Gorillas, the largest living primates, command attention with their imposing size. An adult male gorilla, known as a silverback, can weigh over 180 kilograms (400 pounds) and stand nearly 1.8 meters (6 feet) tall. This remarkable physique raises questions about the biological and evolutionary factors that underpin their immense size.
Fueling Their Enormous Frame
Gorillas primarily sustain their bodies through a herbivorous diet, consuming vast quantities of fibrous plant material daily. Mountain gorillas, for instance, are largely folivorous, relying heavily on leaves, stems, and pith, while Western lowland gorillas incorporate more fruit into their diet. An adult male gorilla can consume between 18 to 34 kilograms (40 to 75 pounds) of vegetation in a single day. This diet, though abundant, is relatively low in concentrated nutrients.
To extract sufficient energy from this high-volume, low-calorie food source, gorillas possess a specialized digestive system. Their digestive tract features an enlarged hindgut, including an expanded large intestine and cecum, which functions as a fermentation chamber. Within this specialized gut, a diverse community of bacteria breaks down cellulose, a complex carbohydrate found in plant cell walls, into digestible short-chain fatty acids. This efficient microbial fermentation process allows gorillas to derive vital nutrients from plant matter that most other animals, including humans, cannot. Consuming large quantities of food also means gorillas spend a significant portion of their day foraging and eating, which directly contributes to their body mass.
Built for Bulk
The size of gorillas is supported by a robust skeletal structure and dense bones. Their anatomy is specifically adapted for power and stability rather than agility. Gorillas possess powerful musculature, particularly in their shoulders, arms, and back, which is essential for activities like knuckle-walking, climbing, and foraging. Their upper body strength is estimated to be six times greater than that of an adult human.
Their primary mode of locomotion, knuckle-walking, is facilitated by specialized wrist and hand bones, allowing them to bear significant weight on their forelimbs. Gorillas’ arms are notably longer than their legs, and their body weight is distributed more towards their upper bodies, further aiding this quadrupedal movement. A broad chest and wide pelvis accommodate their large internal organs, especially the extensive digestive system required to process their fibrous diet. Additionally, gorillas exhibit a lower metabolic rate per gram of body weight compared to smaller primates, which allows for more efficient energy utilization and growth.
The Advantage of Immense Size
Their size provides gorillas with several advantages in their natural habitat. Their size serves as a deterrent against predators, such as leopards. A dominant silverback, weighing up to 180 kilograms, can stand upright and beat its chest to appear even larger, often enough to ward off threats. Living in groups also enhances their collective defense capabilities.
Within their social structure, size correlates with higher dominance rank and increased reproductive success for male gorillas. Larger males are often more successful in competing for resources and attracting females. Their size allows gorillas to efficiently consume and digest abundant, lower-quality plant foods that are inaccessible to smaller animals. This broadens their dietary options and reduces competition for food. Their mass also aids in thermoregulation, helping them maintain a stable temperature, especially in cooler, higher-altitude environments where some gorilla species reside.