We begin our exploration of animal behavior by asking a simple question: Why does an animal exhibit a particular behavior? The Dutch behavioral biologist Niko Tinbergen divided this overarching question into four separate questions, each one focusing on a different aspect of the behavior. Let’s consider an example: Why does a bird sing? While apparently straightforward, this question can be broken down into more specific questions and then answered in a number of different ways:

Tinbergen’s analysis allows us to see that different answers to the question “Why does a bird sing?” can all be correct. The first two, causation and development, are mechanistic explanations for behavior; these are referred to as proximate explanations of behavior because they deal with how the behavior came to be. The second two, adaptive function and evolutionary history, provide evolutionary explanations of how natural selection has shaped a behavior over time; these are referred to as ultimate explanations of behavior because they deal with why the behavior came to be. Tinbergen’s four questions are complementary ways of looking at the same problem, and are a good starting point for the analysis of behavior.

The answers to Tinbergen’s questions rely on an interplay between genes and the environment. The influence of genes is especially clear in innate behaviors, those that are instinctive and carried out regardless of earlier experience. Male silkworm moths of the genus Bombyx exhibit an innate behavior when they fly upwind toward the source of a female-produced pheromone, an airborne chemical signal that communicates with members of the same species, in this case to attract the opposite sex (Fig. 45.1). Pheromones released from a female’s abdominal gland are sensed by small hairs on the male’s antennae. In the presence of pheromones, the antennal sensory hairs fire action potentials (Chapter 35). When about 200 hairs are being activated per second, a male flies upwind distances of a kilometer or more, tracking the increasing pheromone concentration until he finds the female. The male does not need to learn this behavior; he performs it spontaneously. His genes encode molecular receptors to which the pheromone binds, and this binding triggers a cascade of events that result in the moth’s heading up the pheromone’s concentration gradient.

A particular behavior may depend on an individual’s experience, in which case the behavior has been learned. As we will see, even neurologically simple organisms have a considerable capacity to learn. Fruit flies, for example, learn to avoid certain locations or substances if they associate them with an unpleasant experience.

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We can consider, then, the extent to which a particular behavior is genetically encoded (part of the animal’s nature) and the extent to which it is conditioned by the environment in which the animal develops (part of the animal’s nurture). However, as we discussed in Chapter 18, nature and nurture are inextricably linked. For example, most human behaviors are the product of an interaction between nature and nurture. Consider language in the context of Tinbergen’s second question concerning the development of behavior: We have an innate ability for acquiring language, but the specific language that we acquire depends entirely on our nurture. A baby in Italy learns Italian, and one in Finland learns Finnish.