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Hibernation is when an animal spends many weeks in a dormant state in a small, protected area. As it sleeps, the animal uses very little energy. As a result, it does not need to eat to obtain more energy. If it has eaten enough food during the year and stored energy in the form of body fat, the animal will sleep until spring without eating anything. One secret to being able to do this is that the animal succeeds in lowering its body temperature.
Many small mammals hibernate over the winter months because they cannot find enough food to give them the energy they need to keep warm. Such animals rely on vegetation or insects for food. Since the populations of these food sources diminish in the winter months, the animals which eat them would die of hunger if they did not hibernate.
The Chipmunk - a Small Mammal Which Hibernates
Chipmunks, also called ground squirrels, normally have a body temperature of 37°C. They dig underground tunnels and in one of these tunnels they build a nest where they will pass the winter months. In this tunnel, a chipmunk is protected from its enemies and benefits from a constant temperature above freezing. In order to survive this long period of sleep without food, a chipmunk eats enough during the summer months to double its body mass.
Several factors may start an animal thinking about hibernating for the winter.
The chipmunk settles down in its nest of grasses and leaves and curls up into a ball with its fur fluffed up. The following body changes occur:
Hibernation is not always a successful way of passing winter. It has been estimated that two-thirds of the ground squirrels in North America die during hibernation. They die because their body runs out of food reserves or because a predator such as a fox finds them while they are asleep. This is why an animal which is hibernating should never be disturbed. If it wakes up it will lose valuable energy reserves.
The Hedgehog - Another Hibernating Mammal
The hedgehog is a mammal which hibernates from October to April. In winter its body temperature falls to as low as 10 breaths per minute. The normal pulse rate for the hedgehog is about 100 beats per minute, during hibernation this slows down to only 10 beats per minute.
Hibernating mammals carefully control their body temperature. Their body temperature does not fall to the point of freezing, though, as this would kill the animal. The breathing rate and the heart rate slow down too.
The brown bear that sleeps during the winter is not really a hibernating animal. Its body temperature only falls a few degrees and its pulse rate and breathing rate remain the same. Squirrels do not hibernate either; they sleep in nests in trees. They will come out from time to time to search for food. Squirrels are well known for hiding away food in the autumn, which they will find and eat in winter. When the squirrel is looking for its hidden store of food it searches by smell rather than remembering where it hid the food.
Bats hibernate throughout winter when their body temperature drops to about 4°C. However, bats and hummingbirds are animals which become torpid every day. When bats are awake and hunting for food their body temperature is about 40°C. When they return home to roost in a cave or under a roof, their body temperature drops to the temperature of the air. Bats hunt at dawn and dusk so every day they have two sleep periods and two hunting periods. Hummingbirds are active, feeding on nectar, during the day and they become torpid at night. This saves them a lot of energy. When they are flying, hummingbirds may consume 180 cm3 of oxygen per hour but at night they only consume about 6 cm3 of oxygen per hour.
The graph below show the respiration rate of a humming bird at different times of the day and the night (shaded).