Frogs are cold-blooded animals, meaning their internal temperature is regulated by their surroundings. This dependency on external warmth necessitates a period of seasonal dormancy to survive freezing temperatures and scarce winter food. This state is technically known as brumation, a metabolic slowdown distinct from the deep sleep of true hibernation seen in warm-blooded mammals. The timing is dictated by environmental conditions signaling the onset of winter.
Environmental Cues That Trigger Dormancy
The decision for a frog to enter brumation is a gradual process triggered by two primary environmental signals: decreasing daylight hours and dropping temperatures. The shortening of the days in late summer and early autumn serves as a reliable calendar cue, prompting the frog to begin accumulating fat reserves in preparation for the long, foodless winter. This internal readiness precedes the final environmental trigger.
The physical switch to a dormant state is usually activated when ambient temperatures consistently drop below approximately 50°F (10°C). As an ectotherm, the frog’s metabolism slows down naturally as its environment cools, compelling it to seek a protected overwintering site called a hibernaculum. The timing of this transition varies widely across geography; northern-dwelling species may enter brumation as early as September, while those in southern regions may not become dormant until December or January. This reliance on temperature means that unseasonable warm spells can cause a frog to prematurely emerge, only to face a lethal freeze when cold weather returns.
Winter Hideouts: Aquatic vs. Terrestrial Strategies
A frog’s overwintering strategy is determined by its species and tolerance for freezing temperatures, resulting in two distinct hideouts: aquatic or terrestrial. The aquatic strategy is used by species like the Bullfrog and Northern Leopard Frog, which must avoid freezing entirely. They spend winter submerged at the bottom of ponds or lakes, lying on or partially in the mud. They remain near oxygen-rich water rather than burying deep into anoxic sediment, which could cause suffocation.
While submerged, these aquatic species rely on cutaneous respiration, absorbing dissolved oxygen directly through their highly permeable skin. Their low metabolic rate reduces oxygen demand, but they must still have access to sufficient oxygen levels to survive. The terrestrial strategy is employed by freeze-tolerant species, such as the Wood Frog, Gray Treefrog, and Spring Peeper, which overwinter on land under leaf litter, logs, or in shallow soil burrows. American Toads, which cannot tolerate freezing, must burrow much deeper, often over 50 centimeters, to reach soil below the frost line.
The Science of Survival: How Frogs Cope with Freezing
For freeze-tolerant terrestrial species, survival depends on a remarkable physiological mechanism that manages ice formation within the body. When external temperatures drop below freezing, ice crystals first form in the extracellular fluid, such as the body cavity and under the skin. This process can account for up to 65% of the frog’s total body water freezing solid. This external ice formation draws water out of the cells, which could cause them to shrink and collapse.
The moment ice formation begins, the frog’s liver triggers the rapid conversion of stored glycogen into massive amounts of glucose, a sugar that acts as a cryoprotectant. This glucose is distributed throughout the circulatory system to the cells, raising the solute concentration inside the cells. By concentrating the glucose, the frog effectively lowers the freezing point of the intracellular fluid, preventing ice crystals from forming inside the cell walls, which would cause rupture and death. During this state, the frog’s heart stops beating, breathing ceases, and brain activity becomes undetectable, allowing it to survive for weeks or months as a frozen, inert mass until the spring thaw.