Eli5: Why are some animals classified as herbivores when they’ll eat meat given the opportunity?

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The biggest examples I can think of are cows and deer. Numerous times they’ve been caught eating small animals yet are classified as herbivores, why?

In: Biology

Could it be based upon what they mostly eat and the anatomy and physiology of their digestive system?

“A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet.”

Cows and deer eat mostly plants and have adaptations such as wide flat teeth for grinding plant material. They can eat meat and possibly digest it too but they evolved to eat plants.

Simplicity.

Wiktionary [defines herbivore](https://en.wiktionary.org/wiki/herbivore) as:

> An organism that feeds **chiefly** on plants; an animal that feeds on herbage or vegetation as **the main part** of its diet. [emphasis added]

As humans (at least, modernish humans in some societies) we love classifying things – coming up with labels and categories for things and putting them in boxes. But reality doesn’t work that way – a lot of the labels we use, particularly in biology, aren’t absolute but have fuzzy edges (species being a classic example). We use the labels anyway because they are convenient or useful. We simplify things to make it easier to understand and to work with.

We just have to remember that a lot of the simple models we use break down eventually.

I see a lot of the comments haven’t added this so here’s my addition: they don’t eat the animals for the meat. I don’t have any sitations unfortunately but the reason why they eat small animals or bones is for the calcium. It’s the same reason why parakeets in the rainforest nibble on a big calcite cliff. Calcium!

Edit: it’s also why reptile owners feed herbivorous reptiles calcium supplements or dust crickets in calcium powder.