A hibernaculum (plural form: hibernacula) (Latin, "tent for winter quarters") is a place in which an animal seeks refuge, such as a bear using a cave to overwinter. The word can be used to describe a variety of shelters used by many kinds of animals, including insects, toads, lizards, snakes, bats, rodents, and primates of various species.
Insects
Insects range in their size, structure, and general appearance but most use hibernacula. All insects are primarily ectothermic. 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, which occurs at all stages of development and may include migration or hibernation 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. Insects prepare to overwinter through a variety of mechanisms, such as using anti-freeze proteins or cryoprotectants 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. 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 bugs
Some insects, like convergent lady bugs, 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 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 moths. 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. The 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 frogs 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
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