2013

The super powers of forest animals– hibernation

Winter gradually succumbs to spring, which means that “winter sleepers” will soon become active again. Many invertebrates, some fish, nearly all amphibians, all reptiles and several mammals spend the winter season in Latvia in hibernation.

Hibernation is the ability of animals to adapt when sufficient food is scarce and temperatures are low. It is their ability to zone out from active life and sleep through winter. How do poikilothermic animals (animals whose internal temperatures depend on ambient temperatures) spend winter?  Their body temperatures constantly change – increase or decrease depending on whether their surroundings – air, water, soil – heat up or cool down.

All creatures are poikilothermic, except birds and mammals. Can you imagine what happens if these animals do not manage to find a warm place to stay during winter? They freeze to death. They die because their bodies are full of liquid which expands in freezing temperatures, tearing down tissue and cells. Driven by their desire to survive, these animals prepare for winter duly and to the best of their ability.

Many species of beetles, ants, mosquitoes, green lacewings, female bumblebees and wasps, as well as several species of butterflies, moths and other insects, spend winter in their adult forms hiding. They give themselves plenty of time to choose their hideouts. Wintering spots - warm enough for water not to freeze – are sought before the winter season. With frost on the horizon, insects fall into deep winter sleep – a state of suspended animation or anabiosis. Those insects which live only one season – grasshoppers, crickets and many species of butterflies – rush to lay eggs before dying. Many insects spend winter in larval or pupal stages. Houseflies, however, can spend winter in egg, larval or pupal stages.

Among warm-blooded animals – homeothermic animals or animals able to maintain constant body temperatures, only several species of mammals spend winter in hibernation – they are incapable of finding food during the winter period. Hibernating homeothermic animals display heterothermy – they are able to lower their body temperatures and metabolic rates to save energy. Winter sleep is essential for these animals. Therefore their organisms gradually adapt to functioning in hibernation. Hibernating homeothermic animals actively prepare for hibernation – they seek suitable hideouts and store extra food in the form of adipose tissue. These animals have two types of adipose tissue – white adipose tissue which mostly performs thermal insulation functions and serves as a store of energy and brown adipose tissue which quickly generates body heat after hibernation in spring.

Hibernating mammals’ wintering spots must be warm enough for them not to freeze and cold enough to slow down their metabolic rates and ensure that their stored energy lasts until spring and is not used up prematurely. Humidity also plays an important role.

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When hibernating, mammals gain the required amounts of energy to sustain life by consuming previously stored fat. Their weight reduces during the entire period at the expense of adipose tissue and energy stored in various body organs. It is no wonder that some mammals lose 30, 40 or even 50 percent of their weight by spring. Animals do not grow during winter sleep. Their fur and claws cease to grow as well and all organ functions and metabolic processes slow down. Their organisms consume little amounts of oxygen by breathing slowly and irregularly and respiratory rates reduce to a single breath per many minutes. Blood circulation is minimal, and the duration of hibernation depends on species’ resistance to frost.

It is necessary to emphasize that only some animals of our fauna spend winter in obligatory hibernation - deep and lethargic sleep. Those are both species of hedgehogs – southern white-breasted hedgehogs and European hedgehogs, four species of dormice – hazel dormice, fat dormice, garden dormice and forest dormice, northern birch mice and eight species of bats which do not leave for warmer regions in autumn – brown long-eared bats, Daubenton’s bats, pond bats, Brandt’s bats, whiskered bats, Natterer’s bats, northern bats and western barbastelle bats. These mammals are obligatory hibernators – they do so regardless of weather conditions.

There are also permissive hibernators – they have the option to hibernate when suffering from hunger and cold, as opposed to doing so at the appropriate time. They wake up in winter to stretch their legs and try to find something to eat. Their sleep is relatively light and body temperatures are only several degrees lower. Nearly all of their body systems function, however, not as intensively as during the warm period. Permissive hibernators can wake up to disturbances, thaw and noises. Light sleep does not prevent them from storing considerable subcutaneous fat reserves in autumn. In Latvia, brown bears, raccoon dogs and European badgers spend the cold season in permissive hibernation.