Gorillas are herbivorous (plant-eating). Plant material contains cellulose which is indigestible to many non-herbivorous animals. With regard to digestion, herbivorous animals that do not ruminate (re-chew their food as part of the digestive process) rely solely on the microbes (microscopic bacteria) living in their colon. The bacteria function to breakdown the indigestible plant cellulose and turn it into valuable digestible carbohydrates through the fermentation process.
A rainforest is densely vegetated. The abundance of vegetation eliminates the need for herbivorous animals to migrate long distances in search of food.
Hair protects gorillas' skin from biting insects and helps keep them warm. Keeping warm is especially important for the mountain gorillas around the Virunga Mountain Range since night temperatures will often drop below freezing.
On cold days, gorillas will often stay close to their sleeping areas (see Behavior: Daily Activity Cycles) or huddle close together, remaining motionless for extended periods of time to warm themselves.