Some typically great writing by Matt Soniak:
The screams were an alarm call indicating a predator on the ground. The group’s alpha male took off in the direction of the noise and was joined several other adult members of the group, and they all began making facial and vocal threats and shaking and throwing branches toward a dense tangle of liana vines…The mobbing of a predator by capuchins is a real sight. Led by adult and subadult males, the monkeys will make loud threat and alarm calls while breaking large branches from trees and dropping them on the predator like bombs.
…Some monkeys will get closer to predators if the aerial assault doesn’t work. In 1988, biological anthropologist Susan Boinski watched as a group of capuchins in Costa Rica killed a terciopelo, or fer-de-lance, after pinning it to the ground with a heavy branch and then approaching on the ground to beat it with sticks. One monkey rained down a flurry of 55 strikes to the snake’s body and head with a stick that it clumsily wielded like a club. Boinsky later approached the snake and found a mangled mess of bleeding wounds, exposed tissue and broken bones.
Read more about monkeys banding together to fight predators (and use weapons!)