|
Biological clocksMimi Lu
Have you ever experienced extreme exhaustion after traveling long distances? Or wondered how animals knew when it was time to hibernate? Have you ever seen plants capable of raising and lowering their leaves at certain times during the day? These phenomenons can be at least partially explained by internal timing mechanisms known as biological clocks. Biological clocks give organisms a rhythmic pattern to follow daily, monthly or even yearly. In 1729, a Frenchman named M. de Mairam found that those plants that are noted for raising and lowering their leaves rhythmically throughout the day maintain this rhythm even when kept in constant darkness. That is, they maintain this pattern even in the absence of environmental cues, such as light and temperature. Such a rhythm is termed endogenous. Rhythms that stop without environmental "hints" are termed exogenous. The actual environmental cue that is capable of setting the phase of a biological rhythm is a phasing factor or Zeitgeber (German for "time-giver"). Often, in the absence of environmental cues, an endogenous rhythm continues in periods close to, but not exactly, 24 hours. Such rhythms are circadian rhythms, meaning the rhythm continues without exogenous cues, but is only "about" a day in length. In the normal situation, circadian rhythms are corrected by a Zeitgeber (usually daylight) resulting in rhythms with a period of exactly 24 hours. Why are these internal clocks important? Why cannot an animal depend only on environmental cues? One advantage is that biological clocks allow animals to anticipate and prepare for upcoming events. Imagine an animal that is active at night. If it is solely dependent on external cues, it does not know how much time is left until dawn. Clocks also permit a relatively accurate timing of the periods during the day, even when environmental cues are vague, difficult to use or less reliable (like temperature, which can fluctuate). Circadian clocks reacting to changes in day length may be used by hibernating animals to determine the time of year. A hibernating animal must have an internal clock reminding her that winter is approaching, because she might not store enough fat in time for the winter if simply waiting for the weather to get cold. Also, animals depending on daily available light might utilize internal clocks that detect changes in the length of daylight as a sign for reproduction, migration or molting. Some examples of other processes showing circadian rhythmicity include locomotor activity in many vertebrates and insects, variation of body temperature in birds and mammals, and color changes in fish and crabs. What about jet lag? Jet lag results when a person suddenly experiences a shift of many hours in the phase of the environmental cycle, after traveling long distances over a relatively short period of time. The person suffers from a disturbance of the normal relation between her internal biological clocks and the external environmental cycle. Some of the internal rhythms lag behind others in attempting to regain synchrony with the cycle of the outside world. Thus, the person experiences jet lag as her body tries to compensate for the differences between all her internal rhythms and the external cues. Thus, biological clocks serve as automatic internal "watches" for organisms, even in the absence of obvious environmental cues.
Reference R.W. Hill and G.A. Wyse: Animal Physiology (2nd ed, HarperCollins, 1989).
|
