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A hibernaculum ''plural form: hibernacula'' (Latin, "tent for winter quarters") is a place in which a creature seeks refuge, such as a bear using a cave to
overwinter Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activi ...
. The word can be used to describe a variety of shelters used by many kinds of animals, including
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...
s,
toad Toad is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands. A distinction between frogs and toads is not made in scient ...
s,
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...
s,
snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more ...
s, bats,
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s, and
primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...
s of various species.


Insects

Insects range in their size, structure, and general appearance but most use hibernacula. All insects are primarily
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...
. For this reason, extremely cold temperatures, such as those experienced in the winter, outside of tropical locations, cause their metabolic systems to shut down; long exposure may lead to death. Insects survive colder winters through the process of
overwintering Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal act ...
, which occurs at all stages of development and may include
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
or
hibernation Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It mos ...
for different insects, the latter of which must be done in hibernacula. Insects that do not migrate must halt their growth to avoid freezing to death, in a process called
diapause In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions.Tauber, M.J., Tauber, C.A., Masaki, S. (1986) ''Seasonal Adaptations of Insects''. Oxford University Press I ...
. Insects prepare to overwinter through a variety of mechanisms, such as using anti-freeze proteins or
cryoprotectant A cryoprotectant is a substance used to protect biological tissue from freezing damage (i.e. that due to ice formation). Arctic and Antarctic insects, fish and amphibians create cryoprotectants ( antifreeze compounds and antifreeze proteins) in ...
s in freeze-avoidant insects, like soybean aphids. Cryoprotectants are toxic, with high concentrations only tolerated at low temperatures. Thus, hibernacula are used to avoid sporadic warming and the risk of death due high concentrations of cryoprotectants at warmer temperatures. Freeze-tolerant insects, like second-generation corn-borers, can survive being frozen and therefore, undergo inoculative freezing."How insects survive cold weather"
(March 29, 2012). Western Farm Press.
Hibernacula range in size and structure depending on the insects using them. However, insect hibernacula are generally required to be: * Well-insulated from extreme temperature changes * Protected from weather * Dry (except for freeze-tolerant insects)


Lady beetles

Some insects, like convergent lady beetles, reuse the same hibernacula, year after year. They converge with other lady beetles and migrate to hibernacula used by prior generations. They are able to find old hibernacula due to hydrocarbons released by lady beetle feet which create a lasting path. This allows lady beetles to retrace their footsteps to previously used hibernacula. Their tendency to aggregate and overwinter in groups is likely due to their attraction to similar environments and conspecifics. Beetles use rock crevices as hibernacula, often clumping in them, in groups. These rock crevices are found in rock fields the beetle are attracted to for high levels of vegetation and greenery.


Other insects

Other types of insect hibernacula include self-spun silk hibernacula, such as those made and used by
spruce budworms ''Choristoneura'' is a genus of moths in the family Tortricidae. Several species are serious pests of conifers, such as spruce and are known as spruce budworms. Species *'' Choristoneura adumbratanus'' (Walsingham, 1900) *'' Choristoneura af ...
as they moult and overwinter in their second instars. An example is the eastern spruce budworm which creates hibernacula after dispersing during its first instar then overwinter before emerging from the hibernacula in early May. Woolly bear caterpillars overwinter as caterpillars and grow to be
isabella tiger moth ''Pyrrharctia isabella'', the isabella tiger moth, whose larval form is called the banded woolly bear, woolly bear, or woolly worm, occurs in the United States and southern Canada. It was first formally named by James Edward Smith in 1797. ...
s. They use plant debris as makeshift hibernacula, to protect themselves from extreme elements. Some butterflies, like the white admiral butterfly also only mature halfway as a caterpillar before hibernating for the winter. For freeze-avoidant insects, ideal hibernacula are dry, as freeze-avoidant insects are less likely to dampen and freeze in them, however moist hibernacula promote inoculative freezing for freeze-tolerant insects.


Amphibians

Amphibians that hibernate include several species of frogs and salamanders from the northern continental climates of North America and Eurasia and also from extreme Southern Hemisphere climates. These amphibians slow their metabolism during winter to avoid unsuitable conditions, such as freezing. Most freeze avoidance strategies include overwintering in aquatic situations or burrowing in the soil to depths below the frostline. A hibernaculum for amphibians should provide the following: * Optimum temperatures * Maintenance of oxygen and humidity levels * Low-intensity, short-photoperiod lighting * Minimum disturbance Species from cool continental climates hibernate at temperatures from 0 to 4 °C. Some species will not survive hibernation at temperatures that exceed 4 °C. Generally, for amphibians that hibernate under ice, it is necessary for the animal to be submerged in water that is 10 to 15 cm deep and to maintain the temperature between 2 and 3 °C and not above 4 °C. Water should be well aerated, with maintained low-intensity light levels and minimal disturbance of the amphibians.


Frogs

Like other amphibians, frogs show minimal capacities for freezing tolerance and survive winter by using terrestrial hibernacula where they avoid freezing. However, frogs may exhibit greater freeze-tolerance capacity at high latitude range limits, where winter climate is more severe. For example, data suggests that while
cricket frog Cricket frogs, genus ''Acris'', are small, North American frogs of the family Hylidae. They occur in northern Mexico (Coahuila), the United States east of the Rocky Mountains, and in southern Ontario, Canada. They are more aquatic than other m ...
s in South Dakota survive winter by locating hibernacula that prevent freezing, their toleration of short freezing bouts may expand the range of suitable hibernacula. However, overwinter mortality may be high at the northern range boundary due to colder temperatures and might limit cricket frogs from expanding their range northward. The microclimate refers to the climate of a very small or restricted area, like the hibernaculum, especially when this differs from the climate of the surrounding area. Overwinter survival in these cricket frogs among other frogs is dependent on using hibernacula with appropriate physical microclimate characteristics, such as moist soil, that buffer frogs from temperatures that drop below the freezing point of the body fluids for extended periods. Although, determining if frogs can identify sites with appropriate microclimates to support overwinter survival and what factors might inform such choices are still unknown and will require further study. Therefore, it is still not known to what extent various types of prospective hibernacula for frogs might be suitable in the years to come, especially factoring in climate change.


Newts

As part the Highways Agency Biodiversity Action Plan (HABAP) in the UK, the Species Action Plan (SAP) for
great crested newts The northern crested newt, great crested newt or warty newt (''Triturus cristatus'') is a newt species native to Great Britain, northern and central continental Europe and parts of Western Siberia. It is a large newt, with females growing up to ...
aims to maintain and enhance existing newt populations through appropriate management of suitable habitat. As part of steps to implement the HABAP, newt hibernacula (e.g. log piles) have been constructed to improve the quality of the terrestrial habitat through increasing the number of potential overwintering sites. It was also determined that habitat surrounding breeding ponds with plenty of cover and suitable overwintering sites may have less need for provision of artificial hibernacula than landscapes with less woodland, hedgerows, scrub etc. Because great crested newts show high loyalty to over-wintering locations, returning to such established areas year after year, artificial hibernacula could be important in future years to conserve newts and other amphibians. Although, monitoring in the vicinity of these hibernacula in autumn using felt roofing tiles did not reveal the presence of any great crested newts even though they are known to breed in nearby ponds. Common toads and frogs did surround the area however. Therefore, further studies need to be conducted in order to create species-appropriate artificial hibernacula.


Reptiles

Many reptiles undergo hibernation or a process called
brumation Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clos ...
, which is similar to hibernation; both processes require usage of a hibernaculum. Staying inside an insulated hibernaculum is a strategy to avoid the harsh winter months when the frigid outside temperatures may kill an
ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
ic reptile. They depress their metabolism and heart rates to reduce energy consumption so they don't need to exit their hibernacula. Hibernating reptiles are also safer from predation inside of their concealed and protected hibernacula. Various species of turtles, snakes, and lizards all use hibernacula, the forms of which can vary greatly. Hibernacula are typically: * Underground * Below the frost line * Well-insulated * Tunnels (i.e. snakes) or cavities large enough for the body (i.e. lizards)


Turtles

Common snapping turtle The common snapping turtle (''Chelydra serpentina'') is a species of large freshwater turtle in the family Chelydridae. Its natural range extends from southeastern Canada, southwest to the edge of the Rocky Mountains, as far east as Nova Scotia ...
s generally hibernate for about six months from early October to mid-April. They live in lakes during their active months, then travel to small offshoot streams to hibernate. Hibernacula are about 100–150 meters away from the main body of the home lake. Most snapping turtles hibernate by burrowing into the banks of
alder Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
streams or vegetated streams, but some use other structures such as abandoned beaver dens. These streams are typically less than 0.3 m deep and 0.7 m wide, covered by sunken alder roots or fallen trees, and not covered by ice in the winter. Many animals return to the same stream to hibernate in subsequent years.


Snakes

Unlike more solitary snapping turtles, snakes may either hibernate alone or in large aggregations of up to several thousand individuals of the same or different species. They use a wide variety of hibernacula, including: rock piles, debris-filled wells, caves, crevices, unused burrows made by other animals, and ant mounds. The
common European viper ''Vipera berus'', the common European adderMallow D, Ludwig D, Nilson G. (2003). ''True Vipers: Natural History and Toxinology of Old World Vipers''. Malabar, Florida: Krieger Publishing Company. . or common European viper,Stidworthy J. (1974). ...
has actually been observed using all of the hibernacula listed above. Most species seem to prefer finding an already-present suitable site rather than constructing one of their own, but they do expand upon present structures and may make their own burrows if there aren't any quality sites available. Pine snakes and the closely related Louisiana pine snakes are two of the most well-studied hibernating snake species, and share similar hibernacula characteristics. These species sometimes construct their own burrows, or use tunnels formed from the decay of tree roots or by gophers. The tunnels form complex networks, and have side chambers which each house one snake.


Lizards

Mesquite lizards in Mexico and the southern United States have been found hibernating in groups of 2 to 8 in cracks or under slabs of bark in mesquite trees. Common collared lizards spend about 6 months hibernating, almost always solitarily, though pairs of juvenile females have been observed within the same hibernaculum. They use the undersides of rock slabs as hibernacula, digging a small chamber in the dirt just large enough for their bodies with a small tunnel for outside access. Adults use larger rock slabs, dig deeper chambers, and have longer tunnels than juveniles. Perhaps the most extreme example is seen in the
viviparous lizard The viviparous lizard, or common lizard, (''Zootoca vivipara'', formerly ''Lacerta vivipara''), is a Eurasian lizard. It lives farther north than any other species of non-marine reptile, and is named for the fact that it is viviparous, meaning ...
, the most northern of all lizards. They can burrow into the soil, go under leaf litter, or use shelters like rocks as hibernacula. Although the air temperature in West Siberia can drop to −10 °C, the soil temperature at the depths where these lizards hibernate remains higher than −10 °C. This enables them to survive the harshest temperatures of any lizard.


Mammals

Like other animals, mammals hibernate during seasons of harsh environmental conditions and resource scarcity. As it requires less energy to maintain
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
and survive when an individual is hibernating, this is a cost-effective strategy to increase survival rates.Kobbe S. & Dausmann K. H. (2009). "Hibernation in Malagasy mouse lemurs as a strategy to counter environmental challenge". ''Naturwissenschaften''. 96 (10): 1221–1227. Jump up^ Hibernation is usually perceived as taking place during winter, as in the most well-known hibernators bears and bats, but can also occur during the dry season when there is little food or water, as in the mouse lemurs of Madagascar. Given that mammals can spend anywhere from 1 to 9 months hibernating, their choice in hibernaculum is essential in determining their survival. Hibernacula vary greatly, but are typically: * Underground (i.e. ground squirrels, mouse lemurs, bears) or in a protected shelter (i.e. bats, bears) * Concealed * Attuned to environmental conditions, such as temperature * Either reused consistently (i.e. bats, ground squirrels) or very rarely (i.e. bears, mouse lemurs)


Bears

Many bears occupy similar hibernacula to smaller mammals, but theirs are, of course, much larger and can vary greatly across and within species. Most black bears excavate dens into a hillside or at the base of a tree, stump, or shrub, but some make dens at the bases of hollow trees, in hollow logs, or in rock caves or cavities. Den reuse is observed in this species, but very rarely. There were no significant den size differences between age or sex classes, except adult males creating larger entrances.
Grizzly bear The grizzly bear (''Ursus arctos horribilis''), also known as the North American brown bear or simply grizzly, is a population or subspecies of the brown bear inhabiting North America. In addition to the mainland grizzly (''Ursus arctos horri ...
s likewise don't show age or sex class differences in den dimensions. Grizzlies prefer hibernacula sites with abundant ground and canopy cover, and abundant sweet-vetch.
Polar bear The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear spec ...
s differ from black bears, grizzlies, and other bear species where both sexes hibernate in that only females use hibernacula. Like other female bears, polar bears use hibernacula as
maternity den A maternity den, in the animal kingdom, is a lair where the mother gives birth and nurtures the young, when they are in a vulnerable life stage. While such dens are typically subterranean, they may also be snow caves or simply beneath rock ledges ...
s. Also like other species, they tend to dig dens into the earth, although their Arctic hibernacula are usually covered with snow by the time they emerge.


Bats

Bats favor larger hibernacula where large groups may roost together, including natural caves, mines, cellars, and other kinds of underground sites and man-made structures, like ice-houses. Within these hibernacula, the bats are still highly tuned to environmental factors. Little brown bats in northern latitudes hibernate for up to eight months during the winter, and leave their roosts in the warm spring weather when insect prey is plentiful again. Bats gauge the outside temperature by being attuned to the airflow at the hibernacula entrance, which is driven by temperature differences between inside and outside the hibernacula, allowing bats to leave when the temperature begins to warm. Some hibernacula are shared between multiple species, such as
common pipistrelle The common pipistrelle (''Pipistrellus pipistrellus'') is a small pipistrelle microbat whose very large range extends across most of Europe, North Africa, South Asia, and may extend into Korea. It is one of the most common bat species in the ...
s roosting with
soprano pipistrelle The soprano pipistrelle (''Pipistrellus pygmaeus'') is a small species of bat. It is found in Europe and often roosts on buildings. Taxonomy Until 1999, the soprano pipistrelle was considered as conspecific with the common pipistrelle The co ...
s. Behavior other than hibernating can also occur at hibernacula; common pipistrelles produce most of their mating calls and mate with each other in and near their hibernacula.


Other small mammals

Many hibernating, small-bodied mammals hibernate in simple holes in the ground, though some use complex systems of tunnels and burrows.
Mountain pygmy possum The mountain pygmy possum (''Burramys parvus''); also simply known as the burramys, is a small, mouse-sized (weighs ) nocturnal marsupial of Australia found in dense alpine rock screes and boulder fields, mainly southern Victoria and around Moun ...
s in New South Wales, Australia, dig holes in the ground to form hibernacula, with the preferred location being in boulder fields under a layer of snow. During the first few months of hibernation, possums awaken occasionally and leave one hibernaculum in favor of another, seemingly in an effort to find the hibernaculum with the most suitable
microclimate A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often with a slight difference but sometimes with a substantial one. The term may refer to areas as small as a few squ ...
. The
reddish-gray mouse lemur The reddish-gray mouse lemur (''Microcebus griseorufus'') also known as the gray-brown mouse lemur or rufous-gray mouse lemur, is a small, solitary-but-social, nocturnal and omnivorous primate from the Western Madagascar in the region around B ...
also wakes and leaves the hibernaculum spontaneously and for brief periods of time. Their hibernacula are located in holes in large trees with varying levels of insulation. However, the range of insulation levels is relatively narrow, as there are often small numbers of suitably large trees. There can be hibernacula differences even within a species. In
Columbian ground squirrel The Columbian ground squirrel (''Urocitellus columbianus'') is a species of rodent common in certain regions of Canada and the northwestern United States. It is the second largest member of the genus ''Urocitellus'', which is part of the tribe M ...
s, hibernacula size is proportional to the weight of the individual occupying it, with adults having deeper hibernacula than juveniles, unlike black bears. Most juveniles choose to hibernate within 20 meters of their mother's burrow; those that don't have higher mortality rates.


See also

*
Dead hedge A dead hedge is a barrier constructed from cut branches, saplings, and foliage. The material can be gathered from activities such as pruning or clearing, and in traditional forms of woodland management, such as coppicing. Its ecological succession ...
*
Insect hotel An insect hotel, also known as a bug hotel or insect house, is a manmade structure created to provide shelter for insects. They can come in a variety of shapes and sizes depending on the specific purpose or specific insect it is catered to. Mo ...


References

{{Authority control Hibernaculum Ethology Winter