A vast array of periodicities in functions or activities are exhibited by virtually every organism, from single-celled plants and animals to such complex creatures as human beings.8 These rhythms are intrinsic to the organism and enable it to measure the passage of time. For many organisms, the most important interval measured by their internal clocks is the 24-hour cycle of light and dark. Rhythms that coincide with this cycle are called “circadian,” from the Latin circa (about) and dies (day).9 (p. 2, 10)
Other important biological periodicities include’ ‘circatidal” rhythms, matching the period of daily high and low tides, and “circasyzygic” rhythms, which match the cycle of unusually high and low tides occurring each fortnight, when the sun and moon are in the proper alignment. ‘‘Circahmar” rhythms are synchronized with the monthly waxing and waning of the moon. ‘‘Circannual” periodicities are cycles of about a year.9 (p. 9-10) It should be noted, however, that circannual rhythms are not the end of the story; for example, the 7-year and 17-year locusts that emerge from their long puprd incubation in the ground are cycles that span longer periods.
An innate ability to measure the passage of time has adaptive significance. A sense of time helps birds to accurately use the sun, moon, and stars as navigational aids during migration. Internal clocks also enable orgrmisms to synchronize their breeding behaviors with one another as well as with the most favorable environmental conditions for raising young. In fact, biologicrd rhythms help organisms match a number of activities to the times when those activities can be carried out most effectively. For example, as noted by zoologist David S. Saunders, University of Ediiburgh, ScotJand, circadian rhythms allow animals of different spexies tosharethe same foodsources without direct competition because some animals are active ordy during hours of darkness (i.e., they are nocturnaf) while others are active only during the day (diwnuf). The advantage to having a built-in method of responding to light and darkness, rather than relying on actual changes in light as a cue, is that, in effect, the organism is prevented from “sleeping late” and missing the optimal time of day for foraging.9 (p. 12)
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