The main function of hibernation and daily torpor in heterothermic mammals and birds (i.e. species capable of expressing torpor) is to conserve energy and water and thus to survive during adverse environmental conditions or periods of food shortage no matter if they live in the arctic or the tropics. However, the reduced energy requirements also permit survival of bad weather during reproduction to prolong gestation into more favourable periods, conservation of nutrients for growth during development, and overall result in reduced foraging needs and thus exposure to predators, which appear major contributing reasons why heterotherms are often long lived and have lower extinction rates than strictly homeothermic species that cannot use torpor. Known heterothermic mammals and birds are diverse with about 2/3 of mammalian orders and 1/3 of avian orders containing heterothermic species, and their number continues to grow.

Key Concepts:

• Hibernation and daily torpor are the most effective means of energy conservation available to mammals and birds and are crucial for survival of adverse conditions of many species.
  
• Use of torpor often is enhanced by low ambient temperatures and limited food.
  
• Because torpor reduces energy requirements, its opportunistic use allows extension of gestation, nutrient sparing during development, and permits survival in modified and degraded habitats and also reduces the need for foraging and thus exposure to predators.
  
• As the rate of extinction in heterothermic mammals is much lower than in homeotherms, thermal energetics are of concern to conservation biologists because mammals and birds that can use and cannot use torpor differ enormously in their energy requirements and thus foraging needs.