What Animals Do to Survive

What essential functions must animals perform to survive?

Animals display a bewildering variety of body shapes, sizes, and colors. The best way to study and understand this diversity is not to memorize all the body parts of these animals, but to understand how the structures function and why. No matter their appearance, all animals must perform similar functions to stay alive. Like all organisms, animals must maintain homeostasis by gathering and responding to information, obtaining and distributing oxygen and nutrients, and collecting and eliminating carbon dioxide and other wastes. They also reproduce. The body systems that perform these functions are closely linked to one another. Over time, members of different animal phyla have evolved very different body structures that perform these essential functions. You will study these structures in more detail in Chapters 27 and 28.

Maintaining Homeostasis Recall that all organisms must keep their internal environment relatively stable, a process known as maintaining homeostasis. In animals, maintaining homeostasis is the most important function of all body systems. For example, most reptiles, birds, and mammals cannot excrete excess salt very well. Those that hunt or feed in salt water, such as the marine iguana in Figure 25–3, have adaptations that allow them to remove salt from their bodies.

Often, homeostasis is maintained by feedback inhibition. Feedback inhibition, or negative feedback, is a system in which the product or result of a process limits the process itself. If your house gets too cold, for example, the thermostat turns on the heat. As heat warms the house, the thermostat turns the heater off. Your body's thermostat works the same way. If you get too cold, you shiver, using muscle activity to generate heat. If you get too hot, you sweat, which helps you lose heat.

In this unit, you will learn about body systems in various animal groups. You will see how different groups have evolved different ways of ensuring their body systems stay in balance.

FIGURE 25–3 Homeostasis Marine iguanas are reptiles that feed in salt water. Reptile excretory systems are not adapted to process salt water. So these reptiles maintain homeostasis by sneezing a combination of salt and nasal mucus you might call “snalt.” Snalt sometimes coats their bumpy heads and spiny necks, as you can see in this photo.

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