If you were a cow or a hummingbird, your diet wouldn't be a matter of choice. You’d munch on grass, or sip nectar from a flower. Your food sources would be clear cut, and your digestive system would be adapted to process those foods.

Our own species used to eat a narrow range of foods as well. Our ancestors used to hunt for meat and forage for plants to eat. But in today’s world of snacks and fast foods, our 21st Century diet often results from watching TV ads and having too many quick meals at restaurants.

In this activity, we’ll look at the different feeding mechanisms animals use, and the foods they need to grow and nourish their bodies.

A sunflower, a butterfly, and a human all have to absorb nutrients in order to stay alive. Although plants and some bacteria can make organic molecules from inorganic substances like carbon dioxide, animals must obtain organic substances from other sources. Organisms, such as sunflowers, that can build new molecules from inorganic compounds are called autotrophs.

Organisms that cannot manufacture organic molecules from inorganic substances are called heterotrophs. Heterotrophs, which include all animals, need organic compounds, such as sugars, to build new molecules for growth.

Most animals are categorized as meat eaters, or carnivores; plant eaters, or herbivores; or a mixture of both, referred to as omnivores. Organisms that feed on dead or decaying matter are called detritivores. Carnivores include cats and birds of prey. Herbivores include rabbits and deer. Omnivores include crows and bears. Detritivores include earthworms and pill bugs.

Animals rely on other organisms to obtain their food. Herbivores and omnivores eat plants, and carnivores and omnivores eat the plant eaters.

When organisms die, detritivores, such as fungi, bacteria, and some animals, feed on the decomposing organic material.

This entire network of feeding relationships is called a food web.

Detritivores break down most of the decomposing matter into inorganic compounds. These compounds, along with the remaining plant and animal matter, eventually make their way into the soil as nutrients or into the atmosphere as carbon dioxide. These materials provide sustenance to plants, and the cycle starts again.

Animals need fuel, or chemical energy, for cellular respiration. They also need raw organic materials for biosynthesis and essential nutrients—substances the body can’t manufacture for itself.

The main groups of organic molecules are fats, carbohydrates, and proteins. Because animals are heterotrophs, they have to obtain these substances from other organisms.

We’ve already seen the different groups of heterotrophs, carnivores, herbivores, omnivores, and detritivores.

In this activity, we’ll also look at the various mechanisms animals use to obtain food.

We’ll then see how food provides energy for animals, and how this energy is measured and stored in the body.

Next, we’ll look at how food is used to make body components.

Finally, we’ll examine which compounds are essential in the diet and cannot be synthesized from other nutrients.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education