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Essay: Controlled hypothermia

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  • Published: 15 October 2019*
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Many species of mammals and birds allow their body temperatures to fall in a controlled manner under certain circumstances.  Controlled hypothermia is a general term for this sort of phenomenon.  Hypothermia is the state of having an unusually low body temperature, and in controlled hypothermia, the animals orchestrate their entry into and exit from hypothermia rather than being forced.  The most well known and profound forms of controlled hypothermia are hibernation, estivation, and daily torpor.  All three of these terms are states in which an animal allows its body temperature to approximate ambient temperature within a species-specific range of ambient temperatures.  Hibernation, estivation, and daily torpor are generally viewed as being different manifestations of a single physiological process.  They are distinguished by differences in their durations and seasons of occurrence.  Hibernation is when an animal allows its body temperature to fall close to ambient temperatures for periods of several days or longer during winter, while estivation is this process during the summer, and daily torpor is the process for only part of each day, during any season (Hill et al., 1941).

It is extremely important to distinguish the differences between controlled hypothermia and hypothermia.  Any mammal, if exposed to a sufficiently low temperature, will increase its muscular activity, shiver, and increase its metabolic rate.  If the cold becomes too unbearable, or experienced for too long, exhausting will take place and the animal will begin to cool.  When a critical body temperature is reached, death will occur.  This process can occur in both hibernators and non hibernators, however, death takes place at a significantly lower temperature in hibernators than in the later ( ).

Hibernation, estivation, and daily torpor permit mammals and birds to escape the energy demands of homeothermy.  The animals that are capable of these processes are able to switch back and forth between two very different thermal temperatures; they are temporal heterotherms.  When the animals function as normal homeotherms, they are able to benefit from homeothermy; they have physiological independence of external thermal conditions.  However, when the animals suspend homeothermy, they take on many attributes of animals that cannot regulate their body temperature; their tissues are subjected to varying tissue temperatures, but they then have low energy needs (Hill et al., 1941).

Hibernation is known to occur in at least six different orders of mammals; species that hibernate include: hamsters, ground squirrels. Dormice, jumping mice, marmots, woodchucks, bats, bears, marsupials, and montromeres.  Because hibernation occurs in the winter, there is typically long preparation.  Hibernating mammals typically store considerable quantities of body fat during the months before their entry into hibernation.  However, this will talked about later on.  Estivation is not nearly as well understood as hibernation, mainly because it is not easy to detect.  However, it has been reported mostly in species of desert ground squirrels (Walsberg, 2000).  Daily torpor is widespread among mammals and birds, and it is present not only in species facing stress from cold, but also in species occupying tropical climates.  It occurs in numerous species of bats and rodents, certain hummingbirds, swallows, swifts, and so on.  Animals undergoing daily torpor are heterothermic for part of each day, and feed at that time (French, 1993).

Bats are among the animals that may have daily periods of torpor and may also hibernate for longer periods.  If exposed to low ambient temperature, bats may respond in of two different characteristic wats.  In North American Bats, at temperatures below 30℃, these bats can eitheer be torpid and have body temperatures within a degree or two of the air temperature, or they can be metabolically active with normal body temperatures between 32℃ and 36℃ ( ).  They oxygen

When speaking in quantitative terms, the amount of energy saved by controlled hypothermia depends on the ambient temperature at which hypothermia occurs and the duration of the period.  The amount of energy saved per unit of time becomes greater as the ambient temperature decreases.  If a hibernating animal remains in hibernation at low environmental temperatures for long periods of time, its total energy savings can be quite large.  For example, free-living ground squirrels of at least two species, living in cold climates, have been measured to expend only 10%-20% as much energy per month by hibernating as they would if they failed to hibernate; they are able to save this energy throughout their 7-8 month hibernating seasons(   ).

These biological functions also permit mammals and birds to escape the high water demands of homeothermy (Wang, 1989).  Although the escape from energy demands is much more significant, the escape from water demands can be the most important consequence of entering controlled hypothermia.  Homeotherms have relatively high rates of water loss for reasons such as that they must breathe rapidly to acquire the amounts of O2 they need for their high metabolic rates.  Another is that the air they exhale tends to be relatively warm air holds more water vapor.  Entry into controlled hypothermia reduces an animal’s rate of water by reducing both its breathing rate, and the temperature (and therefore the water vapor content) of its exhaled air (Hill et al., 1941).

Recent research has established that, in at least some mammalian hibernators, biochemical downregulation takes place during hibernation.  Until about two decades ago, the understanding was that animals initiate their entry into controlled hypothermia by turning off thermoregulation.  However, according to this view, during the entry into hypothermia, thermoregulation is deactivated, body temperature falls because of cooling by the environment in the absence of thermoregulatory responses, and tissue metabolic rates then decline because the tissues cool.  This lowering of metabolic rate-driven by tissue cooling and following the Q10 principle, and the animal is experiencing the “Q10 effect.”  Q10 is the change of rate over a 10 ~ temperature range (Geiser, 1987).  Researches have established a new view on controlled hypothermia, where the first step in the sequence of events is biochemical down regulation of tissue metabolism.  Body temperature then falls as a consequence of the reduced metabolic rate.  After biochemical downregulation initiates the fall of body temperature, the declining body temperature can potentially exert a Q10 effect that reinforces the biochemical downregulation in depressing metabolism (Hill et al., 2016).

Controlled hypothermia is under accurate physiological control.  In nature, the beginning of the hibernation cycle is usually associated with the time of the year.  The yearly cycle of hibernation is influenced by the duration of the daily light cycle and is associated with endocrine cycles.  Also most hibernators prepare for winter by excessive eating during late summer and autumn.  They take in more energy to sustain routine metabolism.  The excess is stored in two forms: lipids in normal adipose tissue (white fat), and in the form of the lipid of hypertrophied brown adipose tissue.  This white fat is a storage tissue; lipids are deposited in the tissue as fattening occurs and later are mobilized from the tissue to meet metabolic needs; this also includes the need to replenish lipids oxidized from brown fat.  The lipids are stored as triglycerides, which are accumulated in droplets within specialized cells called adipocytes.  Fat-storing hibernators eliminate food ingestion during the hibernation season, and instead rely on the products of this lipid hydrolysis as their primary fuel source (Carey et al., 2003).  These lipid reserves are the primary source of energy during hibernation.  Mature adult ground squirrels may increase their weight by 50-80% during August and September (Musacchia and Deavers, 1981).  There are also hibernators that store food, and they often do not put in weight in the autumn; instead, they must forage for food items.  However, all hibernating mammals possess quantities of brown adipose tissue, because it is crucial for the arousal process, mentioned later ().

The lipids that hibernators are storing reflect in their composition of fatty acids present in the foods that the animals are eating while prepping for their dormant period.  As stated, the lipids are stored as triglycerides.  Each triglyceride molecule is built from three fatty acid molecules (   ).  The fatty acids are saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids.  However, mammals are incapable of synthesizing polyunsaturated fats without the help of eating plants to use the polyunsaturated to use them as substrates of other polyunsaturated fats.  This relationship demonstrates the idea that the lipid composition of the diet of hibernators affects their hibernation.

Along with the buildup of fat or storage of food to prepare for the hibernating it state, it is generally accepted that the endocrine glands undergo changes as well.  At the time that mammals enter the hibernating state and during the hibernating period, the thyroid and adrenal gland are in an inactive state.  The ovaries and testes are atrophic through hibernation .  However, in the last few days of hibernation, the gonads of mammals such as woodchucks and ground squirrels show signs of increased activity (Lyman and Chatfield, ).

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