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
Evolution selected for the herbivores that didn’t kill their food source. Leaves grow back.
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.
Not so. Herbivores eat whatever is in season and has the most nutrients.
In the winter and spring, deer are stuck eating browse – leaves and shoots. Because that’s what’s available.
But when the ground thaws, deer will absolutely tear the crap out of turnips.
And when the oak trees are dropping acorns, that’s where you’ll find them.
And when apples are ripe you have to beat them away from the apple trees.
Bark is *incredibly* difficult to digest. As far as I know, even insects that “eat” bark are just trying to dig through the bark to get to the living, nutritious phloem underneath. That’s what beavers are eating – the phloem, not the bark. The only organism that really eats bark is fungi, which *very slowly* digest the bark with special enzymes. Living trees shed the bark constantly so that few things can digest all the way through before that piece falls off and is replace by a new piece.
Under the phloem, the tree goes back to being pretty worthless, nutritionally. The cells there are just full of water to hold the tree up, and that’s about it – if the cells are alive at all. It’s all structural, made to be extremely tough to hold up the weight of the tree. Once the tree has grown beyond that part – either taller or wider – those cells don’t need to replicate anymore, so there’s no energy going to them and no real active proteins doing anything.
Even just chewing through the bark requires tough mouthparts. Beavers, for example, have evolved to incorporate extra iron in their teeth, making them much stronger than ours. Even then, beaver teeth have to grow continuously because despite being reinforced with iron, they *still* wear down.
Roots tend to be a lot less tough than bark, but they’re also buried. Digging them up takes work, and chewing them requires teeth that can handle chewing some dirt along with the roots. Some plants are fairly easy to uproot, like many grasses, and grazers will often pull the whole plant up. But woody bushes send their roots deep and wide, and it’s generally not worth the effort to dig them all up.
Leaves, on the other hand, are *relatively* soft and extremely nutritious. That’s where the sugars are being made, where most of the business of *being alive* happens in a plant. The leaves still aren’t *easy* to chew, compared to something like meat. They can still wear down teeth. Many plants, like grasses, have evolved to incorporate silicate (essentially glass) into the structure to make them harder to chew. If humans tried to eat a diet of only [raw] grass, our teeth would wear down to nubs and we would starve.
That’s a *lot* of effort to eat the most nutritious part of the plant. It generally is not worth that effort to eat the less nutritious parts. It’s all the same work, all the same wear on mouthparts, for very little reward.
so leaves are like the ultimate fast food for herbivores. easy to digest and packed with nutrients. roots and branches are tougher and take longer to chew. plus leaves are usually easier to reach than the branches. kinda like how we go for chips instead of carrots when snacking. guess it’s all about efficiency and convenience.
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