Dormancy
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. 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, 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 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.
Animal dormancy
Hibernation
Hibernation is a mechanism used by many animals to escape cold weather and food shortage over the winter. This energy saving mode of the animal is characterized by many physiological changes, mainly the decreased body temperature, decreased heart rate (by as much as 95%) and lower metabolic rate. Time migration, as it is often called may last several days or weeks depending on 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. Animals that hibernate include bats, ground squirrels and other rodents, mouse lemurs, the European Hedgehog and other insectivores, monotremes and marsupials.
Diapause
Diapause is a 'sleep time' found mostly in insects and can occurs 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. It can be predictive, that is predetermined by animal's genotype. In such case, diapause can be called as the obligatory part of its life cycle. 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 lay ‘diapausing’ eggs after a few warning-signals. These eggs will have their cycle from egg to adult stopped somewhere. For example, silkworms diapause as early embryo, grasshoppers as moderately grown embryo and gypsy moth diapause as fully formed embryo. 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 photoperiod and for long day insects, shorter photoperiod 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.
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 will find themselves a safe place to sleep—usually underground. The bodies of estivators will slow down. Breathing and heartbeat get very slow. Reptiles use 90-95% less energy when they are estivating. Animals don't 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 of Philipps University of Marburg, Germany, and coworkers presented evidence in the 24 June 2004 edition of Nature that the Madagascan fat-tailed dwarf lemur hibernates or estivates in a small cricket hollow for seven months.
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 call a “true brumation.”
The length of time that 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 further their chances of reproductive success.
Brumation or 'suspended animation', as it is often known, is a survival tactic. For example, the Russian tortoise (Testudo horsfieldi) over much of their range experience summer highs well over 100 degrees Fahrenheit, and in the winter temperatures fall far below freezing. Without the option of brumation, these animals would perish. Instead, 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.[1]
Brumation (and winter slowing in general) does have an affect on reptile reproduction too. In general, cooler temperatures trigger the production of sperm in males, and prepares females for ovulation in spring. Because of this widely accepted theory, many reptile breeders use brumation or slight temperature drops to trigger the reproductive behavior in their herps.
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 or homoiothermic or warm blooded animals lasting for periods ranging from just a few hours to several months. 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. Some animals such as groundhogs, chipmunks, and jumping mice enter this state of hibernation for the duration of the winter. Lungfish switch to the torpor state if their pool dries out. Tenrecidae (common name tenrecs) switch to the torpor state if food is scarce during the summer (in Madagascar). Black bears, although often thought of as hibernators, do not truly enter a state of torpor. While their body temperatures lower along with respiration and heartbeat, they do not decrease as significantly as most animals in a state of torpor. 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.
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. Torpor is also a term used in White Wolf's World of Darkness Vampire system to describe the result of a vampire being staked through the heart. The effects of this torpor are similar to paralysis. 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 is easily dispersible. Therefore, the bacteria may not need dormant forms to overcome the adverse environmental condition. However, there are certain bacteria which produce metabolically inactive forms that can survive 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, etc. 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) upto 15% 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% of the volume and dry weight of the whole endospore. Therefore, endospores are thermoresistant, drought resistant, resistant to many chemical and radiation treatments. It has been reported that even from 50 years old dried soil, 90% 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 degree Celsius under 15 lb per centimeter squar 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 time 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 pass through a phase of innate dormancy coinciding with an unfavourable season. But several species of annual weeds like groundsel (Senecio vulgaris), shepherd's purse (Capsella bursa-pastoris), 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 day length, or a reduction in rainfall. In higher plants, innate dormancy involves seeds, underground organs such as rhizomes, corms or tuber 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-yr-old lotus fruit, recovered from a dry lake bed in northeastern China. [2]
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 coat of some desert plants contains chemicals which inhibit its own seed germination until they are completely wash 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 like incomplete development, i.e., morpholgical or physiological immaturity.
Under normal conditions, the seed matures in time, and the thick seed coat is weakened, 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 temeratures, the process is known as seed stratification. For this to be effective, the seeds require moist pre-chilling at 0 to 5 degree Celsius 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 the 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 growing 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 second wave of foliage in the same season. It seems that late autumn innate dormancy of buds is established and triggered by short photoperiod of autumn. 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. Eventually, however, a 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% 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.[citation needed]
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, etc. Even though there are some risk factors, the dormancy enhances the survivability of the organisms through time migration. The different types of dormancy are not the different stages of organisms' inactivity continuum. They are discrete features being adopted by different types of organisms at 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 loose lot of body energy because of very high surface-volume ratio, they reduce the unnecessary lose by going through torpor. Diapause is making a halt in the untimely reproduction and multiplication. The seed dormancy and bud dormancy are dormancy before and after germination.
Dormancy is not just to escape the adversity of the environment, but also to reduce the exploitation and to promote the sharing of natural resources by the organisms. This exerts also positive effect on the vitality and reproductive capability of the animals in the following season. For this reason, animal breeders are using some sort 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 can be a sort of dormancy.
ReferencesISBN links support NWE through referral fees
Scholar team (2002) SQA Adv. Higher Biology; Environmental Biology. p 93-95 Heriot Watt University
C.J.Clegg and D.G. Mackean (2000) Adv. Biology: Principles and Applications. P 438-439 John Murray (Publishers) Ltd.
Hans G. Schlegel and Christiane Zaborosch (1992) Allgemeine Mikrobiologie. p 79-84 Georg Thieme Verlag Stuttgart
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