Animal behavior includes the actions and reactions of animals to external stimuli. The study of animal behavior involves two main approaches: answering questions about how an animal does something (proximate questions) and why an animal does something (ultimate questions). Though humans have always observed animals behave, animal behavior did not become a field of study until the 1930s, when it was called ethology.

Behavior is determined by both genetics and environmental factors, and is controlled by neural mechanisms. Thus, all animals with nervous systems are capable of behavior, including extremely simple ones such as the flatworm, Caenorhabditis elegans, which responds to light. The study of animal behavior is expanding rapidly and includes taxa and subjects too numerous to list here. Major divisions of the field include learning, cognition, and social behavior.

Founders of animal behavior studies include scientists Karl von Frisch, Konrad Lorenz, and Niko Tinbergen, whose work...