22.13: Animals have some alternative means for processing their food.

Figure 22.24: Ruminants can digest plant material that humans cannot.

Cellulose could feed the world. It is everywhere—it’s the major carbohydrate making up the cell wall around plant cells—and there is a tremendous amount of energy stored in its chemical bonds. But most animals don’t produce (or acquire in any other way) the enzymes that break down cellulose. Those with the appropriate enzymes, though—and some species do have them—gain access to one of the most plentiful sources of chemical energy on the planet. Let’s investigate how some animals do it.

Ruminant animals, such as cows, bison, deer, goats, and sheep, have complex four-part stomachs in which they can digest plant matter that humans cannot (FIGURE 22-24). First, the grazing animals chew on the plant material for a while, grinding the tough cell walls. Then they swallow it, and it passes into the first part of their stomach, which contains a huge pool of enzymes and cellulose-digesting bacteria. There, the plant material gets broken down, and much of the cellulose is digested. To increase their energy-extraction efficiency, the animals then regurgitate the food back into the mouth, chew it some more, and swallow it again.

Called “chewing the cud,” the additional chewing further breaks up the plant cell walls so that the bacteria can have easier access to the cellulose, digesting more of it. From the first part of the stomach, the food then passes through the remaining chambers, where some additional digestion takes place, before moving into the small intestine and continuing the usual path of digestion. Cellulose-digesting bacteria have some of the most dangerous working conditions on earth: although cellulose is the primary component of the ruminant diet, ruminants also get a significant amount of protein every day by digesting many of the bacteria working in their gut.

Some insects, including silverfish, produce cellulose-digesting enzymes. These enzymes make it possible for them to eat books and paper, which are made of plant products containing cell walls made from cellulose.

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Question 22.13

Silverfish are insect pests in libraries rather than in kitchens. Why might this be? (Hint: they are one of only a few species of animals to produce enzymes that digest cellulose.)

There’s more than one way to skin a plant. Some other animals, lacking the large bacteria-filled stomach of ruminants, also employ cellulose-digesting bacteria to enable them to extract energy from cellulose. In these animals, including horses, rodents, rabbits, and koalas, the bacteria live within an outcropping of the digestive tract that receives material from the small intestine, called the cecum. Usually considered the beginning of the large intestine, this is a separate chamber where the food goes for a while and the cellulose gets digested, before the food continues down the intestine. In animals that don’t break down cellulose, the cecum is much smaller. The cecum is almost nonexistent in the meat-eating coyote, for example, while the similarly sized koala has a cecum that is 2 meters long. Within this lengthy chamber, bacteria convert shredded eucalyptus leaves into usable food (FIGURE 22-25). In humans, no cellulose-digesting bacteria live in the cecum. (Connected to the human cecum, and developing from it, is a small pouch-like structure called the appendix.)

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Figure 22.25: Koalas can eat foods that are indigestible for humans. The cecum is the site of cellulose digestion in some animals.

Rabbits and rodents can’t increase their cellulose-digesting efficiency by regurgitating their food and chewing the cud, because the cellulose-digesting bacteria reside in the small intestine, not in the stomach. But they’ve mastered another way to increase their ability to extract energy from their food: they pass it through their entire digestive system twice. Eating some of their feces—a process called coprophagy—allows them to significantly increase their nutritional intake from their cellulose-laden diet. Numerous other species—including pigs, elephants, cats, gorillas, and chimps—also resort to coprophagy from time to time, although the reasons are not always clear and may be related to vitamin and mineral absorption, or the acquisition of bacteria to aid in digestion (FIGURE 22-26).

Figure 22.26: Coprophagy can have nutritional benefits.

TAKE-HOME MESSAGE 22.13

Most animals don’t produce enzymes that break down cellulose. In ruminant animals, complex four-part stomachs have evolved with which the animals can digest plant matter that humans cannot, in part due to the presence of symbiotic cellulose-digesting bacteria. Other animals have alternative methods of utilizing cellulose-digesting bacteria.

Cows swallow their food twice, facilitating the breakdown of cellulose. Explain.

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