Feeding And Digestion

For most mammals, the breakdown of food begins with chewing. Other vertebrates simply swallow their food whole or in large pieces. Chewing speeds up digestion by breaking food into small pieces that provide a large surface area for attack by enzymes. Variations in the size and shape of teeth among different mammalian species reflect differences in diet. Chisellike incisors cut. Pointed canines grip, puncture, and tear. Premolars shear, shred, cut, or grind. Molars grind, crush, or cut. For example, predatory carnivores, such as the bear in Figure 43-7, have a set of teeth specialized for gripping, holding, tearing, and crushing food. Mammalian carnivores are recognizable by their sharp incisors and long canines.

Baleen whales, such as the blue whale, lack teeth. Instead, they have baleen, thin plates of keratin that hang from the skin of the upper jaw like a curtain. As a baleen whale swims, it gulps water, then closes its mouth, and pushes the water out through the baleen. Shrimp and other invertebrates get trapped behind the baleen and then are swallowed.

figure 43-7

Carnivores, such as this bear, have large, sharp incisor and canine teeth that can cut and tear flesh. Bears and many other carnivores also have strong, crushing molars.

Carnivores, such as this bear, have large, sharp incisor and canine teeth that can cut and tear flesh. Bears and many other carnivores also have strong, crushing molars.

figure 43-8

Many herbivores, such as this zebra, have flat teeth that can efficiently grind grasses, grains, or leaves. Herbivores also have digestive systems that harbor symbiotic microbes that help digest plant materials.

figure 43-8

Many herbivores, such as this zebra, have flat teeth that can efficiently grind grasses, grains, or leaves. Herbivores also have digestive systems that harbor symbiotic microbes that help digest plant materials.

Special Adaptations for Digesting Plants

Meat is simple to digest, so most carnivores have short, simple digestive systems. Plants, however, can be difficult to digest because plants contain cellulose, a polymer of the sugar glucose. Animals do not produce enzymes that can break down cellulose. However, the long digestive tracts of herbivorous mammals, such as the zebra shown in Figure 43-8, contain microorganisms that can break down cellulose.

In some herbivorous mammals, the structure that is called a stomach is actually made up of four chambers. One of these chambers is the true stomach. Another chamber, known as the rumen (ROO-muhn), contains symbiotic microorganisms. Plant material that has been chewed and swallowed enters the rumen, where microorganisms begin to break the cellulose into smaller molecules that can be absorbed into the animal's bloodstream. The material is partly digested in the rumen, then regurgitated, chewed again, and swallowed again. The animal may regurgitate and swallow the same food several times. Mammals that have a rumen are called ruminants and include cows, sheep, goats, giraffes, and deer.

In horses, zebras, rodents, rabbits, and elephants, microorganisms that live in the cecum (SEE-kuhm) complete digestion of the food. The cecum is a large sac that branches from the small intestine and acts as a fermentation chamber. Food passes through the stomach and small intestine before entering the cecum. Mammals with a cecum do not chew cud.

Word Roots and Origins cecum from the Latin intestinum caecum, meaning "blind intestine"

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