Climate Change Reshapes White-Faced Capuchin Monkey Groups in Costa Rica

In a Costa Rican dry forest, survival is governed by timing, movement and social decisions. White-faced capuchin monkeys live in groups whose numbers fluctuate over time.

While certain groups remain small, others become much larger. Scientists had long thought they knew the reason: larger groups mean greater competition for food, yet they also offer safety in numbers when facing rival groups.

A decades-long study has revealed that this trade-off is much more complicated. Climate, seasonal conditions and longer-term weather patterns alter the effects of group living. An advantage at one time may become a disadvantage at another.

Decades of capuchin observations

Since 1990, the Lomas Barbudal Monkey Project has monitored capuchins. Over 33 years, researchers followed 12 groups, documenting their feeding behaviour, movements and encounters with neighbouring groups.

“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,” said Susan Perry from the University of California-Los Angeles (UCLA).

The research requires considerable effort. Observers accompany the monkeys for as long as 13 hours each day, gathering information without marking or interfering with the animals.

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

This extended perspective enables scientists to identify trends that shorter studies commonly fail to detect.

Food competition increases

The findings support an established idea: individuals in bigger monkey groups receive less food. When numerous capuchins feed in one tree, its fruit is rapidly depleted.

“This was a clear sign that the group members were competing with each other, which is what we expected for large groups,” said Odd Jacobson for Max Planck Institute of Animal Behavior.

The result aligns with previous studies. A greater number of mouths increases pressure inside the group.

Researchers expected larger groups to travel further each day in order to obtain sufficient food, but the evidence does not confirm this prediction.

The distance travelled daily remains much the same between groups, regardless of their size. This pattern applies during both the wet and dry seasons.

That outcome calls a widely held assumption about group behaviour into question.

Range expansion over time

Rather than travelling further each day, larger groups occupy more extensive areas over longer periods. Their foraging activity is distributed across a broader landscape.

“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 explained.

This strategy conserves energy. Instead of increasing exhausting daily journeys, the monkeys depend on a wider variety of resources.

Bigger groups acquire space at the cost of smaller groups. As a group increases in size, it frequently expands into adjacent territory.

“In doing so, large groups could offset the costs of internal competition,” Jacobson said.

Researchers monitored numerous pairs of groups through time. In the majority of instances, the larger group extended into the range used by the smaller one. This demonstrates how competition between groups influences the way space is used.

Competition and conflict

The dry season transforms these dynamics. Food becomes limited and water is scarce, bringing monkeys together around rivers, where trees continue to bear fruit.

Groups encounter one another more frequently in these places, and conflict rises. Larger groups obtain access to the more favourable sites close to water.

At this point, the usable forest contracts as resources become concentrated in small areas.

Large groups generally keep away from one another, since fights between groups of comparable strength involve substantial risks. As a result, gaps emerge between their territories.

Smaller groups often occupy these gaps, which function as lower-competition safe areas.

Comparable arrangements are seen in other animals. Some species survive by using border areas between dominant groups.

Climate shifts the balance

Climate is a key influence on group success. The researchers used an indicator that measures wet and dry conditions over time, linking these conditions with El Niño and La Niña cycles.

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

Severe conditions damage large groups. In exceptionally dry or wet periods, food is more difficult to obtain, and larger groups are affected more because limited resources must be shared.

However, not every unusual weather pattern harms larger groups. When conditions change only moderately, such as when the dry season is wetter than usual, large groups perform well.

They obtain higher-quality territories and sustain their food intake. These circumstances produce uneven distributions of resources that benefit larger groups.

The findings show that climate does not affect every group equally. The result is determined by the severity of the change.

Climate change may reshape capuchin groups

Climate change could lead to more extreme weather events, potentially making conditions more difficult for large groups. Nevertheless, moderate shifts may continue to benefit them.

“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,” Perry noted.

Capuchins have adjusted to this environment over thousands of years. Their future survival depends on how effectively they respond to changes that are faster and less predictable.

Group living is not static; it changes with environmental conditions. As climate patterns shift, the rules governing social life in the forest may shift too.