Ethology is the study of animal behavior from a biological viewpoint. There are two main approaches to the study of animal behavior, the physiological and the psychological. The former is concerned with mechanisms and the working of the nervous system, and the latter is more concerned with the behavior itself. They are however complementary. 'Behavior' includes all the processes by which an animal senses the environment around it and the internal state of its body, and responds to changes which it perceives. In general, behavior in animals is adaptive. They respond to stimuli in order to feed themselves, find shelter, mate and rear young. There are two basic ways in which animals adapt to their environment: by instinctive responses, which are patterns of behavior built into the animal before birth, and by learning new responses.
The psychological study of animal behavior has mostly been carried out by American experimental psychologists, who are almost exclusively interested in learning and work mostly by performing experiments with animals in laboratories. Another approach, more common in Europe, began in the 1930s with the work of Konrad Lorenz and was later taken up by Niko Tinbergen. They carried out their observations from hides under near natural conditions and much of their work centered around reproductive behavior and courtship or defensive displays.
Numerous other branches of zoology merge into the study of behavior, among them ecology, endocrinology and neurophysiology. Neurophysiology, the study of how the nervous system works, is playing a larger and larger part, as ethologists try to determine how the brain functions in various animals. The techniques of ethology are also used by psychologists to study human behavior.