Researchers studying animal behavior can seek to answer questions using a variety of approaches focusing on different aspects of the behavior under study.
You might ask, for example, why a robin hopping around your yard periodically tilts its head to the side. It turns out that robins do this because their sensory and nervous systems can detect the faint sounds of worms moving through the soil.
Thus, one explanation for the robin's behavior might focus on how the required sensory equipment works. Furthermore, hunting by listening might enable a robin to detect otherwise hard-to-find prey. Hence, a second explanation of the robin's head-tilting behavior might focus on whether listening for worms increases the efficiency of foraging, thus enhancing the bird's survival and reproductive success. If so, then this behavior may have become common over time because it was favored by natural selection.Notice that the first explanation we mentioned addresses a “how” question about behavior: it looks within an individual bird to explain how the head-tilting behavior functions. By focusing on events that take place during an animal's lifetime, this approach seeks to explain behaviors in terms of their immediate or proximate causes. In contrast, the second explanation addresses a “why” question about behavior: it examines the evolutionary and historical reasons for a particular behavior. By addressing previous events that influenced the features of an animal as we know it today, this approach seeks to explain behaviors in terms of their evolutionary or ultimate causes.
Although behavioral ecologists examine both proximate and ultimate causes in their research, they are primarily concerned with ultimate explanations of animal behaviors. We will follow their lead in this chapter, focusing on selected ultimate explanations for why animals behave as they do. We'll begin by examining how natural selection affects behavior.