Organisms everywhere, both plant and animal, are subjected to stress from their environment. In order to avoid this, some animals migrate vast distances, while others are just stubborn enough to stay. These animals change their behaviour and the habitats1 that they stay in in order to try and reduce environmental stresses. However, other creatures become dormant. Taking on an inactive state, they have a lower rate of metabolism2 than normal, which enables them to conserve energy and place few demands on their environment. Most groups of the animal kingdom have species that become dormant.
Types of Dormancy
There are a number of types of dormancy, and the difference between them is rather woolly, scientifically speaking.
Hibernation - Hibernation is often used to describe any winter dormancy in both warm and cold-blooded vertebrates, including bears, shrews and other animals that are inactive over winter. However, there is a difference between dormancy and hibernation. Hibernation involves a huge decrease in body temperature, whereas dormant animals tend to reduce their body temperature and metabolism only slightly. Animals that truly hibernate tend to be small mammals, weighing less than 1kg.
Although bears and other animals are said to undergo hibernation, they are able to wake, move around and eat on warmer days. Some bears are even able to give birth during the winter.
Aestivation - Aestivation is dormancy during summer months, rather than winter months. It occurs with a decrease in water supplies and high temperatures.
Daily Torpor - This involves a period of dormancy that lasts, not unsurprisingly, less than a day. This is a shallow decrease in metabolism, and usually results from a short food supply. Many species of animals undergo daily torpor.
Nocturnal Hypothermia - 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.
In addition, it should be remembered that the degree of metabolic reduction and the period of time that the organism remains dormant vary from species to species. Some animals undergo a series of short-duration 'deep sleeps', while others experience large periods of time with a severe decrease in their metabolic rate.
The process of dormancy appears to have evolved independently in a large number of different species, and, as such, there are a wide number of mechanisms by which a period of dormancy can be entered, depending on the physiology of the organism. In many species, dormancy is an essential part of the life cycle, enabling an organism to change hugely, while undergoing minimal impact. In some cases, it allows organisms to survive huge climatic changes, such as a pond or river drying up. However, the type of dormancy most commonly associated with the word is that encountered when an animal becomes dormant during a period of cold weather.
Dormancy, in short, allows organisms to adapt better to their environment during periods of hardship. It enables species to take advantage of certain environmental niches, which would otherwise be impossible to make habitable.
Causes of Dormancy
Dormancy can be caused by a number of different things. The most important are changes in temperature; slightly less important is the availability of factors such as food, water, oxygen and carbon dioxide. Usually, the inducing factor of dormancy is a slight shift out of the temperature range for the organism. However, as a direct result of the temperature flux, food, water and oxygen are also usually likely to decrease, which reinforces the stimulus to become dormant. As an example, animals in polar regions become dormant during the winter months due to the lack of food. However, during the summer months in the desert, some animals become dormant to compensate for a lack of water. Other factors that can also stimulate dormancy are light intensity and other environmental factors.
Most environmental influences that stimulate dormancy are, not surprisingly, cyclical. There are two cycles that have the greatest effect - circadian3 and annual. Daily changes in light and temperature induce rhythmical changes in metabolism, whereas annual changes affect the availability of food and water. The other factors, oxygen and carbon dioxide, do not vary cyclically, but are brought about by the organism itself.
Strangely enough, it has been scientifically shown that once an organism has become adapted to performing a series of behavioural patterns in response to its environment, it will continue to exhibit these even after the stimulus is removed4. Animals often respond in the same way as before if removed from their original habitats.
Dormancy in Protozoa and Invertebrates - Cyst Formation
Many parasitic and free-living protozoa5 undergo dormancy by excreting a protective cyst. It can be induced by temperature change, pollution, or lack of food or water. Euglena has two types of cysts, both formed in response to stressful conditions. However, one results in a cyst that can contain up to 32 daughter organisms, resulting from asexual reproduction. These daughter cells then emerge from the cyst once the environmental conditions have been restored to normal.
Parasitic protozoa experience cysts in their life cycle. For example, Entamoeba histolytica6 forms cysts in the intestine of infected individuals which then are released from the body through faeces. Once food or water containing the cysts enters the digestive tract of another person, the amoebas are released from cysts and can infect the new host. If it were not for the creation of cysts, many diseases would be easier to control. However, the cysts enable cells to remain viable (alive) for many weeks. E histolytica cysts can withstand temperatures of around 70°C and some chemicals.
Cysts are formed in the life cycle of invertebrate parasites, such as flukes, and are used for the passage from one host to another. However, there is also the formation of cysts during periods of environmental stress. Freshwater sponges and some marine species form gemmules7 within the body of the organism. These have a resistant covering, and are released upon the death and disintegration of the sponge. Once conditions have been returned to normal, the cell mass escapes the cover and a new sponge is formed.
Rotifers8 produce winter eggs that have thick, resistant covers that are similar to the protozoan cyst, enabling it to survive for long periods of time. These eggs can survive drought and freezing conditions, and can be dispersed by wind or other animals. This gives the cyst an added advantage - it prevents too many rotifers living in the one area and introduces the creature to new habitats and niches.
Land snails are mostly dormant during the day, and do not emerge from their shell until night. With unfavourable conditions, they are able to secrete a membrane of mucus and slime that covers the opening of the shell until things improve.
Diapause in Insects
Diapause9 can occur at any stage of the life cycle. It can be recognised in two ways - as a stopping of growth in the immature insect, and as a stopping of sexual activity in the adult insect. Again, it can occur either as a result of environmental conditions, or because it is a part of the life cycle, of which butterflies are a good example.
Many insects are able to survive winter through diapause. Even the adult butterfly or mosquito is able to survive so long as they are in a sheltered spot. Butterflies often remain in shrubbery over the winter, where they can be covered in snow and ice, but still emerge unscathed in spring. Other insects form cocoons, or shelter in already existing hiding places10.
The start of diapause again depends on the influence of a combination of environmental factors upon the regulatory systems of the insect. Temperature and the length of day influence the endocrine functionality of the insect brain11, affecting the release of hormones that influence other organs. The main organs affected are the prothoracic glands. These glands secrete the hormone ecdysone normally: in its absence all growth and metamorphosis is stopped. Once the temperature increases and the days begin to get longer, ecdysone is secreted, enabling the continuation of growth and development.
The time at which diapause comes to an end is species-dependent. Often only favourable conditions are required, but in some species, a further stimulus is required. Also, there are instances where a certain amount of time must have passed before a stimulus is effective12. For instance, the eggs of the mosquito Aedes vexans will remain in a state of diapause until the damp soil on which they are placed floods into a pool that is suitable for the larvae. Eggs of A canadensis, another mosquito, are laid in the same soil as A vexans, but will not leave diapause until subjected to a cold spell. This means that although both species lay eggs in early summer, A vexans will hatch in pools formed in the summer rains, while A canadensis will hatch in the pools formed by spring rain, after winter.
Dormancy in Cold-Blooded Vertebrates
Two kinds of dormancy can be distinguished in vertebrates on the basis of body temperature. Most vertebrates are cold-blooded, or poikilotherms. This term derives from the fact that their body temperature is determined by the temperature of the environment, and is not maintained at a constant temperature by various internal processes or homeostasis. Warm-blooded vertebrates, homoiotherms are able to maintain a constant temperature irrespective of their environmental conditions.
Fish and Amphibians
Cold-blooded animals' metabolism is mostly influenced by temperature, nutrition and photoperiod13. Photoperiod is extremely important for fish and amphibians. Although fish remain active throughout the year, their activity levels alter depending on the temperature of the surrounding waters. Some species can survive periods of brief, superficial freezing or super-cooling where the body fluids don't actually freeze, as the surrounding temperatures are below the freezing point of internal body fluids. However, fish do not reduce their metabolism to the extent of previously described creatures, but maintain some activity throughout the entire year. As such, they cannot truly be said to become dormant.
The other environmental stress that fish can suffer is drought. Lungfish burrow deeply into mud during periods of drought. They cocoon themselves in a layer of slime and become dormant. The gills do not function and an air bladder is used to breathe. Fat reserves provide their energy and urea is excreted rather than ammonia, to preserve water14.
Amphibians find suitable places to remain dormant in until favourable environmental conditions return. Frogs and salamanders often congregate in large numbers where it is moist. Toads often become dormant in burrows and frogs, during periods of drought, may become dormant in a cocoon of mud.
Effects of Temperature on Reptiles
Reptiles depend on external sources of heat to keep warm. As such, they are able to survive low temperatures by finding an environment where the temperature will not fall below freezing. This can be found underground, with the depth depending on the thermal conductivity of the soil and the minimum temperature. It is this dependency that controls where reptiles are found. The Arctic and Antarctic are too cold as the subsoil is permanently frozen. The same is true for neighbouring regions; however, this is compounded with the fact that the length of summer is too short to enable the completion of the life cycle. Snakes are the most robust of reptiles, and have been found at both high altitudes15, and latitudes16.
Low temperatures directly induce dormancy in reptiles. The adder prepares for dormancy at a temperature of 8-10°C, and ends dormancy a few days after the maximum temperature is above 7.5°C. This temperature dependence results in a variation of dormancy period, depending on where the species is found. It can be as long as 275 days in northern Europe, to 105 days in southern Europe. In the UK, because of the Gulf Stream, it can even be as little as a few weeks.
Reptiles also tend to become dormant when the temperature is too high. However, there is a difference in the physiology produced during summer and winter. Animals have a higher tolerance to low temperatures than higher temperatures. There is a wide gap between an animal's normal temperature and the low temperatures that cause death. With high temperatures, this gap is not so wide. This causes reptiles to find cool and shady places, where they can avoid the extreme heat. Although their metabolism doesn't slow, movement is restricted because of their situation. This behaviour can occur daily during the summer in the desert.
The water required by a reptile during dormancy is less than normal, and is provided by the metabolism of fat reserves. If a reptile exists in an environment that has dry and wet seasons, the factor of water availability can result in a longer period of dormancy during the dry season than in the wet season.
Often a large number of snakes are found dormant in the same den. This is due to the limited niches available. It is possible for other reptiles, such as lizards or toads to also share the same den over winter. Although snakes often stay in an evacuated burrow, often that of a prairie dog, it is unlikely that snakes will remain there while the original owner remains.
Effects of Latitude
Latitude affects the period of dormancy and their period of daily activity (due to the difference in daylight). Many snakes are active during the early evening, but in northern countries, such as in Scandinavia, snakes are active during the day and take as much advantage of daylight as possible17, in order for their life-cycle development to occur fully. However, because of the shorter summer, growth and development is severely reduced, sexual maturity is delayed and the reproductive period is over two years, rather than the one year experienced in lower latitudes18.
So, dormancy is a great way for animals to adapt to protect themselves against extreme environmental conditions. It is more energetically sensible than 'toughing it out' over the winter.