Question

Why do herbivores have longer small intestine in mammals and birds why is it necessary to separate oxygenated and deoxygenated blood?

Answer 1

Herbivores eat plants which is cellulose and there is no enzyme to digest cellulose, so herbivores have longer small intestine.
Mammals and birds are warm blooded animals. To maintain their temperature, they need a lot of energy. Separation of oxygenated and deoxygenated blood produces more energy. Therefore, separation is important.

Answer 2

Herbivores eating grass need a longer small intestine to allow the cellulose to be digested. Whereas Meat is easier to digest, hence carnivores like tiger have shorter small intestine.