A number of rodents, bats, insectivores, monotremes (the platypus and echidnas) and marsupials truly hibernate, along with one primate, the fat-tailed dwarf lemur.
Hibernation is characterised by periods of torpor lasting from a few days to five weeks. Body temperature drops to between 2°C and 10°C in temperate hibernators (but as low as -2.9°C in Arctic ground squirrels), and metabolic rates are reduced by up to 99 per cent.
Bouts of torpor are interrupted by breaks of up to 24 hours, when body temperature and metabolism return to near-normal levels, enabling the animals to perform essential functions: they may urinate and defecate, bats may change hibernation sites, and chipmunks may eat stored food.
Though brief, these periods of wakefulness consume 83 per cent of all the energy expended during hibernation. This is why hibernating mammals typically weigh less than 5kg (marmots are the biggest).
Larger mammals would warm up too slowly and consume even more energy when alert.
Instead, bigger animals conserve heat more efficiently - growing longer, denser fur, and laying down more fat. Bears, for instance, undergo a spell of lethargy rather than hibernation: in their dens, their body temperature drops only slightly, from about 37°C to 31°C.