Researchers from the University of Michigan are trying to discover why, and they have four leading hypotheses: https://www.sciencedaily.com/releases/2016/02/160225101603.htm
1. Drought or another environmental stressor creating less access to food or water
2. Poisonous plants that the monkeys are consuming
3. Pesticides or other environmental contaminants
4. A pathogen (e.g. yellow fever or Zika)
As fruit eaters of the forest, howler monkeys help maintain the health of Nicaragua's forests by dispersing seeds. Even though the