The major function of any creature’s digestive system is to provide nutrients to the body tissue. This will involve the ingestion of a food type, chewing, swallowing, secretion of enzymes, acids and bile, absorption and getting rid of any waste products. When mammals emerged, they evolved in different ways depending on the nature of the animal’s diet. For example a carnivore has a relatively simple large intestine whereas an herbivore has a voluminous large intestine.
Also, as mammals are warm blooded creatures, they need a diet that is plentiful and nutritious. Maintaining a warm body temperature is very energy expensive. Carnivorous animals have a relatively simple system because the proteins, lipids and minerals found in meat require very little in the way of specialised digestion. The earliest mammals were therefore probably predators. The smallest mammals tend to be insectivores because they cannot deal with the complex digestive process of an herbivore and they have a high ratio of heat-losing surface area to heat-generating volume. This means they will have high energy requirements and a high metabolic rate. This explains why the only large insectivores, such as anteaters, feed on concentrated populations of insects such as ants or termites.
Larger animals have a better heat-losing surface to heat-generating volume ratio meaning they can afford to digest their food more slowly and accordingly have been able to develop the more complex digestive system required for the herbivore. To be an herbivore the animal must play host to bacteria in their digestive tract. These bacteria usually found in a multi-chambered stomach are designed to break down and ferment plant matter making it readily digestible. The bigger an animal is the more meat it would need if was carnivorous. This would in itself cause problems; a huge amount of energy is expended in the hunting and catching of prey. The bigger the animal the greater the problem which explains why the largest mammals tend to be herbivores.
The third type of feeder is the omnivore such as man; an omnivore is able to eat meat and plant matter. Omnivores are not the next stage in the digestive process; in fact they are a bridge between the two. Omnivores have teeth that can rip and tear meat but they also have other teeth suitable for grinding grains and seeds. Omnivores have intestines that are longer and more complex than carnivores but shorter and simpler than herbivores. This means that omnivores can survive on any type of food imaginable meaning that an omnivore would be able to live in any area as it is not restricted by food type.
Chivers & Langer (Ed.) – The Digestive System in Mammals – Cambridge University Press (1994)