Frogs are ectotherms, meaning their body temperature is regulated entirely by the surrounding environment. Unlike mammals that generate internal heat, a frog’s survival depends on adapting its physiology to temperatures that often drop below freezing. This adaptation is a winter dormancy known as brumation, a state that significantly slows metabolic processes to conserve energy until spring.
Defining Brumation and Habitat Selection
Brumation is the amphibian and reptile equivalent of hibernation, though it differs from the deep, continuous sleep seen in warm-blooded mammals. While a mammal’s body temperature is actively maintained, a brumating frog’s body temperature simply matches its surroundings. Brumation is primarily triggered by environmental cues such as shortening daylight hours and a sustained drop in ambient temperature.
The most immediate decision for a frog in autumn is selecting a winter habitat, as this choice dictates the physiological strategy it must employ. Some species, like bullfrogs and green frogs, choose a fully aquatic environment. Others, such as the wood frog, opt for a terrestrial location like leaf litter or shallow soil. This habitat selection leads to two distinct survival methods: one focused on avoiding freezing and the other on tolerating it.
The Aquatic Strategy: Surviving Beneath the Ice
Frogs that choose an aquatic overwintering strategy, like the American Bullfrog, burrow into the mud and detritus at the bottom of ponds, lakes, or slow-moving streams. This location is selected because the water above the bottom sediment rarely freezes solid, providing a stable temperature just above 0°C. The frog’s metabolic rate and heart rate slow drastically in this cold, dormant state to minimize its need for oxygen.
When submerged, the frog relies entirely on cutaneous respiration, the process of breathing through its skin. Oxygen dissolved in the surrounding water diffuses directly across the thin, highly vascularized skin surface and into the bloodstream. This mechanism requires the water to remain well-oxygenated. Aquatic frogs must select sites where the water is deep or flowing enough to prevent prolonged oxygen depletion. If the water becomes too stagnant or the ice cover persists too long, the frog risks suffocation.
The Terrestrial Strategy: Freeze Tolerance and Cryoprotection
The most extraordinary winter survival method belongs to terrestrial species like the Wood Frog, which spend the winter in shallow burrows or under leaf litter where they are exposed to freezing temperatures. These frogs are freeze-tolerant, allowing up to 65% of their total body water to turn to ice without cellular damage. When the external temperature drops, ice crystals first form in the extracellular spaces, such as the body cavity, causing the frog’s heart and breathing to stop completely.
This external ice formation draws water out of the cells, resulting in massive cellular dehydration and potential fatal damage. To counteract this, the frog’s liver rapidly initiates cryoprotection, converting large stores of glycogen into massive amounts of glucose. This glucose, along with other cryoprotectants like urea, is distributed throughout the circulatory system and into the cells, acting as a biological antifreeze.
The high concentration of these solutes inside the cells lowers the freezing point of the intracellular fluid, preventing ice crystals from forming within the cell structure itself. By keeping the cell interior liquid, this mechanism protects vital organs from mechanical damage and dehydration, even when the frog’s body becomes a solid block of ice. Alaskan Wood Frogs exhibit extreme freeze tolerance, surviving temperatures as low as -16°C, allowing them to endure multiple freeze-thaw cycles throughout the winter.
Signaling the End of Winter
The long period of brumation ends when the environment signals the return of warmer conditions, primarily through rising temperatures and the thawing of the ground. For aquatic species, warming water temperature and increasing daylight prompt a gradual return to a normal metabolic rate and emergence from the mud.
Freeze-tolerant frogs respond directly to the thawing of their surroundings. As the external ice melts, the cryoprotectants are metabolized, and the frog’s internal organs, including the heart and lungs, resume function. This transition from a frozen, seemingly lifeless state to full activity is often rapid, driven by the immediate need to reproduce. These frogs typically emerge and move quickly toward breeding ponds to maximize the short window available for their reproductive cycle.