There are many examples of the amazing evolutionary adaptations in the animal kingdom that aid survival, but the ability of animals to hibernate is one of the most incredible. Most vertebrates must keep warm, because hypothermia or loss of body heat is potentially fatal, but some have evolved to withstand a massive drop in temperature, and even to survive freezing. To avoid inhospitable conditions they enter a state of suspended animation known as hibernation. Cold to the touch, their hearts beat infrequently and they may stop breathing for long periods. They appear lifeless, but can survive in this condition for several months, some even for most of the year.

The long winter sleep of the northern hibernators is triggered by lowering temperatures, shorter days, or reduced food supplies, and in some species by an ‘‘internal clock’’ that controls their activities. In contrast, some animals sleep to avoid the hot and dry conditions of midsummer, which is called estivation. Their lowered metabolism reduces energy consumption, but even deep sleep must be fuelled to sustain life, and the hibernators store fat internally or have food caches nearby. As freezing is fatal to hibernating mammals, they awaken and shiver to warm up a few degrees when their temperature drops too low, but the coldblooded fish, amphibians, and reptiles may freeze and die when exposed to frost, so several have developed unique survival skills. These include the production of anti-freeze compounds to protect their cell water, and the ability to lower the temperature at which they begin to freeze.

This is an account of the animals that are able to survive when their body temperature drops well below the normal danger level. It redefines hibernation, and includes animals such as the bears, which have not traditionally been considered hibernators.