THE LIVING WORLD
Unit Eight. The Living Environment
This dog’s bow is its way of saying to another dog, “Let’s play.” Its rear shoved high in the air, its forelegs flat on the ground, the pup looks up, hopeful that its partner will agree. Every dog invites play in exactly this way, a terrier like this, a retriever—even a wolf. The bow is an innate behavior that they all share, part of what it means to be a canine. In other ways, this dog may behave like no other dog. It may learn to “sit” or “roll over”—or to catch Frisbees, herd sheep, or bring in the morning paper. It can solve surprisingly complex problems. Wolves and other wild canines are social animals, living in packs that cooperate in hunting and raising their young. Of course, there are some things that a dog cannot learn, no matter how much effort is expended in trying. Although a dog can bark, howl, or whine, it will never be able to speak. Behavioral biologists investigate how animals behave, and why they behave in one way and not another. They have learned that there are surprisingly few differences other than language between humans and apes, for example. Even you and this puppy share more behavior than you might expect.
37.1. Approaches to the Study of Behavior
Animals respond to their environment in many ways. Beavers build dams in the fall that create lakes, and birds sing in the spring. Bees search for honey, and when they find it, they fly back to the hive and spread the good news. To understand these behaviors, we need to appreciate both the internal factors that shape the way an animal behaves, as well as the aspects of the external environment that trigger an individual behavior.
Behavior can be defined as the way an organism responds to stimuli in its environment. These stimuli might be as simple as the odor of food. In this sense, a bacterial cell “behaves” by moving toward higher concentrations of sugar. In animals, behaviors are far more complex than this simple bacterial one. This is particularly true for animals with nervous systems. Using eyes, ears, and a variety of other sense organs, they are able to perceive environmental stimuli, process the information, and dictate appropriate body responses, which can be both complex and subtle.
When we observe animal behavior, we can examine it in two different ways. First, we might ask how it all works. How do the animal’s senses, nerve networks, and internal state act together physiologically to produce the behavior? Like a mechanic studying the behavior of a car, we are asking how the machine works. A psychologist would say we would be asking a question of proximate causation. To analyze the proximate cause of behavior, we might measure hormone levels, or record the impulse activity of particular neurons in the brain. The field of psychology often focuses on proximate causes.
We might also ask why it all works the way it does. Why did the behavior evolve in this way? What is the adaptive value of this particular response to the environment? This is a question of ultimate causation. To study the ultimate cause of a behavior, we would attempt to find how it influenced the animal’s survival or reproductive success. The field of animal behavior typically focuses on ultimate causes.
Any behavior can be looked at either way. For example, a male songbird may sing during the breeding season (figure 37.1). Why? One explanation is that longer spring days have induced in his body elevated levels of the steroid sex hormone testosterone, which at these high levels bind to hormone receptors in the songbird’s brain and trigger the production of song. Elevated testosterone would be the proximate cause of the male songbird’s song.
Figure 37.1. Two ways to look at a behavior.
This male songbird can be said to be singing because his levels of testosterone are elevated, triggering innate "song" programs in his brain. Viewed in a different light, he is singing to defend his territory and attract a mate, behaviors that have evolved to increase his reproductive fitness.
Another explanation is that the male is exhibiting a pattern of behavior produced by natural selection to better adapt it to its environment. Seen in this light, the male songbird sings to defend a territory from other males, and to attract a female to reproduce. These reproductive motives are the ultimate, or evolutionary, explanation of the male songbird’s behavior.
A Controversial Field of Biology
The study of behavior has had a long history of controversy. One source of controversy has been the question of whether an animal’s behavior is determined more by an individual’s genes, or by its learning and experience. In other words, is behavior the result of nature (instinct) or nurture (learning)? In the past, this question has been considered an “either-or” proposition, but we now know that instinct and learning both play significant roles, often interacting in complex ways to produce the final behavior. We will begin our study of animal behavior by examining more closely the scientific study of instinct and learning, and the ways in which they interact to determine behavior, both proximately and ultimately.
Key Learning Outcome 37.1. Animal behavior is the way an animal responds to stimuli in its environment. Some biologists study the physiological mechanisms producing the behavior, others the evolutionary forces responsible for its development.