Hibernation is an extended state of dormancy used by animals to survive periods of resource scarcity. This survival mechanism allows certain creatures to endure months without food or movement by shutting down most normal biological functions. To understand which animal achieves the longest rest, it is necessary to first understand the science of this natural pause.
Defining True Hibernation
True hibernation is a precisely regulated, long-term state of controlled hypothermia in warm-blooded animals (endotherms). It is characterized by a dramatic, sustained drop in body temperature and a profound reduction in metabolic rate. This state is distinct from shorter periods of metabolic suppression, such as torpor, which often lasts less than 24 hours. Aestivation is a similar state of deep sleep, but it occurs in response to hot, dry conditions during the summer months. Cold-blooded animals like reptiles and amphibians undergo brumation, where their metabolism slows down based on external environmental temperature.
The Record Holder for Duration
The longest seasonal hibernator in the animal kingdom is the Arctic ground squirrel, known scientifically as Urocitellus parryii. This resilient rodent inhabits the harsh tundra regions of North America and Russia, facing prolonged periods of extreme cold and darkness. The species routinely spends seven to nine months of the year in its subterranean burrow, surviving on stored body fat. Females tend to enter hibernation earlier, often in August, and remain dormant for a longer period than the males. This duration, encompassing nearly three-quarters of the calendar year, is a record for a seasonal hibernating mammal in the wild.
Physiological Adaptations for Extended Sleep
The Arctic ground squirrel’s ability to survive its nine-month sleep is due to extraordinary physiological control over its body systems. During its deep torpor phase, the squirrel’s metabolic rate drops to just 2 to 5% of its active rate, allowing it to conserve energy. The animal achieves the lowest body temperature ever recorded in a mammal, with its core temperature plummeting to as low as -2.9 degrees Celsius (27 degrees Fahrenheit). This ability to withstand sub-zero temperatures without its tissues freezing is called supercooling. The squirrel undergoes periodic, brief arousals back to a normal body temperature. Every two to three weeks, the animal shivers intensely for 12 to 15 hours, using specialized brown fat reserves to warm its body back up to around 37 degrees Celsius (98 degrees Fahrenheit). This rewarming process is necessary to prevent damage to the brain and other organs before the squirrel re-enters its deep torpor state.
Comparing Other Long-Duration Dormancy
The extreme nature of the Arctic ground squirrel’s hibernation is highlighted when compared to the long-duration dormancy of other species. Bears, for instance, are often associated with winter sleep, but they enter a state of winter lethargy, not true hibernation. A bear’s body temperature drops only slightly, typically by less than 10 degrees Celsius, and its metabolic rate is reduced less drastically than that of a true hibernator. This difference means bears can be easily roused and may even give birth during their winter sleep, unlike the deep state of the squirrel. Other long-term sleepers, such as marmots, are true hibernators and can sleep for up to eight months, but they do not achieve the remarkable below-freezing body temperature of the Arctic ground squirrel.