why do most herbivorous animals eat only the leaves?

Question

Some herbvivores also eat the roots, fruits, and the flowers, but most herbivores mainly eat the green leaves right? I understand animals not eating the whole tree trunk, but if some animals can eat roots, surely they can eat the branches too right? I tried googling but I couldn’t find anything other than insects specialized in eat through tree barks.

In: Biology

Answer 1

Leaves and branches are made of very different stuff. Woody structures like branches and twigs are extremely tough and fibrous – made of something like 50% cellulose, which is incredibly difficult to digest. In comparison, leaves are built out of softer, more vascular tissues, which may still be somewhat tricky to digest but are a hell of a lot easier than wood. Plus, because of the photosynthetic structures inside them, leaves tend to be relatively rich in proteins, sugars, and water.

So really, if you’re an herbivorous animal, it’s kind of a no-brainer: why eat the really really tough part that you’ll have to spend a ton of time and energy chewing off and digesting, when you could just eat all the softer, more nutritious parts and then walk over to the next plant?

Of course, in starvation situations where there isn’t a next plant to move on to, it makes sense to be less picky, and indeed you do see animals eating more parts of plants in situations like this, such as deer stripping the bark off of trees during tough winters. But if you’re an animal that’s optimized its digestive system for leaves, your gut probably doesn’t even have to equipment to break down woody tissues properly, so you’ll probably get hardly any nutrition out of it anyway.

Edited to add, because I realized I didn’t fully answer your question: Roots are also made of different stuff than both branches and leaves. While tougher and more fibrous than leaves, roots are typically still less tough than wood (except for maybe thin twigs). And importantly, many plants have specialized “storage roots” – big fat tubers full of starches and sugars – that they grow during rich periods and then feed off of during lean ones. These are kind of a jackpot of energy if you can find them, so it’s a worthy tradeoff for some animals (such as humans!) to invest in digging up and chewing through these relatively tough tissues for the carby goodness they contain.