A biological clock controls a biological rhythm; it involves an internal pacemaker with external (usually environmental) cues. An environmental signal that cues the clock for animals is called a zeitgeber, a German term meaning “time-giver.” Examples of zeitgebers include light and dark cycles, high and low tides, temperature, and food availability. A biological rhythm is a biological event or function that is repeated over time in the same order and with a specific interval. Biological rhythms are evident when an animal’s behavior can be directly correlated to certain environmental features that occur at a distinct frequency. Biological clocks control animal behaviors such as when migration, mating, sleep, hibernation, and eating occur. Some examples of biological rhythms: A tidal rhythm would include the oysters (feeding) and fiddler crab (mating and egg laying)— 12.4 hours; circadian rhythm would include the fruit fly (an adult emerging from a pupa)—twenty-four hours; a circannual rhythm, such as a woodchuck coming out of hibernation or a robin migrating or mating—twelve months; or an intermittent rhythm, such as a lion that needs to feed because it’s hungry, or a river fish called a shiner that reproduces when the river floods—going from days to several years.