Hibernation is a controversial term in scientific circles, as there are questions around what truly qualifies as hibernating. Some scholars have suggested that the black bear–long viewed as the poster child for hibernation–is not a “true” hibernator. Instead, the bear enters a state of “torpor,” which is similar to hibernation but not identical because its body temperature drops only modestly.
Recent research, however, refutes the idea that bears aren’t true hibernators–suggesting that hibernation can occur in animals that experience only modest declines in body temperature. Hibernation, according to their definition, has more to do with metabolic suppression than body temperature. By this definition, black bears qualify because their metabolism has been shown to decrease by 25% and their heart rate drops from an average of 55 beats per minute to as low as nine.
Semantics aside, there are many stunning examples of hibernation, or near hibernation, that exist in the animal kingdom. Here are three that stand out, highlighting the various forms, nuances and complexities of this unique survival strategy.
1. Wood Frog – The “Frozen Hibernator”
The wood frog (Rana sylvatica), native to North America’s temperate forests, showcases an extraordinary adaptation in its hibernation strategy: it can freeze itself nearly solid during the winter months. As temperatures plummet, the wood frog’s body enters a state of suspended animation, effectively halting all metabolic processes. This remarkable ability is facilitated by the frog’s capacity to tolerate the formation of ice within its tissues. Unlike most animals, which would suffer fatal damage from ice crystals, the wood frog has evolved specialized mechanisms to withstand and even thrive in such extreme conditions.
During freezing, the wood frog’s body temperature drops to match that of its environment, often reaching well below the freezing point of water. To survive this harsh condition, the frog’s organs and vital systems remain protected by a combination of glucose and urea, which act as cryoprotectants. These substances prevent ice from forming within the cells, reducing the risk of lethal intracellular damage. The frog’s blood also contains high levels of glucose, which helps to stabilize the tissues and provide energy once the temperature rises and the frog begins to thaw.
As spring arrives and temperatures warm, the wood frog gradually defrosts, resuming its normal metabolic functions with the help of its cryoprotectants. The process of thawing is just as critical as freezing; the frog must carefully manage its internal environment to avoid cellular damage as it transitions from a frozen state back to life.
2. Lungfish – The “Reverse Hibernator”
Lungfish, an ancient group of freshwater fish found in Africa, South America, and Australia, are renowned for their remarkable ability to “estivate” during the hot and dry summer months. Estivation is the opposite of hibernation, occurring when animals enter a state of dormancy during warm weather months.
As seasonal droughts render their habitats inhospitable, the lungfish burrows into the mud or sediment at the bottom of its drying pond or river. It then secretes a mucous cocoon around itself, which hardens into a protective layer that safeguards the fish from desiccation and extreme temperatures.
Inside its mucous cocoon, the lungfish’s metabolic rate drops significantly. This dramatic reduction in metabolic activity allows it to survive on minimal resources while waiting for the environment to become more favorable. Amazingly, lungfish can remain in estivation for multiple years if water levels don’t return.
During estivation, the lungfish relies heavily on its ability to breathe air through its specialized lung-like structures, which are critical since oxygen levels in the trapped water or mud become insufficient.
As the rainy season arrives and water levels rise, the lungfish emerges from its cocoon, rehydrating and resuming its normal activities.
3. Prairie Dog – The “Occasional Hibernator”
The prairie dog has a unique approach to surviving the harsh conditions of winter, which can be likened to a form of short-term hibernation. During the colder months, prairie dogs enter a state of torpor, a period of reduced metabolic activity and lowered body temperature. This strategy allows them to conserve energy and survive when food resources are scarce and temperatures are too cold for their typical foraging activities. Unlike true hibernation, where an animal’s body temperature and metabolism drop significantly, torpor in prairie dogs is less extreme but still significant, helping them to manage their energy reserves more effectively.
In their underground burrows, prairie dogs huddle together in communal nests to maintain warmth and further reduce their energy expenditure. The burrow system itself provides an insulating environment that helps moderate temperature fluctuations, allowing prairie dogs to remain relatively stable even as external conditions become extreme. During torpor, prairie dogs may enter and exit this state intermittently, becoming briefly active to perform essential functions like moving to different parts of their burrow or adjusting their nesting materials, but they do so at a much-reduced level of activity compared to their summer behavior.
Does thinking about animals with unique survival strategies inspire awe in you? Take the Connectedness To Nature Scale to know how your love of the natural world stacks up with others.
