Hibernation
Although traditionally reserved for "deep" hibernators such as rodents, the term has been redefined to include animals such as bears[1] and is now applied based on active metabolic suppression rather than any absolute decline in body temperature.
Many experts believe that the processes of daily torpor and hibernation form a continuum and utilise similar mechanisms.
Before entering hibernation, animals need to store enough energy to last through the duration of their dormant period, possibly as long as an entire winter.
Larger species become hyperphagic, eating a large amount of food and storing the energy in their bodies in the form of fat deposits.
[8] Many insects, such as the wasp Polistes exclamans and the beetle Bolitotherus, exhibit periods of dormancy which have often been referred to as hibernation, despite their ectothermy.
Obligate hibernators include many species of ground squirrels, other rodents, European hedgehogs and other insectivores, monotremes, and marsupials.
The cause and purpose of these arousals are still not clear; the question of why hibernators may return periodically to normal body temperatures has plagued researchers for decades, and while there is still no clear-cut explanation, there are multiple hypotheses on the topic.
[11] Other theories postulate that brief periods of high body temperature during hibernation allow the animal to restore its available energy sources[12] or to initiate an immune response.
[26] Despite long-term inactivity and lack of food intake, hibernating bears are believed to maintain their bone mass and do not suffer from osteoporosis.
[30] In a 2016 study, wildlife veterinarian and associate professor at Inland Norway University of Applied Sciences, Alina L. Evans, researched 14 brown bears over three winters.
Once in their dens, the bears' heart rate variability dropped dramatically, indirectly suggesting metabolic suppression is related to their hibernation.
[31] Ancient people believed that swallows hibernated, and ornithologist Gilbert White documented anecdotal evidence in his 1789 book The Natural History of Selborne that indicated the belief was still current in his time.
It is now understood that the vast majority of bird species typically do not hibernate, instead utilizing shorter periods of torpor.
[33][34] Because they cannot actively down-regulate their body temperature or metabolic rate, ectothermic animals (including fish, reptiles, and amphibians) cannot engage in obligate or facultative hibernation.
They can experience decreased metabolic rates associated with colder environments or low oxygen availability (hypoxia) and exhibit dormancy (known as brumation).
Other animals able to survive long periods with very little or no oxygen include goldfish, red-eared sliders, wood frogs, and bar-headed geese.
[38] Hibernation induction trigger (HIT) proteins isolated from mammals have been used in the study of organ recovery rates.
One study in 1997 found that delta 2 opioid and hibernation induction trigger (HIT) proteins were not able to increase the recovery rate of heart tissue during ischemia.
While unable to increase recovery rates at the time of ischemia, the protein precursors were identified to play a role in the preservation of veterinary organ function.
[45] As the ancestors of birds and mammals colonized land, leaving the relatively stable marine environments, more intense terrestrial seasons began playing a larger role in animals' lives.
[46] This is true for all clades of animals that undergo winter dormancy; the more prominent the seasons are, the longer the dormant period tends to be on average.
In order to conserve energy, the ancestors of birds and mammals would likely have experienced an early form of torpor or hibernation when they were not using their thermoregulatory abilities during the transition from ectothermy to endothermy.
[48] Body size also had an effect on the evolution of hibernation, as endotherms which grow large enough tend to lose their ability to be selectively heterothermic, with bears being one of very few exceptions.
Fish that undergo winter dormancy in oxygenated water survive via inactivity paired with the colder temperature, which decreases energy consumption, but not the base metabolic rate that their bodies consume.
Most fish that are dormant in the winters save enough energy by being still and so there is not a strong selective pressure to develop a metabolic suppression mechanism like that which is necessary in hypoxic conditions.
