How do gorillas and other animals maintain muscular mass while eating mainly plants?

NEGI RK Friday, October 22, 2021

Because their digestive system works differently than ours, and
Because plants do contain protein, you just need to eat a lot of them to get enough. (And gorillas do!)

Let’s look at the first point:

While gorillas are genetically similar to humans, they have very different digestive systems—more akin to those in horses. Like horses, gorillas are “hind-gut digesters” who process food primarily in their extra-long large intestines rather than their stomachs.[1]

This helps them break down fibrous plant material in a way we can’t. But that doesn’t mean their diet is better—ours is more efficient since we can eat more nutritious and calorie-dense food and digest it in our small intestines. Gorillas have to spend most of their day foraging.

The diets of hominids and/or early human populations improved, in part, due to cooking, and the increased abundance of animal products obtained through scavenging, hunting, fishing, and dairy consumption. In contrast, great ape species in the wild derive a significant amount of their total daily metabolic energy needs through the fermentation of lower quality plant materials in their hindguts.[2]

But this ‘low quality’ diet is still better for a gorilla. Captive gorillas who are fed biscuits and simple starches get the same calories in 30 minutes as wild gorillas who’d spend most of their day foraging. But it’s not optimised for their digestive system, and two-thirds of captive gorillas regurgitate and reingest their food on such a diet. (This doesn’t happen with wild gorillas.)

When Less switched the Cleveland gorillas to a new diet—one that cuts out biscuits and simple starches and replaces them with leafy greens, alfalfa, and branches from willow and other trees—she realized gorillas needed to eat about four times as much by weight as they had previously been fed. Not only did gorillas on the “Cleveland diet” get a huge increase in fiber, they spent nearly as much time eating as they would in the wild. Almost immediately, the Cleveland gorillas stopped regurgitating their food. “We thought the diet change might have an impact on regurgitation and re-ingestion,” Less says. “But to see it eliminated completely was shocking.”

Something Mysterious Is Killing Captive Gorillas

For a decade, zookeepers have known that something strange was stopping the giant animals’ hearts—now they’re beginning to trace the culprit to their guts.

https://www.theatlantic.com/science/archive/2018/03/gorilla-guts/554636/

Likewise, a gorilla diet would be a poor choice for a human. Our bodies have evolved to work best on different foods. It’s the same with any animal, and just because we share a lot of similarities with gorillas doesn’t mean we don’t also have considerable differences.

I’ve already alluded to point 2 a bit, but what people don’t understand is that there is quite a lot of protein proportionally (i.e. per calorie) in many plants. They’re just very low in calories, so gorillas need to eat huge amounts to get the same level of protein (and other macronutrients) in their diet as us.

A gorilla can eat 18 kg (40 lbs!) of food per day.[3] The vast majority is vegetation, with some species eating a decent amount of fruit, and occasionally small invertebrates. Their lives consist of foraging, resting a bit, foraging some more, sleeping, then doing it over again.

So a gorilla does get a good amount of protein in its diet. In absolute terms, it’s likely eating more grams of protein than a lot of humans. It just has to eat its way through the jungle in order to get it.