Circadian rhythms
Of great interest are biological rhythms with a period of about a day. They are called so — circadian, circadian or circadian — from lat. circa — about and dies — day.
Biological processes with circadian periodicity are very diverse. For example, three types of luminous mushrooms increase and decrease their glow every 24 hours, even if artificially kept in constant light or in complete darkness. The glow of the single-celled seaweed Gonyaulax changes daily. In higher plants, various metabolic processes occur in the circadian rhythm, in particular photosynthesis and respiration. In lemon cuttings, the intensity of transpiration fluctuates with a 24-hour frequency. Particularly illustrative examples are the daily movements of leaves and the opening and closing of flowers.
A variety of circadian rhythms are also known in animals. An example is the coelenterates, close to sea anemones, the sea pen (Cavernularia obesa), which is a colony of many tiny polyps. The sea pen lives in sandy shallow waters, sucking into the sand during the day and turning around at night to feed on phytoplankton. This rhythm is maintained in the laboratory under constant lighting conditions.
Insects have a well-functioning biological clock. For example, bees know when certain flowers open and visit them every day at the same time. The bees also quickly learn at what time sugar syrup is presented to them in the apiary.
In humans, not only sleep, but also many other functions are subject to the daily rhythm. Examples of this are the increase and decrease in blood pressure and the excretion of potassium and sodium by the kidneys, fluctuations in reflex time, sweating of the palms, etc. Changes in body temperature are especially noticeable: at night it is about 1 ° C lower than during the day. Biological rhythms in humans are formed gradually in the course of individual development. In a newborn, they are rather unstable — periods of sleep, nutrition, etc. alternate randomly. Regular alternation of periods of sleep and wakefulness on the basis of a 24-25 hour cycle begins to occur only from 15 weeks of age.
Correlation and «tuning»
Although biological rhythms are endogenous, they correspond to changes in external conditions, in particular the change of day and night. This correlation is due to the so-called. «capture». For example, circadian movements of leaves in plants persist in complete darkness for only a few days, although other cyclic processes can continue to repeat hundreds of times despite the constancy of external conditions. When the bean leaves, kept in the dark, finally stopped spreading and falling, a short flash of light is enough to restore this rhythm and last for several more days. In the circadian rhythms of animals and plants, the time-setting stimulus is usually a change in illumination — at dawn and in the evening. If such a signal is repeated periodically and with a frequency close to that characteristic of a given endogenous rhythm, the internal processes of the organism are precisely synchronized with external conditions. The biological clock is «captured» by the surrounding periodicity.
By changing the external rhythm in phase, for example, turning on the light at night and maintaining darkness during the day, it is possible to “translate” the biological clock in the same way as usual, although such a restructuring requires some time. When a person moves to another time zone, his sleep-wake rhythm changes at a rate of two to three hours a day, i.e. to a difference of 6 hours, he adapts only after two or three days.
Within certain limits, it is possible to reconfigure the biological clock to a cycle that differs from 24 hours, that is, to make them go at a different speed. For example, in people who lived for a long time in caves with an artificial alternation of light and dark periods, the sum of which differed significantly from 24 hours, the rhythm of sleep and other circadian functions adjusted to the new duration of the “day”, which ranged from 22 to 27 hours, but more strongly change it was no longer possible. The same applies to other higher organisms, although many plants can adapt to «days» that are a whole fraction of the usual ones, such as 12 or 8 hours.
Tidal and lunar rhythms
Coastal marine animals often exhibit tidal rhythms, i.e. periodic changes in activity synchronized with the rise and fall of the water. The tides are driven by lunar gravity, and in most regions of the planet there are two high and two low tides during a lunar day (the period of time between two successive moonrises.) Since the Moon moves around the Earth in the same direction as our planet around its own axis, the lunar day about 50 minutes longer than the sun, i.e., the tides come every 12,4 hours. Tidal rhythms have the same period. For example, the hermit crab hides from the light at low tide and emerges from the shadows at high tide; with the onset of high tide, oysters open their shells, unfold the tentacles of sea anemones, and so on. Many animals, including some fish, use more oxygen at high tide. The color changes of fiddler crabs are synchronized with the rise and fall of the water.
Many tidal rhythms persist, sometimes for weeks at a time, even when animals are kept in an aquarium. This means that in essence they are endogenous, although in nature they are “captured” and reinforced by changes in the external environment.
In some marine animals, reproduction correlates with the phases of the moon and usually occurs once (rarely twice) during the lunar month. The benefit of such periodicity for the species is obvious: if eggs and sperm are thrown into the water by all individuals at the same time, the chances of fertilization are quite high. This rhythm is endogenous and is believed to be set by the «intersection» of the 24-hour circadian rhythm with the tidal one, the period of which is 12,4 or 24,8 hours. Such a «crossing» (coincidence) occurs at intervals of 14-15 × 29-30 days, which corresponds to the lunar cycle.
The best known, and perhaps the most visible of the tidal and lunar rhythms, is that of the grunion, the marine fish that spawns on the beaches of California. During each lunar month, two especially high — syzygy — tides are observed, when the Moon is on the same axis with the Earth and the Sun (between them or on the opposite side from the luminary). During such a high tide, the grunion spawns, burying their eggs in the sand at the very edge of the water. Within two weeks, they develop almost on land, where marine predators cannot reach. On the next spring tide, when the water covers the sand literally stuffed with them, fry hatch from all the eggs in a few seconds, immediately floating into the sea. Obviously, such a breeding strategy is possible only if adult grunions sense the time of spring tides.
The menstrual cycle in women lasts four weeks, although it is not necessarily synchronized with the phases of the moon. Nevertheless, as experiments show, in this case, too, we can talk about the lunar rhythm. The timing of menstruation is easy to shift, using, for example, a special program of artificial lighting; however, they will occur with a frequency very close to 29,5 days, i.e. to the lunar month.
low frequency rhythms
Biological rhythms with periods much longer than one month are difficult to explain on the basis of biochemical fluctuations, which probably cause circadian rhythms, and their mechanism is still unknown. Among these rhythms, annual ones are the most obvious. If trees of the temperate zone are transplanted into the tropics, they will retain the cycle of flowering, leaf shedding, and dormancy for some time. Sooner or later, this rhythm will break down, the duration of the phases of the cycle will become more and more indefinite, and eventually the synchronization of biological cycles will disappear not only in different specimens of the same species, but even in different branches of the same tree.
In tropical regions, where environmental conditions are almost constant throughout the year, native plants and animals often exhibit long-term biological rhythms with a period other than 12 months. For example, flowering may occur every 8 or 18 months. Apparently, the annual rhythm is an adaptation to the conditions of the temperate zone.