Amphibians in temperate regions cannot generate their own internal body heat, so their body temperature closely mirrors their surroundings. To survive the cold months, frogs must enter a state of torpor, or dormancy, known as overwintering. During this time, their metabolism, breathing, and heart rate slow dramatically to conserve energy. The specific location a frog chooses for its winter refuge, or hibernaculum, depends on its species, habitat, and the severity of the local climate. This strategic choice dictates the physiological adaptations the frog must employ to endure months of freezing temperatures until spring.
Overwintering in Aquatic Habitats
Many species of frogs, such as bullfrogs and leopard frogs, spend the winter submerged at the bottom of ponds, lakes, or slow-moving rivers. They seek water bodies deep enough to prevent freezing solid, resting on the sediment or nestled among submerged leaf litter or rocks. Frogs generally avoid burying themselves deep into the mud because the sediment often contains little dissolved oxygen, preferring the better oxygen supply in the water layer just above the bottom.
While dormant underwater, the frog’s need for oxygen is significantly reduced due to its suppressed metabolism. The frog relies entirely on cutaneous respiration, or breathing through its skin, to meet these minimal requirements. Oxygen diffuses directly across the frog’s highly vascularized, permeable skin into its bloodstream. This skin-based gas exchange is sufficient only if the water remains well-oxygenated and the temperature stays consistently low, typically below 5 degrees Celsius.
Overwintering in Terrestrial Habitats
Frogs and toads that are primarily terrestrial, such as American toads, often overwinter on land away from water bodies. They seek a location that offers sufficient insulation to keep the body temperature constantly above freezing. They accomplish this by creating or finding a secure shelter deep enough to reach below the frost line, which is the depth to which the ground freezes.
Good burrowing species, like toads, use specialized hardened knobs on their hind feet to dig down into the soil, sometimes more than 50 centimeters deep, to find stable, unfrozen earth. Poor diggers, such as certain tree frogs, utilize natural shelters instead. These frogs seek out deep crevices in logs, dense root masses, thick leaf litter, or existing rodent burrows. The insulation provided by the surrounding soil or snow cover creates a microclimate that prevents exposure to lethal freezing temperatures.
Surviving Freezing Temperatures
A few exceptional species, notably the wood frog, employ a unique survival strategy by tolerating the freezing of a significant portion of their body. These frogs overwinter in shallow depressions on the forest floor, often under just a layer of leaf litter, exposed to sub-zero temperatures. When ice crystals begin to form on the skin, a rapid physiological response is triggered to protect the animal’s internal structures.
The liver converts large stores of glycogen into glucose, which is then circulated throughout the body at high concentrations. This glucose acts as a cryoprotectant, like an internal antifreeze, preventing ice crystals from forming inside the cells, which would cause them to rupture. Ice formation is limited to the extracellular spaces, between the cells, effectively desiccating the cells but keeping their structure intact.
While frozen, up to 65% of the frog’s body water can turn to ice; its heart stops beating, breathing ceases, and all physical signs of life vanish. The cryoprotectants help stabilize cell proteins and membranes during this suspended state. When spring arrives and the ground thaws, the frog’s body temperature rises, the ice melts, and circulation and organ function gradually resume. The animal can thaw and revive, ready to emerge within hours.