Dormancy

Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity is temporarily suspended. This minimizes metabolic activity and therefore 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.

Hibernation
Hibernation is a mechanism used by many animals to escape cold weather and food shortage over the winter. Hibernation may be predictive or consequential. 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. During hibernation the animal undergoes many physiological changes, including decreased heart rate (by as much as 95%) and decreased body temperature. 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 predictive strategy that is predetermined by an animal's genotype. Diapause is common in insects, allowing them to suspend development between autumn and spring, and in mammals such as the red deer, where a delay in attachment of the embryo to the uterine lining ensures that offspring are born in spring, when conditions are most favorable.

Estivation
Estivation is an example of consequential dormancy in response to very hot or dry conditions. It is common in invertebrates such as the garden snail and worm but also occurs in other animals such as the lungfish. The period of dormancy that bears experience during the winter is also called estivation.

Brumation
Brumation is an example of dormancy in reptiles that is similar to hibernation. It differs from hibernation in the metabolic processes involved.

Torpor
Torpor is a short-term reduction of body temperature to an ambient level during periods of inactivity, often lasting only a few hours. Animals that experience torpor include small birds such as hummingbirds and some small mammals such as bats.

Bacterial dormancy
Certain bacteria produce metabolically inactive forms that can survive intensely adverse conditions unharmed; these are known as cysts or endospores. This is a consequential strategy. Inactivating these resistant forms is usually done using an autoclave (pressurized heating device).

Viral dormancy
Viruses of the family Herpesvirus are notable for remaining dormant within cells in the human body. See for example varicella zoster virus, which in an individual causes first chickenpox then herpes zoster. Concerning viruses this dormancy is often referred to as latency or a latent infection. HIV produces a latent infection in lymphocytes, and at this stage in its life-cycle it is called a provirus.

This should not be confused with clinical latency.

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.

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. After a normal growing season, dormancy can be brought on by decreasing temperatures, shortened day length, or a reduction in rainfall.

Dormant seeds
When a mature seed is placed under favorable conditions and fails to germinate, it is said to be dormant. 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. The second type of seed dormancy is called embryo dormancy or internal dormancy, and is caused by a condition of the embryo which prevents germination (Black M, Butler J, Hughes M. 1987). The oldest seed that has been germinated into a viable plant was an approximately 1,300-yr-old lotus fruit, recovered from a dry lakebed in northeastern China. 

Tree dormancy
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 (Bewley JD, Black M. (1994). )