ELi5: How do some animals get enough nutrients out of eating hay or grass or a single variety of leaf for their entire lives?
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The structural part of plants is made from a substance called “cellulose”, and believe it or not, cellulose is a carbohydrate. Carbs can be simple like sugar, complex like starch, or very, very complex, like cellulose. (Chitin, the stuff the hard parts of bugs are made of, is also a very complex carbyhydrate.)
So that’s how they are living, off mostly carbs. Every animal has a community of microbes living in their gut which helps them digest stuff, and plant eaters are no exception, having microbes living in their gut that can break cellulose up into simpler carbs that the animal can digest.
In things like cows and caribou and buffalo and stuff, the microbes live in a special stomach called a “rumen”. Termites have amoeba living in their absomen that can break down the cellulose in wood.
Gut microbes in cows and bugs deserve an award for turning hay and wood into dinner. #ThankYouGutBugs
To add to DaSaw’s answer, notice how grazing animals are *constantly* eating? Like, how often do you see cows just standing around with their heads up? Almost never, they’re pretty much always munching on something. So in a sense the answer is “by eating a hell of a lot of it”.
To put it simply, they have adapted to be able to subsist on that diet. More specifically, the gut microbe in herbivores help break down that tough plant material. From there the animal can derive all their energy needs, and they can synthesize what they can’t get from the plants. We humans for example, usually consume some form of meat. That’s because some amino acids and vitamins are less prevalent in plants, and we can’t synthesize them by ourselves.
Cows and other ruminants are actually feeding their gut bacteria. It ferments the carbohydrates int o nutrients.
To add to the answers about ruminant digestion, hay is not just cellulose. Even leafy plants a fair amount of complete protein. Hay is 8-14%. Softer stuff like spinach is lower because it has more water.
Pretty much all essential nutrients either come from the plants or microbes, and ruminants have a lot of both.
Most animals like deer still regularly seek out salt deposits to get minerals they lack from their diet. Even many butterflies and birds do the same. And, herbivores like deer are also known to eat meat/eggs when the opportunity is presented.
When you look really really close at the stuff animals need to live it starts to work a lot like Lego. Animals generally need a similar set of different models to work, like everyone needs a yellow car, a red car, and a red house etc. Some animals evolved to try to eat a wide variety of food so that they can just consume those models already assembled and use them as they are. Other animals eat lots of the same food and take in a smaller number of ready assembled models but then their body breaks those models down into their bricks and uses those to build the models they are missing. Maybe you only eat red and yellow cars but you can break up some of the red cars to make the red houses you aren’t getting ready made in your food.
Here’s your reminder that its very common for wild animals to not get enough nutrition, and as a result “their entire lives” are much shorter, not to mention the parasites.
Farm animals eat the plants as well as getting grains, additives, pills, vaccines, and extensive attention that they do not get in the wild.
In addition to the answers already given, many herbivores can also be opportunistic omnivores. For example, I once saw a video of a horse eating a baby chick, and apparently that is pretty common. Similarly, there are probably a lot of bugs in pastures that get eaten along with the plants. So yeah, these animals eat like 99.5% plants, and the microbes in their extensive guts help turn those plants into usable nutrients, but they also sometimes ingest small animals/bugs.
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