The Upside and Downside of Living on Leaves
All of the great apes are plant-eaters. Even chimpanzees, which occasionally hunt and kill small animals and eat them, still eat less meat than nearly any human society. Yet the various great ape species fit into different ecological niches, so they focus on different kinds of plant foods. Chimpanzees are mainly fruit eaters. Although gorillas will eat fruit and nuts whenever they’re available, they mainly eat leaves.
The fact that gorillas mainly eat leaves explains a lot about their behavior and social structure. Leaves don’t run away, so there’s no need to chase them. Leaves are so abundant in the gorilla’s habitat, and so low in calories, that it’s pointless to fight over them. A tree full of ripe fruit or nuts is another matter, entirely. In general, I’d expect animals that mainly eat leaves to be nicer than animals that mainly eat fruit, because they have less to fight over.
Gorillas face the same kinds of challenges as any animal that specializes in eating leaves. Here are a few of those challenges, as explained by Fiona Sunquist (The strange, dangerous world of folivory. International Wildlife; January-February, 1991; pages 4–10):
The demands of living on low-energy and often poisonous food means that most folivores live close to the limit of their energy supply.
- They must conserve energy wherever possible, and this often translates into being very slow.
- It is no coincidence that the sloth, the world’s slowest mammal, is a folivore.
- Besides being slow, folivores also spend much of their time resting.
All this suggests that if you want to be a marathon runner, you’ll want to eat something besides leaves–ideally something starchy. If you simply want to be thinner, you might want to try eating more leaves.
More Than Enough for a Human Being, and Even More Than Enough for a Pig!
Here’s a rather dry and technical article that describes the amount of “available protein,” as opposed to “crude protein,” in the foods that wild mountain gorillas eat.
The mountain gorillas in this study were getting more than enough protein, mainly because they were eating so many leaves. Although leaves are low in calories, a substantial proportion of those calories comes from protein. If gorillas eat enough leaves to get enough calories, the protein takes care of itself. A leafy diet is so rich in protein that the gorillas can afford to snack on some low-protein fruit.
Gorillas are big and strong. Like most of the big and strong land animals, they eat leaves. Animals that mainly eat leaves are called folivores. (Foliage means leaves.) In this article (http://www.jstor.org/pss/4219431), some biologists explain why you would expect a gorilla to be a leaf-eater:
Large mammalian herbivores, according to accepted ecological theory, may be expected to feed on abundant, low quality food as a result of the relationship between their body size, metabolic requirements, and gut capacity. … Abundant low quality food means nonreproductive plant parts, i.e., stems and leaves, because fruit and flowers are too ephemeral to provide a regular source of food in bulk for large animals. So, large herbivores ought to be folivorous in a broad sense, and usually are.
Really big animals need to eat a lot of food. Flowers and fruit may be tasty but are in short supply. For this reason, the really big plant-eaters tend to eat mostly leaves.
Gorillas are practically vegan. They eat plants, mainly leaves. “There is a virtual absence of foods of animal origin.”
In this study (http://jn.nutrition.org/cgi/reprint/127/10/2000), some scientists studied the diet of wild western lowland gorillas, from the Central African Republic. The gorillas ate about 200 different species of plants. However, they were eating practically no meat. Gorillas do not hunt. They do not fish. They do not keep chickens, cows, goats, or sheep. Gorillas do eat a few insects and other creepy-crawlies now and then. In other words, a wild gorillas’ diet is 99.9% vegan.
How much fat, protein, and carbohydrate did this vegan diet supply? By calorie, the diet was 2.5% fat, 15.8% carbohydrate, and 24.3% protein. That’s a lot of protein! Gorillas mainly eat leaves. Leaves are low in calories, but a lot of their calories are in the form of protein. To get enough calories, a gorilla has to eat a lot of leaves. But if it eats enough leaves to get enough calories, it will automatically get enough protein.
The scientists estimated that these wild gorillas were getting 57.3% of their calories from the fiber in their diet. Dietary fiber includes things like cellulose, hemicellulose and pectin, which are found only in plants. These substances are made up of long chains of sugar molecules. But animals cannot make the enzymes to break them down into sugar again. Thus, they will pass through your small intestine intact. But in your large intestine, they will be broken down by bacteria. Bacteria can make the enzymes that break down fiber. This process is called fermentation because it does not use oxygen in the form of O2. This fermentation process produces some short-chain fatty acids, such as butyric acid. These short-chain fatty acids are an important source of energy, particularly for the cells that line the large intestine. To learn about short-chain fatty acids, click here
Photo by jnissa