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How come animals don't have any variety in their diets, yet they seem to be strong and healthy?

  • Gorillas eat only leaves
  • Cows eat only grass
  • Giraffes eat only leaves

Why do humans need such an enormous variety in their diets but animals don't?

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The question is quite broad, so I can only begin to answer, based on the animals you mentioned:

  • Cows and other ruminants (including giraffes) have a totally different digestive system, which allows them to ferment the ingested food and thus acquire nutrients in a way that is not accessible to humans.

  • As for the gorilla, what may only be "leaves" for you are in fact various items (leaves indeed, but also bark, fruits, flowers, roots, seeds and shoots) from hundreds of different species, as you may reed in this article. If we compare these foods to human dietary requirements, these foods provide amounts of protein that would be probably be sufficient for humans, but very low amounts of fat and available carbohydrate, and lots of dietary fiber (see this other article). According to 3, Western Lowland Gorillas derive more than half of their energy from the fermentation of fiber in the colon (under the form of short-chain fatty acids), whereas this source of energy is marginal for humans. Despite very little genetic difference, humans have evolved to eat very differently from apes. A human trying to follow a gorilla diet would quite probably not be able to achieve this level of colonic fermentation and thus would fail to thrive on this diet.

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  • Okay. Interesting. As a follow up question, could humans survive and thrive on a Gorilla's diet? – Lumo5 Mar 7 '17 at 14:31
  • I edited my answer to answer this follow up question. In short: I think humans could survive but not thrive on such a diet. But colonic fermentation in humans does not seem to be well understood yet, and there would probably be interesting experiments to do. – Tetramino Mar 9 '17 at 9:27
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First of all, humans are omnivores, while all the example animals given in the question were herbivores.
You are comparing apples and oranges (or rather, apple and beef)!

If you compare a human's diet with a wild pig's (sus scrofa) diet, you will find out that they both eat a variety of foods.

As to why this is the case, biology.stackexchange might be more suited for your question...

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