22.2: What’s on the menu? Animals have a variety of diets.

All organisms need food—but there are some very different ways to get it. Plants and other photosynthetic organisms (including many bacteria and protists) make their own food, through photosynthesis. But animals do not have the cellular machinery to do this, and so they must eat other organisms. Most animals eat plants (that make their own chemical energy), while some animals eat other animals (that eat plants). Some animals even eat animals that eat animals that eat animals (that eat plants)—but that’s pretty rare (see Section 15-7). In every case, though, while some organisms are able to capture solar energy to make food, all animals must acquire that energy indirectly by consuming other organisms.

In the animal world, species fall into three groups based on their diets (FIGURE 22-2).

Carnivores Predatory animals that consume only other animals are carnivores. They include spiders and snakes; mammalian species such as wolves, seals, bats, and cats; and hawks, owls, and other birds of prey. Some carnivores don’t actually kill their prey but instead just suck nutrient-rich fluids from them. Mosquitoes and ticks are carnivorous fluid-feeders.

Herbivores Food: easy to get, but hard to digest. That sums up life as a herbivore, an organism that consumes only plants. Because plants are plentiful in most habitats and can’t run away, they are easy targets for predation. To protect themselves, however, plants tend to carry many toxic compounds that are difficult or impossible for animals to break down chemically. Nonetheless, many herbivores have developed digestive adaptations to overcome these difficulties. We’ll explore a couple of them in detail in Section 22-13. Examples of herbivores include beavers, tortoises, and caterpillars, as well as many species of seed-eating birds and nut-eating squirrels and a variety of large grazing mammals such as cows, horses, and deer. And just as there are fluid-feeding carnivores, there are fluid-feeding herbivores. Aphids, for example, pierce the surface of plants and suck their sugary sap.

Omnivores Most humans, as omnivores, eat plants and animals and can digest both efficiently. We share this diet with numerous other species, including cockroaches. Omnivores come in all sizes, from bears to raccoons, from chickens to flies and wasps.