Why do some animals hibernate?

Some animals spend the entire winter asleep. Sounds pretty good, doesn't it? But how do they actually do this and why? Some animals, such as hedgehogs, bats, hazel dormice and hamsters, hibernate deeply, and are referred to as ‘true’ hibernators. Other animals, such as bears and badgers, engage in another form of hibernation referred to as ‘winter sleep’. What is the actual difference between true hibernation and winter sleep? We consulted a few experts to find out. 

Behavioural biologist Claudia Vinke: ‘During the cold winter months, many animals have a difficult time finding food. In order to deal with this lack of food, some animals engage in winter sleep. This helps them to get through this difficult period using the least amount of energy possible. This means that winter sleep is functional behaviour and part of the natural biorhythm.’

True hibernation or winter sleep? 

Margriet Montizaan of the Dutch Wildlife Health Centre (DWHC): ‘There is a difference between true hibernation and winter sleep. During hibernation, the body temperature drops quite far (to a few degrees above or even around freezing point), while the respiration and heart rate are set at a low level to conserve energy.’ 

Some animals sleep throughout the winter, while others wake up a few times. ‘How harsh the winter is also influences this. Like for true hibernators, the heart rate and respiration rate of animals engaging in winter sleep also drop. The body temperature, however, only drops by a few degrees. In addition, animals in winter sleep wake regularly to eat a bit. Bears and badgers are good examples of this, while squirrels will take cover for longer periods, especially during very cold days, and then leave their den on better days to retrieve food from their stockpiles,’ Montizaan explains. 

Do not confuse a hibernating animal with a dead one

This time of the year, you can encounter hibernating animals in the strangest places. Take hedgehogs, for example. Montizaan: ‘People often come across hibernating hedgehogs by accident, when gardening in the winter months, for instance clearing leaves and branches. Hedgehogs might very well have created their winter nest among these, since they do not hibernate out in the open.’ Sleeping hedgehogs are easy to confuse with dead ones, since they breathe very infrequently and irregularly in hibernation. They also feel cold and can even be as rigid as if dead. In addition, the animal will hardly respond to stimuli, even if disturbed. They will only move their spines very subtly and sometimes make a soft blowing sound. Montizaan: ‘In such cases it is important to leave the hedgehog alone and to cover it up again. It will then return to its state of hibernation of its own accord.’ 

Specialist veterinarian Yvonne van Zeeland is well acquainted with this description, albeit in relation to hamsters and golden hamsters. ‘Hamsters going into hibernation when it becomes too cold indoors in a home, and then being thought to be dead, are a well-known problem. So it's important to check for signs of life and in some cases perhaps have a vet take a look to determine whether an animal is actually truly dead.’