A 33-year study in Guanacaste, Costa Rica, found that extreme droughts can change how white-faced capuchin monkeys live. Scientists watched 12 groups of these monkeys in the dry forest and recorded their movements, food choices, and competition with other groups. In normal years, bigger groups can defend better feeding areas, which gives them more food.

This changed during the strong El Niño drought in 2015. Rivers dried up, trees had less fruit, and most places had little food. Because of this, large groups could not use their size to get more food than smaller groups.

This led to more fights for food inside big groups. Some baby monkeys died and mothers became very tired because food was hard to find. The study shows that severe drought can break the usual ways that animal groups help each other survive.