 Dear students, in this topic we shall discuss the behavioral rhythms of animals. Animals respond rhythmically to cyclic fluctuations in their environment. The major cyclic fluctuations of environment are the changes in seasons and daily light and dark periods. The rhythmic behaviors are controlled in animals by biological clocks. The biological clocks regulate the daily and seasonal cyclic activities of animals. These clocks have a molecular genetic basis and they are inherent and are fitted in the nervous system of the animal. Genes for time sense have also been identified in humans and many other animals. The biological clocks are of three types, circadian clocks, circannual clocks and lunar clocks. These clocks are influenced by the dark and light periods and lunar cycles. Dear students, we shall discuss all these clocks one by one. First, we shall discuss in detail the circadian clock and the circadian rhythms which are generated due to circadian clock. Circadian clock is the most important biological clock in all animals. It regulates and coordinates the daily behavioral activities. Its output is a circadian rhythm. The circadian rhythm is exhibited as a daily cycle of rest and activity. The most common example is the daily rest and activity of animals which are cyclic. The circadian clock is synchronized with light and dark cycles of the environment. However, it can also maintain rhythmic activity under constant environmental conditions. The effect of light and dark periods is that the animal is on the surface and is taking active life. That effect is on the daily basis of light and dark periods. However, some animals which have been hibernated, for which the light and dark period of daily life is not present, the circadian clock is active in them. The circadian clock and circadian rhythms are linked to yearly cycle of seasons. This clock spans events which are repeated on a 12 month basis and correspond to the tilt of the earth's spin axis that results in seasonal changes. The annual hibernation behavior is present in the examples of circanval behaviors. Migration behavior is present on the annual basis of a season in a region after migrating another season. Similarly, the reproductive activities of many animals are also seasonal. The third type of biological clock involves lunar cycles and tidal rhythms which are controlled by the phases of the moon. Because the gravitational pull of the moon affects the tidal rhythms in marine environments. When the full moon is present, there are high tides. When the low moon is present or the moon is not present, then that is the time of low tides. It means that the phases of the moon that are present change the tidal movements accordingly. So the lunar cycles and tidal rhythms affect the reproductive behaviors of many marine animals. These rhythms are linked to the new or full moon. This behavior links reproduction to the times of greatest tidal movements. The effect of these greatest tidal movements is that they disperse the larvae to deeper waters. Deeper waters are relatively safer environment to complete early development of the embryos.