They say it takes a village – and that’s particularly clear in the case of white-faced capuchins, who are highly social and live in groups of up to 40 individuals.
Living in large groups means that animals have more allies to help defend themselves against predators and compete with rival groups for resources such as food.
Communal living does have its drawbacks, however. More individuals mean more mouths to feed, and more competition within the group.
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Now, a study co-authored by researchers at the University of California, Los Angeles (UCLA) and the Max Planck Institute for Animal Behavior (MPI-AB) has revealed how white-faced capuchins navigate this complex balance – especially in the face of challenging climate conditions.
Susan Perry, a UCLA anthropologist, has led the Lomas Barbudal Monkey Project (LBMP) in Costa Rica for 35 years, making it one of the longest-running primate field studies in the world.
The white-faced capuchins live in Guanacaste’s tropical dry forest, which encompasses different land types such as privately-owned farms, cattle ranches and government-protected forest. For most of the year, they feed abundantly on fruits, insects and the occasional small vertebrate.
Perry’s data collection team spent 12-13 hours each day tracking 12 groups of capuchins through the site with a GPS device. This method allows them to record travel routes and mark sleep sites without interfering with the animals. It also means the researchers can observe the capuchins at close-range.
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The team followed the capuchins’ travel routes and recorded how much food they ate. The study’s authors combined this observational data (collected over 33 years) with climatological data and satellite imagery, which allowed them to track how the capuchins’ habitat changed across seasons and climate cycles.
One of the main findings was that the capuchins who lived 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 the study’s lead author Odd Jacobson, a postdoctoral researcher at the MPI-AB.
However, the team also realised that the capuchins adapted their behaviour throughout the seasons. The larger groups expanded their range in the forest, claiming areas (and therefore more food resources) from smaller groups.
“In doing so, large groups could offset the costs of internal competition,” explains Jacobson.
But when the environment changed, so did the capuchins’ behaviour.
Shifting behaviour
The tropical dry forests in and around Guanacaste’s Lomas Barbudal Biological Reserve experience significant seasonal variation, which affects the availability of critical resources such as food, water and shade. And in January, the dry season begins.
The capuchin groups were forced into closer contact as these critical resources became concentrated around rivers.
The researchers started to observe that the capuchin groups overlapped less in space but had more frequent encounters – suggesting more intense competition with neighbours and active defence of resources.
Larger groups tended to avoid overlapping with other large groups and in 84 per cent of cases, the group that caused the overlap increase was the group that became relatively larger over time.
The larger capuchin groups gained access to less-frequently visited areas during the wet season and to higher-quality patches during the dry season – while smaller ones were pushed into less productive areas.
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Climate cycles
The research was carried out during El Niño and La Niña events, which brought severe drought and unusually heavy rainfall.
This intensified competition for food and weakened the traditional advantages that being part of a larger group provides.
“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 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.”
While El Niño and La Niña are natural climate cycles, climate change is likely to make extreme weather events more frequent – making it important to understand how these social groups may be affected.
“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 says.
“Long-term data sets such as this one are so valuable scientifically that they make the hardships seem worthwhile.”
Read the full paper here: Environmental fluctuations alter the competitive trade-offs of group size in a social primate
Top image: landscape of Lomas Barbudal Biological Reserve. Credit: Susan Perry / University of California Los Angeles
