Spider monkeys are very agile creatures and avoid many dangers by spending their time in small groups in the forest canopy. If necessary, they sound an alarm to call a larger group together that is able to attack humans and other larger animals. The larger group consists of 40-50 monkeys, but it often breaks into smaller groups for specific activities during the day.
Spider monkeys have very strong tails and are able to grip tree branches with their hands. While they do not have opposable thumbs, they have a strong grip. This allows them to stay away from threats. One researcher, who the monkeys obviously considered a threat, reports a smaller group emitting rough barks to call others and then moving to lower tree limbs. While they remained at almost 40 feet from him, they broke off tree branches and threw them at him.
Spider monkeys also make a great deal of noise in order to bluff a perceived threat. While they are relatively defenseless, this bluff often scares humans and many other animals away. Monkeys that have not encountered humans before are more likely to attempt a bluff. However, those with negative experiences of humans are more likely to attempt to sneak away without allowing the human to notice them.