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What are differences of herbivores vs omnivores? I do not mean dietary differences (obviously), but physical ones.

E.g., afaik herbivores have a much longer digestive tract than carnivores; then there is the difference in teeth, possession of carnivorous instincts or lack thereof, etc.

But where is the distinction between herbivores and omnivores? Is there a clear distinction at all, or is the border between the two groups somewhat fuzzy? Or are omnivores merely herbivores who are less picky?

EDIT

As pointed out in the comments, the last sentence seems to ask for dietary differences. It is not meant to; the only difference I am aware of is dietary, so it asks whether there are any anatomical distinctions, or the dietary is the only one. I.e. are they anatomically similar, and not distinguishable in the same way herbivores and carnivores are; hence are they herbivores that simply do not mind eating non-plant based food in a while?

I hope this serves to clarify.

EDIT 2

I realize that this question is quite broad, so while I am trying to get a good view of the bigger picture, partial answers will be more than welcome!

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closed as too broad by anongoodnurse, March Ho, James, fileunderwater, kmm Mar 2 '16 at 1:33

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ at first you're asking about anatomical differences, but your last paragraph seems more focused on the dietary behavior for which you said you were not asking about. If this is not the case, please clarify your question. $\endgroup$ – theforestecologist Feb 27 '16 at 14:22
  • $\begingroup$ @theforestecologist I just did, I hope this clarifies! $\endgroup$ – foaly Feb 28 '16 at 3:07
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    $\begingroup$ This is a bit broad. Teeth, gastrointestinal differences (not alike in all herbivores), hooves/claws, eyes, etc, not to mention instincts... it's pretty broad. $\endgroup$ – anongoodnurse Feb 28 '16 at 4:10
  • $\begingroup$ @anongoodnurse I realize that it is. However, I lack the knowledge to narrow it down. Also, I am basically interested in the big picture, i.e. pretty much anything having an impact on how they feed. $\endgroup$ – foaly Feb 28 '16 at 4:16
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    $\begingroup$ @foaly Instead of making additional sections like EDIT1 or EDIT2 while retaining the original content, please actually edit the question so that it becomes short and precise. $\endgroup$ – WYSIWYG Feb 29 '16 at 9:06
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"Is there a clear distinction at all, or is the border between the two groups somewhat fuzzy? Or are omnivores merely herbivores who are less picky?"

On second thought, I'll tackle those questions.

The "border" between herbivores and omnivores is very fuzzy among mammals, birds and at least some reptiles. Amphibians are a treat - they're all carnivores. Among reptiles, snakes are exclusively carnivorous.

Most lizards are also carnivores, though there are some herbivores. I'm not even sure about omnivorous lizards, though.

Omnivores are all over the map. If some are described as herbivores that are less picky, others could be described as carnivores that are less picky.

Note that certain species can be herbivores, carnivores or omnivores in certain parts of their range or during certain seasons. For example, bears may feast on salmon or ungulate calves when they're available but spend most of their time foraging for plants and small animals.

The most familiar omnivores include humans, pigs, raccoons and (some) rats, so you might look for some similarities between these species. Humans and raccoons are plantigrade, for example (as are bears - and rats, too, I believe). Humans, pigs and rats are known for their intelligence. Pigs and rats are very popular in medical research, for some interesting reasons.

Another thing to note is species that defy their group's general dietary tendencies. For example, the giant panda and palm nut vulture are herbivores, whereas other vultures are carnivores and most bears are omnivores - except for the polar bear.

There's another general rule; omnivores may be more rare in habitats that are lacking in plants. I don't think any marine mammals are considered omnivores, for example. All cetaceans (whales and dolphins) are carnivores, as is the sea otter. However, sirenians (manatees, dugong, etc.) are all herbivores.

That might give you something to work with.

EDIT: To my surprise, there are some good resources comparing omnivores and carnivores. Check out The Comparative Anatomy of Eating. The source might look a little flaky, but it looks very accurate to me. (It uses some of the same examples I did. ;))

To my surprise, there is apparently a closer correlation between carnivores-omnivores than herbivores-omnivores. But bear in mind that this article apparently focuses on mammals. Thus, differences in jaws and teeth can hardly be applied to birds.

Regarding my comment on omnivore intelligence, see Intelligence, Evolution of the Human Brain and Diet, though I can't vouch for its accuracy. Intelligence can be related to diet; it takes more brainpower to capture active prey than sneak up on a clump of grass, for example. This would suggest a preponderance of "intelligent" organisms among carnivores.

Intelligence can also be related to anatomical differences, like bipedalism, opposable thumbs and binocular vision.

In fact, most of the animals that are renowned for their intelligence appear to be carnivores or omnivores. Exceptions include gorillas, orangs, gibbons and the giant panda.

While whales, wolves/dogs, squids and octopuses are carnivores. Crows and ravens - regarded as among the most intelligent birds - are omnivores, as are humans, chimpanzees, pigs and at least some rats. As far as I know, pigs are generally regarded as the most intelligent ungulates (hoofed mammals); they're also among the few that are omnivores. (I can't think of any other omnivorous ungulates offhand.)

Pigs are also more compact than other ungulates, with stocky bodies and short legs. In fact, there don't appear to be many long-legged omnivores (among mammals, at least). Similarly, most (if not all) long-legged birds are carnivores (or insectivores, vermivores, etc.).

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    $\begingroup$ Links to external material are strongly encouraged on biology SE but you rarely use them. It serves two functions: 1) supports claims and arguments you make in the text 2) allows readers to find good follow up material so they can read more if they like. biology.stackexchange.com/help/how-to-answer $\endgroup$ – rg255 Feb 28 '16 at 7:35
  • $\begingroup$ I also don't think you try to answer the question at all, the op asks about anatomical differences between omnivores and herbivores which you haven't answered at all $\endgroup$ – rg255 Feb 28 '16 at 7:38
  • $\begingroup$ This slightly reads as if "omnivorous" is not an actual anatomical property, in contrast to "herbivorous" or "carnivorous". However, it would be more interesting to actually point out anatomical distinctions (or the lack thereof by comparison to the distinctions in herb. vs carn.). $\endgroup$ – foaly Feb 28 '16 at 7:47
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    $\begingroup$ The OP asked a lot of things - more than anyone can really handle. In closing, he wrote, "partial answers will be more than welcome," so I offered him some scattered ideas to help him get a better view of the big picture. As I think I mentioned, the anatomical differences between carnivores and herbivores can be very confusing; the differences between carnivores and omnivores are even tougher. $\endgroup$ – David Blomstrom Feb 29 '16 at 2:42
  • $\begingroup$ Please could you add some supporting scientific material that reinforces your answer and allows further reading. $\endgroup$ – rg255 Mar 8 '16 at 7:39

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