How do grazing animals get so large on their diet?

Cows, horses, camels, giraffes, etc…They’re all huge, muscle-bound, and powerful beasts that seem to feed mainly on grasses and leaves. How do they recieve the proper amounts of protein, calcium and vitamins from their diet?

They are generally able to digest plant material such as cellulose, so their food is more nutritious to them than it would be to us.

They eat pretty much constantly.

When they aren’t running away from a predator, they conserve resources by not doing much at all.

This is true of even small herbivores - we’ve just acquired a pair of guinea pigs and all they do is eat and sit.

Actually, it helps to be large if you’re a herbivore. Larger animals have relatively lower metabolic rates, so that they require a smaller amount of food per unit of mass.

There are very few small endotherms (“warm-blooded” animals, such as birds and mammals) that subsist entirely on vegetation. Because of its relatively low energy content, they just can’t eat enough of it to keep themselves going. Small endotherms mostly eat meat (including insects) or seeds, which have a much higher energy content than leaves or other vegetation. (Hamsters and other rodents and sparrows would be examples of small eed-eaters). Some, like rabbits, may favor high energy plant parts such as buds or food-storing roots (e.g. carrots).

One thing pure herbivores have trouble obtaining from their diets is enough sodium and calcium, which are in low concentrations in plants. That is why deer, cattle, etc. seek out salt licks.

Grazing – eating grass, etc., indiscriminately – is a relatively late adaptation among herbivores, with a number of specialized digestive system modifications to make it a “profitable” econiche in terms of biophysics. (Only most ruminants and some perissodactyls graze.) Most herbivores in terms of evolutionary history and in terms of species today are browsers, eating only that which is especially tasty and nutritive among plant parts.

And AFAIK geese are the only successful true herbivores (as opposed to seed-eaters and such) among the birds.

Actually, the bizarre Hoatzin of Amazonia is the only obligate folivore (leaf-eater) among the birds, and is also the only fore-gut fermenter, having adaptations similar to the ruminants in mammals. But geese are more “successful” in terms of number of species and global range, though not as specialized for herbivory.

Actually, vegatation eating animals like cattle and gorillas don’t eat all the time. Most of their daytime hours consist of just digesting their food. So they eat a bunch of stuff. Let it ferment, etc. in their bellies, chew cud, whatever, then several hours later eat some more.

Being big is a by-product of this activity. The animal needs a large volume to hold all the material while it digests since it takes so long. A large digestive system means larger bones, more muscles to support it all etc. It also affects the skull since grinding up the material takes a lot of muscle and strong jaws.

Think of these animals as very large composting systems with supporting infrastructure (plus the usual reproductive parts).

Note that grazers technically can’t digest the cellulose in grass. Instead they chew up the grass into pulp and let it ferment in their large stomachs. The microbes in their guts break down the cellulose, and the grazers digest the microbes. The larger the stomach you have the more efficient the bacterial processing is. Ruminants (such as cows) take the process further…they eat grasses without much chewing, soak and preprocess it in the first part of their stomachs, then regurgitate the stuff, rechew the cud, and eat it again. Rabbits have an even more disgusting process…they have to eat their own droppings and reprocess them to extract all the nutrition.

Most grazers don’t need much in the way of vitamins or specific amino acids, since they can manufacture what they need (or get it from their stomach bacteria). Even though grass is low in protein they eat a lot of it.

Grazers also face problems from tooth wear. Grass contains tiny glass particles that wear away teeth like sandpaper. Many grazers will starve to death when they get old because their teeth have worn away to nothing. And of course, many herbivores starve to death with full stomachs. They can find plants to eat, but the plants are too low in nutrients to sustain them even eating non-stop.

The ruminants have evolved a particularly efficient means of ensuring that they can obtain sufficient protein from their diet. They excrete their urea waste not primarily through the kidneys but via special salivary glands. IOW instead of pissing out nitrogen they effectively excrete it back into the mouth where it is re-swallowed. Unlike animals the microbes in the stomach are able to utilise inorganic nitrogen of this type to produce proteins. As a result ruminants are able to thrive on diets far lower in protein than any other animals could cope with simply because they recycle the nitrogen. In fact commercial cattle and sheep herds on low quality feed are often fed common garden-fertiliser urea as part of their diet. Instead of fertilising the grass farmers can fertilise the animals directly and produce the same results more efficiently.

Other grazers get around the problem of the low quality of grass by throughputting huge volumes. Because they only have a single chambered stomach and the food is only processed once they are able to put through far higher volumes of feed than ruminants and make up for their inability to recover as much nitrogen by sheer mass. As a result ruminants tend to do better on mid-quality pasture while other grazing animals can often perform better on very high and very low quality.

If by that you mean that the only mammals that graze are ungulates then it should be pointed out that many macropod marsupials and wombats are also grazers. Add to that list a few species of grazing primates and rodents. I suspect there are also other grazing mammals although the ungulates have certainly diversified the most by using a grazing lifestyle.

That’s really only true of the macropods and ruminants. Other grazers utilise what is known as hindgut fermentation where the food is fermented in the large intestine and cecae. In those species the stomachs are more or less ‘normal’ size and it is the intestines and cecae that are enlarged.

Beavers do that, too.

Kinda puts the whole carnivore thing in perspective, don’t it? (burp)

My theory for years has been that cattle are not so much herbivores as bacteriovores. Cattle can even “make” protein from urea (and many cattle are fed a supplement with 30% urea in it) because the bacteria can use urea as food, and multiply, and pass into the cow’s gut for digestion.

Great explanation of foregut/hindgut fermentation and how it works physiologically. And I wasn’t aware of marsupial grazers, though I probably ought to have been. However, I now propose the Blake-Polycarp Law of Mammalogy: In any statement regarding the uniqueness of a group of mammals, there is to be an understood “except for [marsupial group X], which convergently does the same thing.”

Flippin’ Aussie copydasyprocts! :wink:

Nice try, but dasyprocts are from South America, and they’re rodents. I think you mean “Flippin’ Aussie copydasyurids”. :wink:

Indeed. Don’t forget that there’s even a grazing member of order Carnivora. Has to spend at least 12 hours a day eating grass.

You’re right! :o ::: envisions Desmostylus with a sig line of “Polycarp doesn’t know the difference between a marsupial tail and a rodent asshole!” :::

You can’t really call a panda a grazer. It does indeed live on grass, but it’s not an herbaceous grass so a panda is a browser, not a grazer.

And geladas eat other stuff besides grass, so I guess it all comes down to your definition of grazing. ::shrug::

Poly: You’re lucky I can’t think of a decent joke about hairy asses. :wink:

What revolting creatures. Bang goes the ‘cute’ factor. A distasteful hijack - How do they know which droppings have been through the system once already and are therefore not worth eating again? And do they nibble on each other’s poo, or stick to their own?

All grazing animals eat stuff besides grass, so if that’s your standard there are no grazers on the planet and your comment is even less accurate. However there is a difference between grazing, which is eating herbage, and browsing which is selectively eating specific plants and young shoots. Pandas are browsers and in no wise grazers. While geladas, cattle and kangaroos will all eat stuff besides grass they all primarily eat herbage, not the shoots of pseudo-woody plants like bamboo.

Aside from the ability to use fermetation as part of the digestion process, and/or having multiple stomachs, being big has other advantages too. Large animals can graze over larger areas - they can migrate further once an area is used up. Also, assuming they live on open plains, sheer size and brute strength are good protection. An angry bison or cape buffalo can be deadly.