The cycle of seasons causes the climate to change all over the world sometimes causing extremes of weather from freezing winters to boiling summers. In temperate regions, seasons are clearly divided into four seasons: summer, autumn, winter and spring. But in tropical regions, temperature doesn’t fluctuate greatly, though rainforests can experience periods of heavier rain and even drier periods. In areas in the far north, seasonal changes between summer and winter are so quick that it is almost as though there are just two seasons.
Torpor in Mammals
Mammals must adapt or react to these seasonal changes and in some causes they hibernate or go into a state of torpor as a way to avoid these extremes in temperature or lack of food. Small mammals such as bats and shrews can allow their body temperatures to drop significantly when they sleep or when they become inactive. This is known as daily torpor and it is a way for these mammals to save energy.
Hibernation in Mammals
Hibernation is common in mammal orders Rodentia, Insectivora and Chiroptera. In the northern temperate regions, other mammals such as marmots, jumping mice, squirrels and woodchucks go into hibernation in winter to avoid the cold weather and the scarcity of food. They prepare by storing body fat prior to this period of dormancy. Body temperature drops slowly, then raises to normal, and then drops again as they prepare for hibernation. Metabolism decreases, as does respiratory rate and these mammals use shivering and non-shivering thermogenesis to produce heat.
Mammals in true hibernation would appear dead because their breathing is so slow and their temperature is so low, they would be cold to touch. Many mammals keep their temperature as low as 1 degree Celsius above ambient temperatures.
Large Mammals Don’t Hibernate
Contrary to popular belief, large mammals such as bears don’t actually hibernate. They enter a state of ‘winter sleep’ in which their body temperature drops just a few degrees. Though they sleep for long periods of time over winter, they may wake up occasionally to eat or urinate, whereas mammals in true hibernation will stay asleep throughout the whole of winter and rely on their body stores to keep them alive.
Due to their large mass, larger mammals can store energy reserves and go into winter sleep without the need to reduce their body temperature drastically. Winter sleep saves less energy than hibernation, however, they have less of a need to save metabolic fuels and because a large mammal would use too much energy trying to get its body temperature back up to normal.
Resource and further reading:
Randall, Burggren & French, 2002, ‘Energetic Costs of Meeting Environmental Challenges’, in Animal Physiology, Mechanism and Adaptations, ed. W.H. Freedman and Company, New York, chapter 17.
Hickman, Roberts, Larson & I’Anson, 2004, ‘Homeostasis’ in Integrated Principles of Zoology, 12th edition, ed. McGraw Hill, New York, chapter 30.
Roberta Goli - Roberta has a Bachelor of Science Degree (Zoology), has worked with animals for over 10 years and enjoys writing short stories and flash ...
