

Many ‘strict herbivores’ will eat animal matter on occasion - curtis
http://scienceblogs.com/tetrapodzoology/2010/12/20/carnivory-in-cows-and-deer/

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bane
One of the main differences between herbivores and carnivores (and the various
omnivores that rank in between) is what their digestive system is optimized
for.

Herbivores tend to have longer (relative to body length), more complex and
less acidic digestive tracts than Carnivores. Their systems tend to make more
use of grinding and physically breaking down plant fibers to get at the sugars
in the material. Even then, herbivores get surprisingly little nutrition from
plants and almost always have to by grazing or eating. About 50% of the food
herbivores eat is excreted as waste.

Modern farming techniques solves some of this by providing easier to digest
feed to the animals, usually high in protein. The feed is often made up of
other animals (one of the sources of issues during the mad cow disease problem
a few years ago). Herbivores generally lack the mechanical mouth parts to be
effective meat eaters, but will eat animal products more or less just fine if
presented in a digestible format (they don't produce all the right enzymes all
the time and their stomachs aren't quite as acidic, but in some small amounts
it can actually be healthy for the animal). If you've ever worked near or on a
farm, you'll know about various herbivorous animals eating non-plant items
pretty frequently. This article is a no duh that's common knowledge to any
farmer.

Herbivores come in a few types as well, ruminants have complex multi-
compartmented stomachs. As with anything complex, they tend to have a wide
variety of failure modes when eating as well. The complex flora in their
digestive systems often creates gaseous environments that don't often have a
good way to escape. A surprisingly common vet procedure is to basically just
punch a hole in the side of a ruminant to provide a vent for outgassing.

Carnivores tend to have shorter (relative to body length), less complex and
more acidic digestive tracts. Protein simply processes faster than cellulose
and can be broken down by acids and a few enzymes. Anybody who's owned a dog
or cat can tell you that carnivores eat plants all the time. It doesn't digest
well though as they simply lack the mechanisms to digest cellulose and usually
it comes out more or less like it went in. True carnivores also tend to not
have as much or any gut flora, which aids in breaking down and digesting plant
matter.

As with many things in nature, life is a spectrum. Most things fit somewhere
on that spectrum, cows are more herbivore, while goats (as goat owners can
tell you) are slightly less, while lions are on the other end as pretty much
carnivores. Bears, apes, humans, dogs, pigs etc. all fit in various places on
this spectrum.

Humans have digestive systems that are basically somewhere between a simple
herbivore and a carnivore (a horse and a dog). Humans, like brown and black
bears have digestive systems that aren't particularly optimized to process
plant or animal products, but are optimized to switch "modes" over a short
period of time. A diet of largely plants will turn out systems to become more
acidic and we'll start cultivating gut bacteria to aid in digestion. A diet of
largely meats and we'll start to switch to a more alkaline intestinal
environment. One problem is that our systems never really complete either
transition, so we're not particularly efficient or complete either way.

Our mouth parts are more like a carnivore than an herbivore, but evolution has
reduced some of the more useful hunter parts, our canines are pretty small at
this point. However, our molars are still meant for ripping and not flat for
grinding like a herbivore's. Our jaws provide vertical movement like a
carnivore, not rotary grinding motions like a herbivore. Our stomach capacity
relative to our body size is almost exactly within carnivore parameters and we
pass food out of our stomach and into our intestinal tract about as quickly.
We have strong stomach acid compared to herbivores and our stomachs provide
almost no digestive activity (a tremendous amount of digestion occurs in an
herbivore's stomach).

Our intestines are closer to a carnivore's, and our feces are produced and
have consistency like a carnivore's. We have developed gallbladders (helpful
for breaking down animal proteins) and our pancreas is a major contributor of
enzymes needed for digestion (unlike herbivores).

In cases where humans need surgical intervention, a surprising amount of our
digestive tract can be removed and we'll survive. Herbivores need much more of
their tract relatively to survive. Humans for example can live without a
stomach, colon and cecum. Herbivores cannot.

What's actually surprising about humans is how well we can live on diets made
up mostly or solely of plant or animal material. There's some nutrients we
don't get as well out of one kind of material, and our bodies don't produce
some of the nutrients that animals on the extremes of the spectrum make
naturally. It's likely we were scavengers and opportunists, eating pretty much
whatever we could get our hands on. We've had weapons and meat processing
tools in our inventory since before we were homo sapiens a quarter of a
million years ago. The oldest unambiguous weapons are throwing spears from the
Palaeolithic 300,000 years ago in a cache full of thousands of animal bones.
Meat processing tools have been found much older than Homo erectus (up to 2
million years ago). We've been hunting and eating animals as part of our diet
long enough for evolutionary forces to have an affect. If our ancestors were
ever herbivores, it was long _long_ before then (millions upon millions of
years), and indications are that we've moved rapidly towards the carnivorous
end of the spectrum (i.e. our mouth parts aren't anything like an herbivores
and neither are any of our known ancestors). In fact it's known that meat was
a major part of Homo erectus diets, but cooking was unknown to them. This diet
was so effective that Homo Erectus lasted for well over a million years.

~~~
Retric
I would just add that insects often make up a small but important part of many
herbivores diet.

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akavel
Comments are interesting too, e.g.:

 _On Mann, or one of those other remote British islands, bird conservationists
noticed (some years back, before the internet, so I don’t have a link) that
there were a lot of gulls or terns or whatever with one wing or a shortened
wing. On further inspection they also found a lot of chicks with zero heads
(they did not survive this). The enigma was studied, and it turns out that the
grazing sheep there were regularly grazing of bits of nest-bound pre
fledglings probably because of a lack of calcium and /or other minerals in the
local soil, and thus, generally, food chain (and dirt … grazers also eat dust
every time they eat anything, which includes minerals)._

 _Because the sheep were good at snipping, the amputations were often clean
enough that the chicks would grow to one-limbed adults._

or:

 _About Indian cows: Europeans are unaware that lots of city cows in India
rarely have a chance to nibble at any grass or leaves. They feed on paper,
torn cloth, household scraps and small quantities of food stolen from market
stalls. It was amazing that if you drop a bit of paper in India, the nearest
cow turns with interest, comes and eats it._

 _Re: Well, I’m one European who is certainly aware of this :) Some of my
friends watched an Indian cow consume paper and other material from a burning
fire… you have to feel sorry for these animals if they are really this
desperate._

~~~
Excavator
In regards to those remote British islands, might it be this?

[http://news.nationalgeographic.com/news/2003/08/0825_030825_...](http://news.nationalgeographic.com/news/2003/08/0825_030825_carnivorousdeer.html)

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Steuard
I learned this a few years ago from a colleague in biology. He and his
students routinely collect birds in nets for tagging and measurement, and he
mentioned that it's important for them to check the nets every few hours when
they're set up. If they leave the birds trapped in ground-height nets too
long, the local deer will find them and eat them. (This behavior was alluded
to in the article.) I got the feeling that a lot of herbivores out there would
happily be omnivores if they were _able_ to catch appropriately-sized prey.

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sidcool
I remember watching a very disturbing video once, where a deer ate a sparrow.
I can't quite find the link. Will post if I am able to.

I personally feel that some animals understand their nutritional requirements
and select their food accordingly. It's for survival, nothing personal.

~~~
vijayr
Someone posted a video on Reddit of a baboon eating a baby deer, alive. It was
like 2-3 mins, couldn't watch more than a few seconds. It was horrible. I
remember thinking, why can't the stupid baboon kill the deer and then eat it,
instead of eating it alive?

Animal kingdom is cruel

~~~
ColonelPanic001
I think "indifferent" is probably more accurate. The baboon didn't do it out
of malice, the idea probably just didn't occur to it.

That actually strikes me as even more grim, in some ways. Nature isn't out to
get you, nature just doesn't care at all. It just is.

~~~
vijayr
May be, but many (most?) carnivores first kill their prey before eating them.
There are exceptions of course, like the Python.

Either way, it is horrible to watch.

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chrisBob
I had a veterinarian teach me that carnivore has nothing to do with diet, and
means that the animal is in the order Carnivora, which are mostly
distinguished by having canine teeth. The kinkajou
([http://en.wikipedia.org/wiki/Kinkajou](http://en.wikipedia.org/wiki/Kinkajou))
is a carnivore by this definition, but does not eat meat. I am sure that there
are herbivores in a similar situation.

~~~
FreeFull
While carnivore does mean of the order Carnivora (which includes animals like
the Giant Panda), the term carnivorous does simply mean that the animal mostly
eats meat.

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okonomiyaki3000
Sounds like most of the 'vegetarians' I've known.

~~~
supervacuo
An omnivore eating less meat is good for all the reasons being vegetarian is
good - it's not all-or-nothing.

That more-vegetarian-than-previously omnivore calling themselves vegetarian is
helpful to other vegetarians in that it raises the profile of those issues and
normalises pro-social dietary choices.

It's obvious that people making huge lifestyle changes struggle - I'd say
support is more personally and societally useful than sneering disparagement
about them not "actually" being vegetarian. Technically correct: the worst
kind of correct.

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JoeAltmaier
Also carnivorous sapiens will occasionally eat a bit of vegetable matter, but
only if its hiding under a burger or has cheese and sour cream on it.

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vince_refiti
I observed this in a few humans myself.

