When fruit and tender leaves become rare during the dry season, howler monkeys turn to a diet largely consisting of older leaves, which are more fibrous and fewer in nutrients. Helping them to metabolize this hard food, the highly evolved digestive systems of howler monkeys allow more efficient fermentation and breakdown of challenging plant fibers, hence facilitating their processing. Their hindgut has widened. This adaptation enables them to acquire vital minerals from leaves that other animals would find challenging for breakdown.

Apart from their digestive adaptations, howler monkeys selectively forage, choosing plants with certain chemical compositions either for medicinal uses or to aid digestion. Sometimes, they consume some plants believed to have antibacterial properties, for example, those used to assist prevent digestive disorders.

Keystone species in Costa Rican forests, this nutritional adaptability highlights how flexible howler monkeys are against environmental changes. Their ability to change their diet not only helps them to survive during occasional food scarcity but also influences the structure of the forest since their selective foraging effects limit plant development patterns and maintain ecological equilibrium.