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Question: Many animals have more than one stomach and humans only have one. Why is this?
The animals that you are referring to are called ruminants. They don't actually have more than one stomach, but have one stomach with four chambers. Cows, camels, llamas, deer, sheep and goats are examples of ruminants.
As you may have guessed, ruminants are animals that eat only plants, or herbivores. Plants contain cellulose, which is difficult to digest. In fact, many animals (humans included) cannot digest cellulose. Creatures that are dependent on a plant-based diet therefore need special adaptations to be able to digest this material. The four-chambered stomach is one such adaptation.
The ruminant stomach is composed of the rumen, reticulum, omasum and abomasum. When food is first swallowed, it enters the rumen. The rumen contains microorganisms, which produce cellulase, an enzyme no mammal is able to produce by itself. Cellulase breaks cellulose down into fatty acids, which are absorbed in the stomach and used for energy. This process is most efficient when food is broken down very well, as by chewing. Chewing food more than once then sounds like a good idea. In fact, ruminants regurgitate their food and chew it again. We call this chewing cud, or ruminating. Cud comes from food stored in the rumen and reticulum. Once food is chewed thoroughly it passes into the omasum, where water and minerals are absorbed, and finally into the abomasum, which is the equivalent of our stomach.
Several years ago, I attended an open house at the Cornell veterinary school. There I met a very special cow with a hole in her side. The hole, called a fistula - is created by a veterinarian. It is a three-inch diameter opening into the rumen and can be closed with a plug. Sometimes cows do not have enough microorganisms in their rumens and become very ill because they cannot properly digest their food. If this happens, vets can replace the microorganisms in the sick cow's stomach with some from the fistulated cow. Apparently the cow does not even notice the fistula, and provides a very important service to cows that could otherwise die.
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