically remain in their birth group for life, while males disperse to join other groups. Capuchins have one of the largest brain-to-body ratios of any primate and are known for their problem-solving abilities, social traditions and diverse diet of fruits, insects, and small animals.

The new study combines detailed behavioral observations of feeding rates and travel routes of 335 white capuchin monkeys in 12 neighboring groups with climatological data and decades of satellite imagery measuring the density of the surrounding forest, allowing the team to track how habitat conditions shifted across seasons and climate cycles.

"The combined approach allowed us to disentangle how group size, neighbor interactions and climate variability jointly shape space use and competition," Perry said.

Perry, a field primatologist who has led research and training at the site in Guanacaste, Costa Rica, for 35 years, making it one of the most comprehensive, longest-running primate field studies in the world, designed long-term protocols that would facilitate investigations of questions about the costs and benefits of group living.

"But when I started observing a single capuchin group back in 1990, I had no idea three decades later that the study would have expanded to this size, or that the monkeys would experience such extreme climate disruption, or that there would be such dramatic within-group variation in size over time," Perry said.

The longitudinal approach of the newly published study allowed for a variety of factors to be considered, including fluctuating group sizes, a wide range of climate extremes and the annual transitions between rainy and dry seasons.

Unlike many group-movement studies, which rely on radio-collared or tagged individuals, this study was completely non-invasive. In order to gain sufficient trust among the monkeys to enable the kinds of close-range observation of feeding tactics and social interactions that their core protocol requires, Perry's data collection team does not catch or otherwise interfere with the animals. This means that group movement data are collected by humans following these monkeys with a GPS device to record travel routes and mark their sleep sites. This is extremely laborious, as it requires 12-13-hour workdays in difficult terrain for the observers, day after day and year after year.

"Long-term data sets such as this one are so valuable scientifically that they make the hardships seem worthwhile," Perry said.

Strength in numbers

The capuchins in the study live in one of Costa Rica's last remaining fragments of tropical dry forest, which encompasses government-protected forest, cattle ranches and privately owned farms. They feed on a wide range of fruits, insects and occasionally small vertebrates. For most of the year, food is abundant and widely distributed.

By following capuchins and recording how much food they ate, the scientists found that capuchins living in larger groups generally consumed fruit at a slower rate, and that this cost to large groups was most extreme in the most extreme climatic conditions. "This was a clear sign that the group members were competing with each other, which is what we expected for large groups," said lead author Odd Jacobson from MPI-AB.

Capuchins, however, had a solution in large groups. 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," Jacobson said.

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 one another 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 amplified foraging costs for large groups, intensifying competition for food and eroding the advantages of larger group size.

Life in a big group has costs, and normally these can be buffered by out-competing other groups for the better foraging spots, Perry said. "But under climatic extremes, that buffer reaches its limits, and monkeys may adjust by making changes to group size, for example, by dispersing to other groups."

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.

In addition to Perry and Jacobson, Margaret Crofoot, Brendan Barrett and Genevieve Finerty were co-authors of the study. Jacobson and Barrett were previously research assistants at LBMP under Perry's supervision, and later did Ph.D. work at the site under her co-supervision.

Funding for Perry's data collection at LBMP has come from the National Science Foundation, the National Geographic Society, the Leakey Foundation, Wenner-Gren, Templeton World Charity Foundation, the Max Planck Institute for Evolutionary Anthropology, UCLA and the University of Michigan. Salary support for the MPI-AB collaborators was funded by the Alexander von Humboldt-Stiftung funds awarded to M. Crofoot.