The Green Tree Frog (Hyla cinerea) enters a state of dormancy to survive cold temperatures. This seasonal slowdown in frogs and other ectotherms is correctly termed brumation, not hibernation, as it is biologically distinct from the deep torpor exhibited by mammals. Brumation is necessary in temperate zones where winter temperatures drop below the level required for active metabolism. This strategy allows the Green Tree Frog to conserve energy and reduce reliance on external resources during periods of cold weather and food scarcity.
Defining the Winter Slowdown
Brumation is a state of metabolic suppression in ectothermic animals, whose body temperature is regulated by the ambient environment. This state differs from the true hibernation seen in endothermic mammals, which generate their own body heat. While a hibernating mammal enters a profound, non-responsive torpor, an animal undergoing brumation remains capable of movement and may wake up periodically.
The process in amphibians is temperature-dependent; their metabolic rate slows down directly in response to falling environmental temperatures. Ectotherms in brumation may occasionally emerge to drink water or shift position, though they do not actively feed. This ability to rouse distinguishes brumation from the deep, sustained physiological depression of mammalian hibernation, which involves a much more drastic drop in core body temperature and brain activity.
Physiological Adaptations for Survival
Survival during brumation, especially where freezing occurs, requires the Green Tree Frog to possess remarkable internal mechanisms. As temperatures drop, the frog’s metabolic rate is significantly suppressed, conserving energy reserves stored as fat. This metabolic slowdown is accompanied by biochemical preparations that protect the animal’s cells from damage.
For tree frogs exposed to sub-freezing temperatures, the primary defense is the production of cryoprotectants, which function as biological antifreeze. These low molecular weight compounds, primarily glucose and sometimes glycerol, are distributed throughout the body tissues. Glucose is rapidly released into the bloodstream from stored liver glycogen upon the onset of freezing, a process known as glycogenolysis.
The accumulation of glucose and glycerol in the cells and body fluids prevents cellular damage by two main actions. First, these solutes reduce the total amount of ice that forms within the tissues. Second, they stabilize proteins and cell membranes against the stresses of massive dehydration that occurs when ice crystals form outside the cells. This process allows for freeze tolerance, meaning the frog can survive the formation of ice in its extracellular fluids, with up to 65 percent of its total body water potentially freezing solid. Some tree frogs also accumulate glycerol in their tissues during cold acclimation, with the kidneys actively reducing filtration to retain the compound.
Seasonal Timing and Shelter Selection
Brumation is triggered by predictable environmental changes, primarily sustained drops in temperature and the shortening of daylight hours. The frog’s hormonal system signals the shift toward dormancy, prompting a search for a suitable overwintering location (hibernaculum). Selecting a shelter is paramount for survival, offering protection from predators and extreme cold.
Green Tree Frogs are arboreal and often seek elevated, protected spots. They utilize natural locations and man-made structures.
- Under loose bark of trees
- In tree crevices
- Within dense leaf litter near the ground
- Man-made structures like birdhouses, drainpipes, or small spaces in sheds
The chosen location must provide a stable microclimate that is insulated from the coldest air temperatures and moist enough to prevent fatal desiccation.
The duration of brumation varies widely depending on the local climate, lasting longer in northern parts of their range compared to milder southern regions. Emergence is signaled by the consistent rise of temperatures in the spring, which stimulates an increase in the frog’s metabolism. Once the risk of prolonged freezing passes, Green Tree Frogs re-emerge to begin feeding and reproductive activities.