The study of animal behavior begins with understanding how an animal's physiology and anatomy are integrated with its behavior. Both external and internal stimuli prompt behaviors - external information (e.g., threats from other animals, sounds, smells) or weather and internal information (e.g., hunger, fear). The study of behavior helps us to understand when and why animals, including humans, act. At one extreme, behavior is the link between the nervous system and the world outside an animal. At the other extreme, behavior is the way in which an animal establishes and maintains itself in its ecological niche. Behavior is a key link between an animal and its environment. Animals behavior includes communication, navigation, foraging, mating, parenting, and social cooperation. Each of these topics is a major subject within the study of behavior, and we will deal with each in a separate chapter. These issues all involve action and interaction; they are the externally expressed component of animal behavior. Behavior has internal underpinnings, as well. Motivation, learning, cognition and emotion shape an animal's behavioral decisions and therefore are critical in the study of behavior. Each of these topics has a neurobiological basis, so that their study links externally expressed behavior with the internal state of an animal. Scientists are drawn to the study of animal behavior for varied reasons and the field is extremely broad, ranging from research on feeding behavior and habitat selection to mating behavior and social organizations. Many scientists study animal behavior because it sheds light on human beings. Research on non-human primates, for instance, continues to offer valuable perspectives into the causes and evolution of individual, social, and reproductive human actions. Understanding why some animals help others at the potential cost of their own survival and reproduction, for example, not only gives us insight into their behavior but could also potentially help us to understand the underpinnings of our species' ideas of altruism and sacrifice.