Hibernation is a state of inactivity and metabolic depression in endotherms. Hibernation refers
to a season of heterothermy that is characterized by low body temperature, slow breathing and
heart rate, and low metabolic rate. Although traditionally reserved for "deep" hibernators
such as rodents, the term has been redefined to include animals such as bears and is now
applied based on active metabolic suppression rather than based on absolute body temperature
decline. Many experts believe that the processes of daily torpor and hibernation form a continuum
and utilize similar mechanisms. Hibernation during the summer months is known as aestivation.
Some reptile species are said to brumate, or undergo brumation, but any possible similarities
between brumation and hibernation are not firmly established.
Often associated with low temperatures, the function of hibernation is to conserve energy
during a period when sufficient food is unavailable. To achieve this energy saving, an endotherm
will first decrease its metabolic rate, which then results in a decreased body temperature.
Hibernation may last several days, weeks, or months depending on the species, ambient
temperature, time of year, and individual's body condition.
Before entering hibernation, animals need to store enough energy to last the entire
winter. Larger species become hyperphagic and eat a large amount of food and store the
energy in fat deposits. In many small species, food caching replaces eating and becoming
fat. Some species of mammals hibernate while gestating young, which are either born while
the mother hibernates or shortly afterwards. For example, the female polar bear goes into
hibernation during the cold winter months to give birth to her offspring. She loses
15-27% of her pre-hibernation weight and uses stored fats for energy during times of food
scarcity, or hibernation. It is evident that pregnant female polar bears significantly
increased body mass prior to hibernation, and this increase is further reflected in
the weight of their offspring. The fat accumulation prior to hibernation in the female polar bear
enables them to provide a sufficient and warm nurturing environment for their newborn.
Hibernating Animals Primates
While hibernation has long been studied in rodents, namely ground squirrels, no primate
or tropical mammal was known to hibernate prior to the discovery that the fat-tailed
dwarf lemur of Madagascar hibernates in tree holes for seven months of the year. Malagasy
winter temperatures sometimes rise to over 30 °C, so hibernation is not exclusively
an adaptation to low ambient temperatures. The hibernation of this lemur is strongly
dependent on the thermal behaviour of its tree hole: if the hole is poorly insulated,
the lemur's body temperature fluctuates widely, passively following the ambient temperature;
if well insulated, the body temperature stays fairly constant and the animal undergoes regular
spells of arousal. Dausmann found that hypometabolism in hibernating animals is not necessarily
coupled to a low body temperature. Bears
For many decades it remained controversial whether bears actually hibernated, because
over-wintering bears only experienced a modest drop in core-body temperature compared to
smaller animals. What defines hibernation, however, is not the degree of temperature
reduction, but the metabolic suppression. Adult bears can, however, lower metabolic
rate to some 75% below basal metabolic rates, which indicates that bears are hibernators.
Indeed, northern-most bears will neither eat nor drink for periods as long as 8 months,
relying only on stored body-fat reserves for energy and water. Though it is believed that
bear hibernation is very different from either rodent or primate hibernation and involves
temperature-independent metabolic suppression, because the modest decreases in core temperature
do not account for the large decrease in metabolic rate, this belief does not consider the effect
of metabolic reductions that can occur through extensive peripheral vasoconstriction. For
example, it is known that peripheral tissues contribute as much as 50% to metabolism. This
discrepancy alone would be sufficient to account for the `missing´ proportion and without
having to resort to more esoteric physiologic mechanism. This effect has been observed in
other torpid metabolic states, like diving. In diving penguins and seals, for example,
metabolic rate can be lowered without resorting to any core temperature decreases merely through
extensive vasoconstriction of peripheral tissue beds.
They are able to recycle their proteins and urine, allowing them to both stop urinating
for months and stop muscle atrophy. Obligate hibernators
Obligate hibernators are defined as animals that spontaneously, and annually, enter hibernation
regardless of ambient temperature and access to food. Obligate hibernators include many
species of ground squirrels, other rodents, mouse lemurs, the European hedgehog and other
insectivores, monotremes, marsupials, and even butterflies such as the small tortoiseshell.
These undergo what has been traditionally called "hibernation": the physiological state
where the body temperature drops to near ambient temperature, and heart and respiration rates
slow drastically. The typical winter season for these hibernators is characterized by
periods of torpor interrupted by periodic, euthermic arousals, wherein body temperatures
and heart rates are restored to euthermic levels. The cause and purpose of these arousals
is still not clear. The question of why hibernators may experience
the periodic arousals has plagued researchers for decades, and while there is still no clear-cut
explanation, there are myriad hypotheses on the topic. One favored hypothesis is that
hibernators build a 'sleep debt' during hibernation, and so must occasionally warm up in order
to sleep. This has been supported by evidence in the arctic ground squirrel. Another theory
states that the brief periods of high body temperature during hibernation are used by
the animal to restore its available energy sources. Yet another theory states that the
frequent returns to high body temperature allow mammals to initiate an immune response.
Hibernating arctic ground squirrels may exhibit abdominal temperatures as low as -2.9 °C,
maintaining sub-zero abdominal temperatures for more than three weeks at a time, although
the temperatures at the head and neck remain at 0 ˚C or above.
Historically there was a question of whether or not bears truly hibernate, since they experience
only a modest decline in body temperature compared with what other hibernators undergo.
Many researchers thought that their deep sleep was not comparable with true, deep hibernation.
This theory has been refuted by recent research in captive black bears.
Facultative hibernation Unlike obligate hibernators, facultative hibernators
only enter hibernation when either cold stressed or food deprived, or both. A good example
of the differences between the two types of hibernation can be seen among the prairie
dogs: the white-tailed prairie dog is an obligate hibernator and the closely related black-tailed
prairie dog is a facultative hibernator. Hibernating birds
Historically, Pliny the Elder believed swallows hibernated, and ornithologist Gilbert White
pointed to anecdotal evidence in The Natural History of Selborne that indicated as much.
Birds typically do not hibernate, instead utilizing torpor. One known exception is the
common poorwill, first documented by Edmund Jaeger.
Dormancy in fish Fish are ectothermic, and so, by definition,
cannot hibernate because they cannot actively down-regulate their body temperature or their
metabolic rate. However, they can experience decreased metabolic rates associated with
colder environments and/or low oxygen availability and can experience dormancy. For a couple
of generations during the 20th century it was thought that basking sharks settled to
the floor of the North Sea and became dormant. Research by Dr David Sims in 2003 dispelled
this hypothesis, showing that the sharks actively traveled huge distances throughout the seasons,
tracking the areas with the highest quantity of plankton. The epaulette sharks have been
documented to be able to survive for long periods of time without oxygen, even being
left high and dry, and at temperatures of up to 26 °C. Other animals able to survive
long periods without oxygen include the goldfish, the red-eared slider turtle, the wood frog,
and the bar-headed goose. However, the ability to survive hypoxic or anoxic conditions is
not the same, nor closely related, to endotherm hibernation.
Hibernation induction trigger Hibernation induction trigger is a bit of
misnomer. Although research in the 1990s hinted at the ability to induce torpor in animals
by injection of blood taken from a hibernating animal, further research has been unable to
reproduce this phenomenon. Despite the inability to induce torpor, there are substances in
hibernator blood that can lend protection to organs for possible transplant. Researchers
were able to prolong the life of an isolated pig's heart with a HIT. This may have potentially
important implications for organ transplant, as it could allow organs to survive for up
to 18 or more hours, outside the human body. This would be a great improvement from the
current 6 hours. This supposed HIT is a mixture derived from
serum, including at least one opioid-like substance. DADLE is an opioid that in some
experiments has been shown to have similar functional properties.
Human hibernation
Hibernation, and the species that are able to utilize it, have become fantastic models
for many different human diseases. Hibernators make natural models for stroke, ischemia-reperfusion
injury, diabetes, obesity, and depression. References
Further reading Carey, H.V., M.T. Andrews and S.L. Martin.
2003. Mammalian hibernation: cellular and molecular responses to depressed metabolism
and low temperature. Physiological Reviews 83: 1153-1181.
External links Hibernaut
Do Black Bears Hibernate? Freeze avoidance in a Mammal: Body Temperatures
Below 0°C in an Arctic Hibernator Potential medical usage
Harvested human Lung Preservation With the Use of Hibernation Trigger Factors