These tree-dwelling herbivores mainly consume tree and vine leaves, flowers and tropical forest fruits. Mammals do not have the enzymes capable of digesting cellulose, the carbohydrate that composes the leaf cell wall. Instead, with the help of bacteria contained in a sacculated stomach, all monkeys in the subfamily Colobinae (e.g. colobus monkeys) receive energy rich gases from the bacteria triggered reaction (fermentation). Unlike colobines, howler monkeys do not have the sacculated stomach, but rather a simple acid stomach that also contains two enlarged sections in the cecum and colon in which fermentative bacteria are found. As with colobines, the gases serve as the energy source. Howlers also eat flowers and fruit, which are far less abundant than leaves and require greater energy expenditure to forage.

They do not need to travel far to find leaves. For this reason, their total home size is about 77 acres (31 hectares) for 15 to 20 animals, and they typically move about 1,300 feet (400 meters) per day. In comparison, a spider monkey, which feeds primarily on fruit, has a home range of 1,000 acres (300 hectares).

At the Smithsonian’s National Zoo, they eat primate biscuits, browse, spinach, romaine lettuce, kale, other greens, broccoli, grapes, bananas, apples, melons, carrots, sweet potatoes and green beans.