Biology For Dummies
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Different groups of animals eat different foods, but what do animals physically do with the food they consume? The different methods of breaking down food all make up mechanical digestion, which occurs from the time the animal consumes the food until it enters the stomach. This is opposed to chemical digestion, which is what happens once chewed or ground-up food is flooded with enzymes and acids to break it down further.

Ruminating over ruminants

Ruminants are mammals that can break down cellulose. Humans have one stomach that fills with hydrochloric acid and enzymes to help break down food. Ruminants, such as cows, have a stomach with several compartments.

Cows, like all herbivores, eat grasses and other plant material. Plants contain cellulose, which is very hard to digest (even for an herbivore). So, when a cow swallows some grass, the chewed grass first enters the compartment of the stomach called the rumen. The rumen contains a salty solution and bacteria that helps to break down the cellulose.

Cows then regurgitate (spit up) the material from the rumen, called cud, back into their mouths. They “chew their cud” to help break down the cellulose even further. The cud is swallowed again, and it re-enters the rumen. This cycle repeats as necessary until the material is broken down far enough to be churned up and passed into the true stomach. From there, digestion continues through the intestines and excretory system of the cow.

The daily grind of a gizzard

Gizzards are a sac-like structure in animals that help to break down food. Many animals have teeth to help tear apart or chew food. But some animals don’t chew. Do chickens have lips? No. Do ducks have teeth? No. Birds generally swallow things whole, including stones. Yes, stones. The stones that are swallowed end up in the gizzard to help grind the other food that the bird eats. The walls of the gizzard are extremely muscular, and when the walls of the gizzard rub back and forth, the contents of the gizzard get ground up. Earthworms also have gizzards.

The truth about teeth

Not all teeth are for chewing, but all teeth are for maceration. Maceration is the action of physically breaking down food into pieces. Chewing is a grinding action that only herbivores and omnivores do.

A lion’s huge dagger-like teeth are intended for killing an animal and tearing its flesh. The lion then swallows the chunks of meat whole. No chewing involved. Because lions often don’t chew, they do not have many grinding teeth (like molars). Herbivores, on the other hand, have many grinding teeth, which are flat, as well as incisors (think scissors), which can clip grasses and plants. Humans and other omnivores have a combination of these kinds of teeth: canine teeth for tearing food, incisors for biting off pieces, and molars for grinding up food.

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