Dormancy

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Dormancy is a general term used to describe a period in an organism's life cycle when metabolic activity is minimized and active development is temporarily suspended. Thus, it helps an organism to conserve energy. Among types of dormancy are hibernation, estivation, diapause, brumation, and torpor.

Diversity is one of the factors that enhances the human experience of nature. Dormancy, in its various forms, not only provides a source of diversity in itself, but this phenomena allows organisms to prosper in particular environments where they might not otherwise be able to survive. They can persist through difficult times of the year and delay energy consumption and reproduction to times when conditions are more optimal.

Dormancy tends to be closely associated with environmental conditions. Organisms can synchronize entry to a dormant phase with their environment through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase before the onset of adverse conditions. For example, shortening photoperiod and decreasing temperature are used by many plants to predict the onset of winter. Consequential dormancy occurs when organisms enter a dormant phase after adverse conditions have arisen. This is commonly found in areas with an unpredictable climate. While very sudden changes in weather conditions may lead to a high mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources.

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Animal dormancy

Hibernation

Main article: Hibernation

Hibernation is a mechanism used by many animals to escape cold weather and food shortage over the winter. This energy-saving mode is characterized by many physiological changes, mainly decreased body temperature, decreased heart rate (by as much as 95 percent), and lower metabolic rate.

Time migration, as it is often called, may last several days or weeks depending on the species, ambient temperature, and time of the year. An animal prepares for hibernation by building up a thick layer of body fat during late summer and autumn, which will provide it with energy during the dormant period. Hibernation may be predictive or consequential.

American Black Bear

Animals that hibernate include bats, ground squirrels and other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes, and marsupials. Birds typically do not hibernate, instead utilizing torpor. An exceptional bird known as the Poorwill does hibernate (Jaeger 1948). Many experts believe that the processes of daily torpor and hibernation form a continuum.

One animal that some famously consider a hibernator is the bear. However, during a bear's winter sleep state, the degree of metabolic depression is much less than what is observed in smaller mammals. Many prefer to use the term "denning" in place of hibernating. The bear's body temperature remains relatively stable (depressed from 37° C to approximately 31° C) and it can be easily aroused. In contrast, hibernating ground squirrels may have core body temperatures as low as -2° C. Still, there is much debate about this within the scientific community; some feel that black bears are true hibernators that employ a more advanced form of hibernation.

Estivation

Estivation or aestivation (from Latin aestas, "summer") is a rare state of dormancy similar to hibernation, but during the months of the summer when environmental temperatures become too high for healthy physiological function to occur.

Animals that estivate spend a summer inactive and insulated against heat to avoid the potentially harmful effects of the season (such as the increase in body temperature or relative lack of water), or to avoid contact with other species with which they may otherwise be in competition or for which they are prey. Some animals, including the California red-legged frog, may estivate to conserve energy when their food and water supply is low.

When hot and dry times come, estivators find a safe place to sleep—usually underground. The bodies of estivators slow down. Breathing and heartbeat get very slow. Reptiles use 90-95 percent less energy when they are estivating. Animals do not move, grow, or eat during this time. Animals that estivate include mostly the cold blooded (poikilothermic) animals of hot and arid climatic zone, like North American desert tortoises, salamanders, and lungfishes. The lungfish estivates by burying itself in mud formed at the surface of a dried up lake. In this state, the lungfish can survive for many years. Other animals estivate in their burrow and wait for autumn to come.

Snails also estivate during periods of heat of the day. They move into the vegetation, away from the ground heat, and secrete a membrane over the opening to their shell in order to prevent water loss.

Both land-dwelling and aquatic mammals undergo estivation. Until recently no primate, and no tropical mammal was known to estivate. However, animal physiologist Kathrin Dausmann and co-workers of Philipps University of Marburg, Germany, presented evidence that the Madagascan fat-tailed dwarf lemur hibernates or estivates in a small cricket hollow for seven months (Dausmann et al. 2004).

Diapause

Diapause is a "sleep time" found mostly in insects and can occur at any stage of the life cycle. It is characterized by suspension of growth and development in the immature insects between autumn and spring and by stopping of sexual activity in the adult insect. It is a way for animals to live through extreme temperature, droughts, or lack of food.

Diapause can be predictive, that is predetermined by the animal's genotype. It may be an "obligatory" part of its life cycle (they necessarily go through this period). However, in others, it can be "facultative" and the animals undergo dormancy only if something bad is going to happen. The adult butterfly or mosquito is able to survive winter cold so long as it is in a sheltered spot. Other insects form cocoons. In some other cases, the female lays "diapausing" eggs after a few warning-signals. These eggs will have their cycle from egg to adult stopped along the way. For example, silkworms diapause as early embryos, grasshoppers as moderately grown embryos, and gypsy moths diapause as fully formed embryos.

In mammals such as the red deer, diapause is seen as a delay in attachment of the embryo to the uterine lining to ensure that offspring are born in spring, when conditions are most favorable.

Change in temperature and photoperiod beyond the critical values works as the warning signals for animals to undergo diapause. For example, for short-day insects, longer photoperiods, while for long-day insects, shorter photoperiods are the warning signals. These signals affect the endocrine functionality of the insects causing the suppression or release of growth hormone, thus leading to start or termination of diapause.

Brumation

Brumation is a term used for the hibernation-like state that cold-blooded (ectothermic) animals undergo during very cold weather.

In nature, these animals typically find hibernaculums within their environment in which they can be somewhat insulated. Burrows, rock crevices, caves, and leaf litter are a few examples of hibernaculums documented in nature. Some temperate species can even brumate under water.

Reptiles will enter brumation in response to endogenous stimuli (like change in hormones, amino acid concentration, etc.) or to exogenous stimuli (drop in temperatures and shortening of photoperiod) in the late fall and come out of brumation in spring, triggered by exogenous stimuli like increased temperatures, longer days, and changes in barometric pressure. When a reptile brumates, it becomes lethargic, sometimes not moving at all for the duration of the cold season. Sub-tropical animals, as well as those found near the equator, often do not undergo what herpeteculturists (reptile breeders) call a “true brumation.”

The length of time a reptile brumates is extremely variable. Temperate and desert species tend to brumate much longer than sub-tropical and tropical species. Male reptiles typically emerge prior to females, allowing them ample time to establish breeding territories to enhance their chances of reproductive success.

Brumation or "suspended animation" as it is often called is a survival tactic. For example, the Russian tortoises (Testudo horsfieldi) over much of their range experience summer highs well over 100°F, and in the winter temperatures fall far below freezing. Without the option of brumation, these animals would perish. Thus, they construct burrows, sometimes as deep as six feet, and remain in them until conditions become favorable. During this time, the tortoises' physiological functions grind to nearly a halt. Feeding obviously ceases, as does digestion and defecation. Heart and respiratory rates also drop (LLLReptile 2004).

Brumation (and winter slowing in general) does have an affect on reptile reproduction as well. In general, cooler temperatures trigger the production of sperm in males, and prepares females for ovulation in spring. Because of this widely accepted pattern, many reptile breeders use brumation or slight temperature drops to trigger the reproductive behavior in their herps (reptiles).

Torpor

Torpor is a short-term reduction of body temperature to an ambient level especially during periods of inactivity. Thus, torpor may be defined as a state of regulated hypothermia in the endothermic, homoiothermic, or warm blooded animals lasting for short periods of time, perhaps just a few hours, but perhaps the temperature reduction lasts several months and may still be labelled by some as torpor. Some consider torpor and hibernation to form a continuum.

Animals that go through torpor include small birds such as hummingbirds and some small mammals, such as bats. During the active part of their day, these animals maintain normal body temperature and activity levels, but their body temperature drops during a portion of the day (usually night) to conserve energy.

Torpor is often used to help animals survive in a cold climate, since it allows the organism to save the amount of energy that would normally be used to maintain a high body temperature. Lungfish switch to the torpor state if their pool of water dries out. Tenrecidae (common name tenrecs) switch to the torpor state if food is scarce during the summer (in Madagascar).

Torpor is alternately used as a reference to any non-physiological state of inactivity. As an example, recently naturalists have learned that the female crocodile enters a deep torpor without aggression during their short egg laying period. This definition is also commonly used to describe the "chill out" effects of a number of psychotropic drugs, such as psychedelic mushrooms and LSD. Many birds display nocturnal hypothermia. The body temperature drops by roughly 3-5°C overnight, and their metabolism also reduces to about half of the daily, active rate.

Most animals are in danger during torpor or hibernation. They are so slow and unaware of what is happening around them that they are easy to catch.

Bacterial dormancy

Each bacterial group is ubiquitous, occurring nearly everywhere and easily dispersible. Therefore, bacteria may not need dormant forms to overcome the adverse environmental conditions. However, there are certain bacteria that produce metabolically inactive forms that can survive severely adverse conditions unharmed.

The species of Gram-positive genera Bacillus, Clostridium, Desulfotomaculum, Sporolactobacillus, and Sarcina form endospores on confronting the adverse environmental conditions, like lack of water or depletion of essential nutrients, and so forth. This is a consequential dormancy. The endospores are formed in the spore mother cells, one per one mother cell. Each endospore contain endospore specific dipicolic acid (pyridin-2,6-dicarbonic acid) up to 15 percent of the dry weight. The endospore is surrounded by exosporium, outer covering, inner covering, cortex, cell wall, and cell membrane; thus, the protective coverings alone form about 50 percent of the volume and dry weight of the whole endospore. Therefore, endospores are thermoresistant, drought resistant, and resistant to many chemical and radiation treatments. It has been reported that even from 50 year-old dried soil, 90 percent of the endospores could germinate into viable cells. Pasteurization is not enough to kill the endospores; these resistant forms are inactivated usually by sterilization in an autoclave by heating at 121°C under 15 pound per centimeter square steam pressure for 15 minutes.

Besides endospores, some bacteria develop exospores (e.g., Methylosinus trichosporium) or undergo encystment to form cysts (e.g., the species of genera Methylocystis and Azotobacter). Many species of Azotobacter can withstand drying of the soil for significantly long times without undergoing any structural or physiological change.

Plant dormancy

In plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant species, which enables them to survive in climates where part of the year is unsuitable for growth, such as winter or dry seasons.

Innate dormancy occurs whether or not external conditions are suitable. Most plants of temperate regions, such as maples, pass through a phase of innate dormancy coinciding with an unfavorable season. But several species of annual weeds like groundsel (Senecio vulgaris), shepherd's purse (Capsella bursa-pastoris), and chickenweed (Cerastim spp.) show imposed dormancy only in the very cold weather.

Plant species that exhibit dormancy have a biological clock that tells them to slow activity and to prepare soft tissues for a period of freezing temperatures or water shortage. This clock works through decreased temperatures, shortened photoperiod, or a reduction in rainfall. In higher plants, innate dormancy involves seeds, underground organs such as rhizomes, corms, or tubers, and the winter buds of woody twigs.

Seed dormancy

If a seed fails to germinate even when it is placed under favorable conditions, then the seed is said to be dormant. Many seeds, especially those of wild plants, do not germinate as soon as they are formed and dispersed. If seeds germinated as soon as they were shed in late summer or early autumn, they would produce plants that might succumb to harsh winter conditions before they could reproduce. Thus, dormancy helps to avoid this hazard. The oldest seed that has been germinated into a viable plant was an approximately 1,300-year-old lotus fruit, recovered from a dry lake bed in northeastern China (Shen-Miller et al. 2002).

There are two basic types of seed dormancy. The first is called seed coat dormancy, or external dormancy, and is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo (e.g., Acer sps.). Seed coats of some desert plants contain chemicals that inhibit their own seed germination until they are completely washed away by sufficient rain; this dormancy is to avoid scarcity of water. The second type of seed dormancy is called embryo dormancy, or internal dormancy, and is caused by a condition of the embryo, such as incomplete development, that is, morphological or physiological immaturity.

Under normal conditions, the seed matures in time, and the thick seed coat is weakened via a process called scarification by abrasion in the soil, by the action of soil microorganisms, or by the enzymatic action in the digestive track of animals. However, the internal dormancy of seeds of many species can only be overcomed by low temperatures, a process known as seed stratification. For this to be effective, the seeds require moist pre-chilling at 0° to 5°C for two or three weeks. This is the reason that such seeds are sown in Fall to make them undergo cold stratification in the winter.

Bud dormancy

In the temperate zones, the growing parts of the stems of trees and shrubs are well protected from winter cold by special buds called winter buds. In these winter buds, the growing meristem and the immature foliage leaves are covered by many layers of scale leaves externally. They are formed in summer while the conditions are still favorable. Although formed in the growing season, these buds do not normally break until after passing through the cold winter. However, if the trees suffer from unexpected defoliation, then the buds will grow and produce a second wave of foliage in the same season.

It seems that the late autumn innate dormancy of buds is established and triggered by the short autumn photoperiod. Tree species that have well-developed dormancy needs may be tricked to some degree, but not completely. For instance, if a Japanese Maple (Acer palmatum) is given an "eternal summer" through exposure to additional daylight, it will grow continuously for as long as two years (Walston 1997). Eventually, however, the temperate climate plant will automatically go dormant, no matter what environmental conditions it experiences. Deciduous plants will lose their leaves; evergreens will curtail all new growth. Going through an "eternal summer" and the resultant automatic dormancy is stressful to the plant and usually fatal. The fatality rate increases to 100 percent if the plant does not receive the necessary period of cold temperatures required to break the dormancy. Most plants will require a certain number of hours of "chilling" at temperatures between about 0°C and 10°C to be able to break dormancy (Lionakis and Schwabe 1984).

Summary

Dormancy is the purposeful inactivity of organisms, a form of adaptational (behavioral and/or physiological) tactic to cope with adverse weather change, food shortage, habitat insuitability, and so forth. Even though there are some risk factors, dormancy in general enhances the survivability of the organisms through time migration.

The different types of dormancy are not different stages of organisms' inactivity continuum. They are discrete features being adopted by diverse species of organisms under different types of environmental conditions and different stages of their life cycle. Hibernation and estivation are especially utilized by warm blooded organisms for two opposite temperature extremes. If the poikilothermic or ectothermic organisms do the same, then it is known as brumation. Small endothermic or homoiothermic animals can lose a lot of body energy because of a very high surface-volume ratio, but they can reduce the unnecessary loss by going through torpor. Diapause is a halt in what would be an untimely reproduction and multiplication. Seed dormancy and bud dormancy are dormancy before and after germination.

Dormancy is not just to escape the adversity of environmental conditions, but also to reduce exploitation and to promote the sharing of natural resources by the organisms. This exerts also a positive effect on the vitality and reproductive capability of the animals in the following season. For this reason, animal breeders are using types of induced dormancy in their practices. Dormancy is not well known in human beings. However, the deep mystical state arrived by some yogi for several months may be classified by some as a sort of dormancy.

References



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