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Hypobiosis

Diapause and Quiescence

Dormancy

(hypometabolism)

- Hibernation

- Estivation

- Diapause

- Quiesence

Cryptobiosis

(ametabolism)

- Anhydrobiosis

- Cryobiosis

- Anoxybiosis

- Osmobiosis

Figure 1 Schematic summary of different hypobiotic (metabolism less than normal) states, including hypometabolism and ametabolism. Adapted from Keilin (1959) The problem of anabiosis or latent life: History and current concept. Proceedings of the Royal Society of London 150: 149-191.

Dormancy can also involve a developmental arrest (diapause). Cryptobiosis, which literally means 'hidden life', is a more extreme state than dormancy, with almost no detectable activity or metabolism. It is most prevalent in lower vertebrates, and is often a seasonal survival strategy to cold or desiccation.

Cryptobiosis

This state of'suspended animation' has been observed for a variety of invertebrate animals and plants during extreme environmental conditions. It was first described for invertebrate animals that survived an absence of water by becoming inactive and allowing their tissues to become desiccated (anhydrobiosis, e.g., rotifers). Two other forms of cryptobiosis also involve an altered state of cellular water, freezing temperatures (cryobiosis, e.g., a frozen insect), and high osmotic concentration (osmobiosis, e.g., brine shrimp eggs in a salt lake). Another form of cryptobiosis is survival of a lack of oxygen (anoxybiosis, e.g., killifish eggs sealed inside their egg capsule). The best-known example of cryptobiotic animals is probably the eggs of brine shrimp (Artemia), which can survive extended periods of complete desiccation, high salt concentration, or anoxia; their desiccated eggs are also remarkably resistant to extremes of temperature. Various 'resurrection' plants are well-known examples of cryptobiotic plants, being able to recover from desiccation for extended periods. Seeds of some plants are also spectacularly resistant to desiccation, sometimes for very long periods of time (e.g., seeds more than 1000 years old of the Indian lotus from an ancient lake bed in China).

All of these forms of cryptobiosis involve complete inactivity. Ecological advantages of cryobiosis include survival of harsh environmental conditions, and dispersal of highly resistant life stages. However, the physiological adaptations required by these animals and plants to survive extreme conditions at no detectable metabolic rate are generally complex and specialized.

Diapause is an ecological strategy for the avoidance of harsh conditions that involves the cessation ofdevelopment of a subadult life stage. It is essentially a time-delaying tactic to synchronize further stages of the life cycle with appropriate environmental conditions. Diapause is especially common in insects but is also observed in a wide variety of other invertebrate animals (e.g., brine shrimp embryos) and vertebrate animals (e.g., annual killifish embryos), as well as many plants (e.g., buds, bulbs, rhizomes, and seeds). Some plant seeds require drying out before they can develop, ensuring that adverse dry seasons pass before the embryo starts to develop. Diapause is also a reproductive strategy in a variety of mammals for the delayed implantation and development of embryos (e.g., macropod marsupials, mustelids, and deer). Quiescence is a period of inactivity, similar to diapause, but is a facultative response to an immediate change in environmental conditions that is terminated simply by the resumption of more favorable environmental conditions, rather than a programmed and obligate response. It may be a response to harsh environmental conditions such as low or high temperature, or drought. Many invertebrates and plants (particularly their seeds) become quiescent.

Hibernation (Winter Dormancy)

Hibernation is when an organism spends the winter in a state of dormancy; it is long-term multiday torpor. Many plants survive extended periods of cold and desiccation, either as aboveground trees or shrubs, or as underground structures. Protective scales around stem tips allow buds of aboveground plants to endure winter conditions without damage. The aboveground structures of other plants die back in unfavorable conditions, leaving dormant underground bulbs, rhizomes, tubers or corms, for which the soil buffers environmental extremes. Many plants accumulate solutes in their fluids to prevent freezing during winter, while others can tolerate freezing of water in their xylem and other extracellular water pools. For ectothermic animals, hibernation is primarily a behavioral state with reduced body temperature, hence activity and metabolic rate. Some use supercooling or antifreeze solutes to avoid freezing, or tolerate freezing of their extracellular fluids (e.g., weta crickets and wood frogs). Many endothermic mammals also hibernate (Table 1). Mammalian hiberna-tors typically use multiday torpor for weeks or even months (e.g., Figure 2), and attain very low body temperatures (Tb's) (e.g., 0 to —5 ° C). Only one bird, the poorwill, is known to hibernate, although many other birds (and mammals) readily use single-day torpor during winter.

Dormancy

(hypometabolism)

- Hibernation

- Estivation

- Diapause

- Quiesence

Table 1 Summary of torpor patterns in monotreme, marsupial and placental mammals, and birds, for single-day torpor (T), hibernation (H), or estivation (E)

Taxon

Torpor pattern

Table 1 Summary of torpor patterns in monotreme, marsupial and placental mammals, and birds, for single-day torpor (T), hibernation (H), or estivation (E)

Taxon

Torpor pattern

Monotremata

Tachyglossidae

H

Ornithorhynchidae

Metatheria

Didelphidae

T

Microbiotheriidae

H

Dasyuridae

T

Myrmecobiidae

T

Petauridae

T

Burramyidae

H

Acrobatidae

H

Tarsepididae

T

Eutheria

Rodentia

T,

H, E

Insectivora

T

Chiroptera

T,

H

Carnivora

T,

H?

Primates

T?

', H/E

Macroscelidae

T

Aves

Coliiformes

T

Trochiliformes

T

Strigiformes

T

Caprimulgiformes

T,

H

Columbiformes

T

Coraciformes

T

Passeriformes

T

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