Animal behaviors often involve the relationship between an individual and its environment. Such behaviors, ranging from movement to synchronization with daily, monthly, and yearly environmental cycles, require that an animal be able to observe aspects of its environment and respond appropriately to them.
Movement, the simplest example of an animal's interaction with its environment, falls into two general categories, kinesis and taxis. Kinesis refers to random, undirected movement, while taxis refers to movement in relation to a stimulus. Taxis is stimulus dependent: different stimuli produce different types of taxis. Some common types of taxis include phototaxis (a reaction to light), chemotaxis (reaction to chemicals), and magnetotaxis (reaction to magnetism). Movement toward a stimulus is called positive taxis and movement away from a stimulus is called negative taxis.
Circadian rhythms and other cyclical behaviors are more complex examples of behavior tuned to the environment. In general, cyclical behaviors are regulated by endogenous time-keeping mechanisms, called biological clocks, which are in turn regulated by external environmental stimuli that act to set the internal clock. Such stimuli are known as Zeitgebers and can include patterns of light and darkness and temperature variations.