key concept 30.3 Animals Use Diverse Forms of Movement to Feed

As we noted in Key Concept 30.1, animals are heterotrophs, or “other-feeders.” Although some animals rely on photosynthetic endosymbionts to nourish them (see Investigating Life: Can Corals Reacquire Dinoflagellate Endosymbionts Lost to Bleaching? in Chapter 26), most animals must expend energy to obtain an outside source of nutrition, otherwise known as food.

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  • Plant eaters are called herbivores.

  • Predators possess features that enable them to capture and subdue their prey.

  • Parasites obtain nutrients by living on or within a host organism.

The need to locate food has favored the evolution of sensory structures that provide animals with detailed information about their environment as well as nervous systems that can receive, process, and coordinate that information. Furthermore, in order to acquire food, animals must either move through the environment to where food is located, or move the environment and the food it contains to them. Animals that move from one place to another are motile; animals that stay in one place are sessile.

The principal feeding strategies that animals use fall into five broad categories:

  1. Filter feeders (or suspension feeders) strain small organisms from their environment.

  2. Herbivores eat plants or parts of plants.

  3. Predators capture and eat other animals.

  4. Parasites live in or on other, generally much larger, organisms, from which they obtain energy and nutrients.

  5. Detritivores feed on dead organic material.

Each of these strategies can be found in many different animal groups, and none of them is limited to a single group. Individuals of some species employ more than one of these feeding strategies, and some animals employ different feeding strategies at different stages of their life cycle. The constant and ongoing need to obtain food, the variety of nutrient sources available in a given environment, and the necessity of competing with other animals to obtain food means that a variety of feeding strategies can be found among all the major animal groups.