Tree frogs, like all amphibians, are ectotherms, meaning their internal body temperature mirrors the temperature of their surroundings. This physiology poses a significant survival challenge when winter brings freezing conditions to their environment. Unlike mammals that can generate internal heat, a tree frog must enter a specialized state of dormancy called brumation to survive the season. This metabolic slowdown is not enough on its own, and the animal must employ complex biological defenses to prevent lethal damage from ice formation.
Finding the Winter Hideout
As autumn temperatures drop and daylight hours shorten, tree frogs descend from their summer arboreal habitats to seek terrestrial refuge. The winter location, or hibernaculum, must provide a stable environment that offers insulation from the harshest cold. These locations typically include spaces beneath loose bark, inside rock crevices, under fallen logs, or shallowly buried within thick leaf litter and soil.
The depth of the chosen hiding spot is often surprisingly shallow, placing the tree frog within the zone where temperatures can fluctuate and even fall below freezing. Unlike frogs that burrow deep below the frost line, species like the Gray Tree Frog are equipped to survive this “freezing zone.” They select microhabitats that retain moisture, which prevents desiccation during the long dormant period.
The insulating properties of snow cover and deep leaf litter help buffer rapid temperature changes above ground. These layers slow the rate of freezing, allowing the frog’s internal defense mechanisms time to activate. The selection of a secure, moist, and insulated terrestrial spot is the first step in the tree frog’s survival.
The Physiological Strategy for Survival
The ability of some tree frog species to survive in shallow, freezing locations is due to a biological adaptation known as freeze tolerance. This strategy allows the frog to withstand the freezing of up to 65% of its total body water, with a complete cessation of breathing, heart function, and blood flow. The frog remains alive at a cellular level, even as its body becomes rigid and icy.
The mechanism centers on the rapid production and distribution of cryoprotectants, which act as a form of biological antifreeze. The liver converts stored glycogen into a low molecular weight sugar alcohol or sugar, such as glycerol or glucose, which is then circulated throughout the body. Gray Tree Frogs (Hyla versicolor and H. chrysoscelis) primarily mobilize glycerol, though glucose is also utilized.
The cryoprotectant prevents ice crystals from forming inside the cells, which would rupture the cellular membranes and cause death. The concentration of these solutes increases the osmotic pressure within the cells, drawing water out into the extracellular spaces. This process effectively dehydrates the cells, concentrating the cryoprotectant and limiting the area where lethal ice can form.
Ice formation is restricted to the spaces outside the cells and organs, where the damage is less catastrophic. When the first ice crystals form on the skin, a signal triggers the liver to begin the rapid mobilization of these protective compounds. This response is non-anticipatory, meaning the frog only begins to produce the cryoprotectant once the freezing process has already started.
The cryoprotectants also stabilize proteins and cellular membranes during dehydration and low temperatures. Once the frog is frozen, its metabolic rate drops to a bare minimum, entering the state of brumation, which conserves energy reserves for the duration of the winter.
Preparation and Spring Emergence
Preparation for brumation begins in the late summer and early fall with a period of intense feeding (hyperphagia). The tree frog must consume large quantities of insects to build up fat reserves, which are then converted into the glycogen stored in the liver. This stored glycogen is the raw material necessary to produce the cryoprotectant needed for freezing.
The environmental cues that signal the onset of brumation include falling temperatures and the reduction in daylight hours. These changes prompt the frog to seek out its hibernaculum and reduce its activity as its metabolism slows. The length of the brumation period depends on the regional climate, lasting until warming begins.
The end of the dormant period is signaled by rising temperatures in early spring, which causes the frozen animal to thaw. As the external temperature warms, the ice crystals melt, and the frog’s body begins the process of rehydration and reanimation. This thawing process is slower than the initial freezing, but the cryoprotectants are retained to stabilize the cells during the transition.
Within a day of thawing, the heart and lungs resume full activity, and the tree frog recovers from its frozen state. The immediate priority for the newly emerged tree frog is to migrate to a breeding pond to begin the mating season. For males, the distinctive mating calls fill the evening air as they seek out females.