Circadian and Circannian Rhythms

Certain mammalian behaviors serve to promote the survival of the individual. One striking aspect of animal behavior is the rhythmic, or cyclic, pattern of activity. Some species are active at night(nocturnal) and some during the day(diurnal); others are active primarily at dawn and dusk. The activity periods tend to be at regular intervals. The time of emergence of a particular species of bat may differ by no more than two or three minutes night after night. Animals also exhibit other kinds of cyclic behavior. The timing of reproduction is cyclic, and in some mammals, such as some rodents and bats, daily or seasonal shifts occur between highly active and torpid states. Migratory movements are also cyclic. Daily activity rhythms, those based on a 24-hour cycle, are termed circadian rhythms and are better understood than other types of rhythms.

Circadian rhythms differ markedly from one species to another. Most mammals are nocturnal, but even in two nocturnal species there are contrasts between the patterns of activity. In general, small mammals that are especially vulnerable to predation, such as rodents, tend to be nocturnal, whereas less vulnerable species, such as many ungulates(mammals with hooves), may be active during the day. The activity cycles of carnivores seem to be geared to the circadian cycles of their prey or to the period when hunting is most rewarding.

Circadian rhythms are also influenced by interactions between species with similar environmental needs. In some cases, competition between species is reduced or eliminated because their activity cycles are out of phase. Two species of fishing bats, both of which feed over water, avoid interfering with one another partly by foraging at different times of the night and also by eating different prey. Clearly. the circadian rhythm of an animal is part of its total adaptation to its particular mode of life and environment and has evolved just as have the form and structure of its body parts.

The question of whether circadian cycles are endogenous (internally controlled) or exogenous(ultimately regulated by external stimuli) has occupied the attention of many biologists. Clearly, some strong endogenous control is present in many species. As an example, careful work on the flying squirrel showed that even under constant environmental conditions, including continuous darkness flying squirrels maintained regular activity periods that deviated only around two minutes from the mean value (average) for activity periods under natural conditions. When a laboratory animal whose circadian cycle is not in phase with the natural 24-hour light-dark cycle is again exposed to normal day and night conditions, its cycle rapidly shifts and becomes synchronized; that is, it becomes adjusted and locked to the 24-hour cycle. Studies using large outdoor enclosures indicated that circadian activity of root voles, though not affected by weather conditions, followed a rhythm based on the Moon. Circadian cycles and other animal behaviors are regulated by intricate and as yet poorly understood interactions between endogenous and exogenous factors.

As might be expected if circadian cycles are adaptive, they often shift seasonally and depend on an animal’s ability to track some environmental variable, such as light or temperature. Attending the seasonal changes in environmental temperatures are changing metabolic demands put on small mammals, and some shifts in circadian rhythms may allow the animals to avoid activity during times of most intense temperature stress. Three types of seasonal changes in circadian rhythms were reported by researchers for small mammals in Canada: overall decreased winter activity changes in percentage of nocturnal and diurnal activity, and changes in peak daily activity. Seasonal shifts in activity probably result in a considerable saving of energy. Although less studied than circadian rhythms, circannian rhythms play an equally prominent role in the lives of some mammals Among vertebrates, such vital activities as breeding, migration, and hibernation are phased on an annual cycle or a circannian rhythm. A circannian rhythm is an endogenous cycle that has a length of approximately one year. Circannian rhythm is a major key to the survival of some Temperate Zone and Arctic mammals. According to researchers, these rhythms allow an organism to anticipate, and thus prepare for, a future annually occurring environmental condition such as cold weather, drought, food scarcity. or optimal breeding time.the rhythm also ensures some flexibility of response to cyclic environments that may differ markedly from year to year.?

1.Certain mammalian behaviors serve to promote the survival of the individual. One striking aspect of animal behavior is the rhythmic, or cyclic, pattern of activity. Some species are active at night(nocturnal) and some during the day(diurnal); others are active primarily at dawn and dusk. The activity periods tend to be at regular intervals.?The time of emergence of a particular species of bat may differ by no more than two or three minutes night after night. Animals also exhibit other kinds of cyclic behavior. The timing of reproduction is cyclic, and in some mammals, such as some rodents and bats, daily or seasonal shifts occur between highly active and torpid states. Migratory movements are also cyclic. Daily activity rhythms, those based on a 24-hour cycle, are termed circadian rhythms and are better understood than other types of rhythms.

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