Referring to the howler monkey as primarily a folivore implies that their food mainly comprises leaves. Nonetheless, among them, in terms of eating patterns, they do not have a commonality in all habitats. Still, they have an extraordinary ability to adapt and adjust according to the nature and level of available food sources in their environment.
Howler monkeys, which find their food supply in the mature leaves that dominate year-round in forests such as tropical rainforests, can consume many mature leaves.

It is common for the leaves to have a high fiber and low protein content, necessitating an efficient digestive system so that the nutrients can be properly digested. Howler monkeys developed large, highly compartmentalized stomachs that accommodate symbiotic bacteria, which help with cellulose breakdown; this maceration of plant cell walls then would liberate energy through all the acceptable chemical reactions while at once becoming less susceptible to predators like herbivores as these cellulose structures are not palatable either in themselves or after digestion. It was an adaptation that allowed them to extract maximum nutrients from their leafy diet even when the nutritional value of mature leaves was relatively low, keeping their energy needs well sustained.

Seasonal and geographical factors may influence the occurrence of mature leaves, but howler monkeys can modify their feeding behavior to include other food resources in such cases. Mature leaves could be very limited in some places, such as dry forests or seasonally dry areas. Howler monkeys respond by eating other plant parts, like flowers, fruits, or young leaves, which are usually richer in nutrition and more accessible to digest.

Howler monkeys can survive variations in food availability and maintain good health and high levels of activity despite living in a demanding ecosystem, as they can assimilate their diet in this way. The relationship between diet and habitat for howler monkeys goes beyond what is available for food; it covers the larger ecology in which these primates live. Factors like forest type, structure, and diversity of plant species are essential for howler monkeys' selection of food sources, and these factors reflect the climate, soil, and disturbance regimes that vary spatially.

It is worth noting that howler monkeys, on their part, determine the structure and functioning of forest ecosystems by feeding on plants and contributing to seed dispersal events. This interplay sustains biological diversity as well as ecosystem dynamics. Like numerous other animal species, howler monkeys are challenged with higher pressures due to human activities, such as the destruction of their habitat due to fragmentation or climate change.

Changes in forest habitats can disrupt howler monkeys' food sources, which will cause nutritional stress, poor reproductive performance, and a decrease in population numbers. Consequently, preservation of the forest where the howler monkeys are found has to be placed at the top of the agenda of any conservation efforts aimed at maintaining healthy populations of these species.