Why Gorillas Are So Gentle

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.

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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.

How Much Protein Do Gorillas Get From Eating 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.

http://www.arts.mcgill.ca/anthro/chapman_files/cweb/Pdf/229_FiberNitrogen.pdf

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 because they are leaf-eaters

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.

Are Gorillas Vegan?

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