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Recently I saw Inside nature's gaints episode on horse, and was fascinated about its internal organisation. And my question is that they have a very large lungs to accommodate, but a relatively smaller stomach. But other herbivores like the hippopotamus have a very large digestive system even though they need less energy than horse. How does horse manage this problem? How do they get that much of energy even though it had small digestive system and more over it is an herbivore?

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    $\begingroup$ Horses (and rhinoceri) are hindgut fermenters unlike cows which are foregut (stomach) fermenters. I don't know about hippos, but the hindgut fermentation in horses allows it to eat more frequently. $\endgroup$
    – WYSIWYG
    Dec 31, 2016 at 8:21
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    $\begingroup$ So, the horses eat a lot frequently. $\endgroup$
    – sreekara
    Dec 31, 2016 at 11:50

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The digestive system of a horse is by no means small:

  • They have 15 to 21 m (50 to 70 ft) of small intestine, with a capacity of 38 to 45 L.
  • They have a 1.2 m (4 ft) long caecum that holds 26 to 30 L.
  • They have 3.0 to 3.7 m (10 to 12 ft) of colon, capacity up to 76 L.
    (Figures sourced from Equine Anatomy (Wikipedia))

Certainly horse stomachs are comparatively small as they are hindgut fermenters, but that is not where most energy and nutrients are absorbed. Some food passes from the stomach to the small intestines before it is fully digested, which enables more continuous eating to keep up with their energy and nutrition requirements. Most proteins, carbohydrates, and fats are absorbed in the horse's small intestine. Some nutrients are absorbed in the large intestine as well, see Is there nutrient absorption in the large intestine of hindgut fermenters?.

The hippopotamus is a pseudoruminant, having a 3 chambered stomach (unlike the 4 chambers of a cow). Hippos are therefore considered to be foregut fermenters, and are not comparable to horses. In terms its digestive tract, the hippo is much more similar to a cow than a horse.

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