Biological rhythms and behavior (zoology)

Circadian and other biological rhythms have been observed and described in so many processes and behaviors in so many diverse organisms that their presence in higher plants and animals is considered a basic characteristic of life. The term “circadian” (from the Latin circa, meaning “about,” and diem, “day”) was coined by Franz Halberg to describe these approximately twenty-four-hour rhythms, which, in time, were found to exist not only in plants but also in animals and humans. The circa prefix is also used with words denoting other time periods (such as “circannual”).

Circadian and Circannual Rhythms

Circadian rhythms enable animals to time precisely their daily activities. Animals are broadly classified as diurnal if they are active by day, nocturnal if they are active at night, and crepuscular if they are active at both dawn and dusk. Many species schedule their activity to start within minutes of the same time each day. Thus, the Swiss psychiatrist Auguste Henri Forel, in 1906, noticed that bees adapted to his schedule of eating breakfast on the terrace: The bees came each morning at breakfast time to feed on the jam. One of Karl von Frisch’s coworkers, Ingeborg Beling, found that she could train bees to visit a feeding station every twenty-four hours but that the bees could not be trained to come every nineteen hours. Individual species of flowers produce nectar only at certain times of the day, and bees have been observed to plan their visits according to the time of nectar flow.

The activity rhythms of caged flying squirrels have been studied in detail by Patricia DeCoursey. She found that the time of the onset of activity in this nocturnal rodent was very uniform from day to day but that the time gradually drifted during the year from about 4:30 PM in January to 7:30 PM in July and then back to 4:30 PM by the following January. Such a pattern is called a circannual rhythm. Circannual rhythms are particularly evident in migratory birds, which show seasonal changes in their physiology and behavior. Many mammals have distinct reproductive seasons. Mammals that hibernate, such as ground squirrels and woodchucks, gain fat in the fall, enter hibernation, and then wake up in the spring according to a circannual rhythm.

Although the annual changes in temperature might be expected to be the environmental factor that would signal a change in season to a plant or animal, it is now known that many plants and animals respond to changes in the length of the photoperiod. This response is called photoperiodism. Photoperiodism was found first in plants in 1920, in insects in 1923, in birds in 1926, and in mammals in 1932. In a typical experiment, light was artificially added to the short days of late fall to create a longer photoperiod—similar to that characteristic of spring. As a result, the organisms came into reproductive development months early.

Circadian rhythms play an essential role in photoperiodism. What became known as the Bünning Hypothesis or the External Coincidence model postulated that a circadian rhythm was involved in the organism’s mechanism, which measures the length of the photoperiod. It was hypothesized that the first twelve hours of the circadian rhythm were light-requiring, and the last twelve hours were dark-requiring. Short-day effects occurred when the light was limited to the light-requiring phase, but long-day effects occurred when light was present during the dark-requiring phase.

Light plays two roles in this scheme—a zeitgeber to synchronize rhythms and an inducer to stimulate reproductive responses. Later experiments demonstrated that short photoperiods followed by a brief flash of light in the middle of the dark were interpreted by the organism as long photoperiods, and the organisms became reproductively developed months early. Thus, the important thing is not how long the photoperiod is but rather when light is present with respect to a circadian rhythm of sensitivity to light.

Marine Rhythms

Some of the most dramatic examples of biological rhythms are found in marine organisms. The periods or lengths of the rhythms are rather diverse and include circadian, circatidal, circalunar, and circannual rhythms, as well as various combinations of them. Perhaps most famous is the rhythm of reproductive activity of the South Pacific marine worm called the palolo worm (Palola viridis). This species spawns at the last quarter of the moon in October and November (spring in the Southern Hemisphere). The worm lives buried in coral reefs, and at spawning, the last twenty-five to forty centimeters of the worm, which bears the gametes, breaks off and rises to the sea's surface. The gametes are released into the seawater, where fertilization takes place. Spawning always occurs at daybreak. The exact timing of spawning is an adaptation that increases the chances for successful reproduction in this species.

Similarly, the California grunion, a small smelt about fifteen centimeters long, spawns in the spring at about fifteen minutes after the time of the high tide each month. During the spawnings, or “grunion runs,” the fish ride the waves onto the sandy beaches, where the females burrow the posterior end of their bodies into the sand. The male curls around the female’s body and releases sperm as the female lays her eggs. The fish return to the sea and the eggs continue to develop until approximately fifteen days later, when the high tide returns and uncovers the hatching young. During the grunion runs, the adult fish are caught by fishermen (legally only by hand) and are eaten. Neither the palolo worm nor the grunion has been sufficiently studied to determine what environmental factor—moonlight, gravity, magnetism, or another factor—synchronizes their rhythms so precisely.

Another study found that snails living near the sea adjust their circadian rhythm to correspond with the tidal cycle, showing the dynamic nature of these rhythms and behaviors. Even the smallest and simplest worms and other marine creatures have individual rhythms. These rhythms adjust with any changes that occur in the sea or other bodies of water, making climate change a threat to the disruption of these animals' behavioral patterns.

Chronobiology

The broad field of the study of biological rhythms is called chronobiology. A rhythm is the cyclical repetition of a property or behavior, whether it concerns the level of body temperature, enzyme activity, or hormone level in the blood or describes an activity of the whole animal, such as feeding patterns, daily or seasonal migrations, or seasons of reproduction. The period of the rhythm is the time it takes to complete one full cycle. This could be measured from crest to crest or trough to trough. The frequency of the rhythm refers to how many cycles occur per unit of time (such as a heart rate of seventy beats per minute). The amplitude refers to the strength of the rhythm (for example, one-half of the height of the rhythm when shown on a graph).

The properties of biological rhythms are fascinating. They are ubiquitous, innate, probably endogenous, free-running, self-sustaining, entrainable, relatively temperature-independent, and relatively unsusceptible to chemical perturbations. Biological rhythms are said to be ubiquitous because they are found everywhere—at all levels of life, from cell organelles to cells, tissues, organs, whole organisms, and populations. They are found in all kinds of living things, with the possible exception of the prokaryotes. They are said to be innate because the rhythms are not learned and are largely programmed by the genetic makeup of the organism. Biological rhythms are probably endogenous, with an oscillator inside the cells of the organism, but it should be noted that Frank A. Brown has published extensive evidence that the timing information may be exogenously derived from geophysical fluctuations. Biological rhythms are entrainable, which means that they usually are kept in synchrony with day/night or other environmental schedules. Entrainment is maintained by an organism’s responses to environmental factors called synchronizers, zeitgebers, entraining agents, or time cues. Light, temperature, noise, and feeding are some of the zeitgebers that have been identified. The rhythms are called self-sustaining because they continue in the absence of any obvious zeitgebers. Biological rhythms have been found to be relatively temperature-independent, which is important because they often function as clocks. Biological rhythms can free-run when isolated from zeitgebers. When a rhythm free-runs, its period is found to be slightly different from the entrained period.

Despite years of investigation, biological rhythms are poorly understood. The search continues to find biological bases for the rhythmic processes so commonly seen. The innermost rhythmic process is sometimes referred to as the “biological clock” since it represents the seat of the cell’s or organism’s timekeeping mechanism. Some progress has been made, such as research that uncovered a gene, Per2AS, in mice that regulates their sleep and wake cycles. Another study in humans found positively charged amino acid blocks to be associated with these cycles.

Studying Biological Rhythms

One of the earliest scientific observations of a biological rhythm was reported in 1729 by Jean Jacques d’Ortous de Mairan, a French astronomer. He made detailed observations of the reactions to the constant darkness of a so-called sensitive plant that normally has its leaves unfolded during the daylight hours and folded during the night. De Mairan wondered whether the leaves respond directly to the presence of the sunlight and, therefore, open at dawn and close at dusk. Placing the plant in constant darkness to see how it responded, de Mairan found that the plant continued to show the rhythmic folding and unfolding of its leaves. The curious results were published in a brief report in the Proceedings of the Royal Academy of Paris.

Several years later, in 1758, Henri-Louis Duhamel repeated de Mairan’s experiment and further observed that warm temperatures failed to alter the pattern of the rhythmic opening and closing of the leaves of the sensitive plant. Studies in the nineteenth century revealed that in the sensitive plant, Mimosa pudica, the rhythmic opening and closing of leaves in constant dark completed a full cycle in 22 to 22.5 hours. It was found that plants supplied with lamps during the night and kept in darkness during the day adapted to the new schedule within a few days and unfolded their leaves only during the artificial day. Charles Darwin did experiments that convinced him that plants survived frosts more successfully when they could fold their leaves at night.

The extent to which circadian or other biological rhythms are endogenous (originate inside the organism) has been a subject of debate. Frank A. Brown spent most of his research career trying to resolve this question. Brown found that even when organisms were placed in heavy metal chambers that were airtight, it was virtually impossible to isolate an organism from its rhythmic, geophysical environment. Normally, circadian rhythms keep in synchrony with the day/night cycle (supposedly, they are reset slightly each day since they are not exactly twenty-four hours long). Brown studied in detail free-running rhythms—that is, rhythms found in organisms in a seemingly constant environment. When he averaged oxygen uptake data over many months, he found exact geophysical rhythms of twenty-four hours as well as exact lunar and annual rhythms in the metabolism of many different organisms, such as potatoes, carrots, hamsters, and rats. Furthermore, he showed that many animals, such as snails and flatworms, are influenced by subtle changes in the Earth’s magnetic field. Therefore, he concluded that the actual timing information that underlies circadian and other biological rhythms may well be exogenous and derived from the rhythmic, geophysical environment that pervades the organisms’ everyday surroundings. Despite such evidence for exogenous influences, most biologists today regard biological rhythms as the product of essentially endogenous processes.

Endocrine and Nervous System Rhythms

When looking for some basis for endogenous rhythms, researchers often investigate the endocrine and nervous systems because of their large roles in integrating and controlling biological functions. In a fascinating study by Albert H. Meier, he found that endocrine rhythms play a role in regulating the seasonal changes of physiology and behavior in the migratory white-throated sparrow and other vertebrates. He successfully induced seasonal changes by injecting birds with the hormones corticosterone and prolactin in different time relationships. In early studies, he found that prolactin injections either caused fat gain or fat loss, depending simply on whether the injections were given in the morning or afternoon. Migratory birds gain fat before they migrate and use this fat as an energy source for their flights. If the birds are given daily injections of corticosterone and prolactin four hours apart, they gain fat, try to fly from the south side of their cages, and do not have well-developed gonads—all characteristics of the normal fall bird. If the birds are given daily injections of corticosterone and prolactin eight hours apart, they remain lean, do not show any directed flight, and do not have well-developed gonads—traits characteristic of the normal summer bird. Conversely, if the birds are given daily injections of corticosterone and prolactin twelve hours apart, the birds gain weight, try to fly out the north sides of their cages, and have gonads that will grow in response to a lengthening photoperiod—traits characteristic of the normal spring bird. Some assays using radioactive isotopes of corticosterone and prolactin have been made in wild populations of white-throated sparrows, and the results show that the timing of the peaks of the hormones is roughly similar to the time relationships just discussed. Further research by this group centered on modifying brain chemistry to bring about seasonal changes in vertebrates.

The methods used to study biological rhythms range from the simple methods used by pioneers in the field to the latest innovations in molecular biology. The field is attracting many new researchers; new discoveries are being published almost daily. Yet much remains to be done before the essential nature of biological rhythms can be understood.

Implications of Biological Rhythms

There are many implications to the fact that plants and animals possess circadian and other biological rhythms. Some scientists have speculated about whether humans can survive living in space vehicles that leave the geophysical environment of the earth-moon complex. Will it be necessary, they ask, to try to duplicate parts of the terrestrial geophysical environment—by, for example, installing a rhythmic magnetic field in the space vehicles?

More mundane applications of a better knowledge of rhythms are to be found in animal husbandry. The annual rhythm of the reproduction of farm animals can be manipulated to result in higher productivity. It is a standard practice to lengthen the photoperiod in the henhouse to increase egg production and minimize the winter decrease in production. Sheep are treated with the hormone melatonin, naturally produced in more abundance in the winter months and the early fall to hasten the reproductive season.

Scientists had long asserted that the hypothalamus was the primary mechanism of circadian rhythm regulation, but further study revealed that, in mammals, each cell has its own circadian clock. Information from each cell is processed in the central nervous system. Cancer cells are an example of this phenomenon. The circadian rhythm impacts the sensitivity of cancer cells to therapy, but the exact details of this relationship require further study. However, it is possible that adjusting the time of day a patient receives treatment could improve the effectiveness of the treatment and overall patient outcomes.

Other conditions, such as eating disorders and obesity, are associated with the disruption of natural rhythms. Treating insomnia, jetlag, other sleep disorders, and many more benefits of a better understanding of biological rhythms await discovery.

Principal Terms

Biological Rhythm: A cyclical variation in a biological process or behavior, often with a duration that is approximately daily, tidal, monthly, or yearly

Circadian Rhythm: A cyclical variation in a biological process or behavior that has a duration of about a day—from twenty to twenty-eight hours

Endogenous: Refers to rhythms that are expressions of only internal processes within the cell or organism

Entrainment: The synchronization of one biological rhythm to another rhythm, such as the twenty-four-hour rhythm of a light-dark cycle

Exogenous: Refers to rhythms that originate outside the organism in the environment

Free-Running: Denotes a rhythm that is not entrained to an environmental signal such as a light-dark cycle

Frequency: The number of repetitions of a rhythm per unit of time, such as a heart rate of seventy beats per minute

Period: The length of one complete cycle of a rhythm

Photoperiodism: The responses of an organism to seasonally changing day length, that cause altered physiological states

Zeitgeber: “Time giver” in German, it is also referred to as a synchronizer or entraining agent

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