Animals change rhythmically with the periodic changes of the environment, which is called the rhythmic behavior of animals. The most common is the circadian rhythm.

The activities and physiological functions of animals are related to the daytime of the earth. For example, all kinds of bees, songbirds and most mammals are diurnal, while rats, bats and owls are nocturnal. Nighthawks and other animals are mostly active in the morning or evening, which is called morning-fainting animals. These phenomena occur in a 24-hour cycle.

The activities of marine animals are related to the ebb and flow of tides, and are closely related to the behaviors of animals such as foraging and reproduction. Animals such as oysters feed at high tide, while crabs feed in large quantities after low tide. Obviously, the activity law of animals living in intertidal zone is mostly the law of adapting to the ebb tide, which is called monthly rhythm.

Animals with seasonal rhythm, such as birds and mammals in temperate regions, breed in spring, mammals molt in spring and autumn, tropical animals sleep in summer, animals hibernate in cold regions, bird migration. These rhythms are displayed one by one according to the internal procedures of animals. Many experts believe that the migration behavior of birds, in particular, really depends on a certain rhythm inside birds, and all internal rhythms must be consistent with the external natural rhythms, which is the result of long-term adaptation to nature.

With the periodic changes of the earth, the sun and the moon, the periodic and rhythmic behavior gradually formed is the rhythmic behavior. Such as circadian rhythm, monthly rhythm, seasonal rhythm, etc.

Some creatures have a periodic rhythm every 24 hours, and this activity and physiological function are related to the day and night of the earth, which is called circadian rhythm, such as diurnal (bees, butterflies, songbirds), nocturnal (mice, bats, owls) and morning fainting (nighthawks).

The monthly rhythm means that the activities of marine animals are related to tides and the ebb and flow of tides. Tide is a periodic gravity fluctuation phenomenon with the displacement of the sun and the moon. Most of the activities of animals in intertidal zone adapt to the tidal law. For example, barnacles and oysters feed underwater at high tide, and crabs climb out of caves to feed on the beach at low tide.

Some creatures have periodic behavior with the change of seasons, which is called seasonal rhythm. For example, many birds move to warm areas in the south for winter before winter comes, amphibians and reptiles hibernate in winter, while fish migrate.