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Cows, camels, sheep, goats, etc being ruminants must chew their food repeatedly by regurgitating their food from their first stomach compartment and chewing their 'cud'. This then finer chewed material makes its way to the various stomach compartments to be digested.

These animals are eating plant material, the same plant material animals such as elephants, horses and hippos eat as well. However, these animals only have one stomach compartment.

  • Why does one need a multi-compartment stomach and one does not if they are all eating the same/similar food?
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There is more difference than just the parts of plants that are eaten.

Two of their stomachs - rumen and reticulum are not used for digesting food at all. Multiple rounds of chewing and mixing with saliva, wich result in very small particles of undigested food help the bacteria in those stomachs reach, digest and use that cellulose-rich plant material. Cows "eat" the bacteria from their stomachs and the grass is just food for these bacteria. You could say that cows are not herbivores but secondary consumers.

Look at this picture (from Wikipedia article on ruminants, a good place to check all this in more detail but still simpler than most professional books): ruminant digestion system versus single stomach animals

See that the ruminants have the same second and third part of their digestive system (though the bacteria use up most of carbohydrates, so some difference in absorbed material), but have an additional part in the front - where the microbes digest and ferment the food, that the ruminant has provived them. The reticulorumen needs to be large in order to provide space for all that plant material and bacteria.

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It is not a biology question but simply a logistic problem. The digestion of cellulose just because of the presence of celluase producing microbes. Rumination itself, by the action of chewing, can enhance the cellulose digestion efficiency by incresing the surface area. Still nothing related to the multiple compartments of the ruminant stomach. Actually, the multiple compartments allowed the anti-directional peristalysis and multi-tasking of the stomach. That is Ruminants need A Multi-Compartment Stomach to perform rumination for the reason of logistic.

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If you ever look at a huge pile of elephant/horse dung you will see it is full of poorly digested vegetation. I had an (unfortunate) 4 day project on a field trip in south africa which involved handling a lot of elephant s***, more than once I found whole amarula fruits!

Think about this in comparison to ruminants which have a much better digested pile of dung, e.g. a cow-pat. The ruminant digestive system allows them to digest cellulose (found in plant material) much more efficiently than other vertebrates thanks to a specialized gut flora. You may want to look at this and this for more details.

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They are not all eating the same/similar food.

Elephants, horses and hippos are herbivores that have different diet to ruminants. Elephants, for example, have trunk to access fruits that is less difficult to digest.

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  • $\begingroup$ 100 kg for a 5000 kg elephant is 2% of body mass. $\endgroup$
    – kmm
    Jan 3, 2013 at 15:47
  • $\begingroup$ ok, i have edited. $\endgroup$
    – neo
    Jan 3, 2013 at 15:56

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