The American Bullfrog (Lithobates catesbeianus) is the largest true frog species found throughout much of North America, typically reaching lengths of 9 to 15 centimeters. These substantial amphibians are strongly associated with permanent water bodies like large ponds, lakes, and slow-moving rivers. As cold weather arrives, this ectothermic creature cannot simply migrate or remain active. The bullfrog enters a profound state of dormancy to endure the freezing months.
The Bullfrog’s Dormant State
What is often casually referred to as hibernation in the bullfrog is more accurately described by the biological term brumation. This is the specific dormant state for cold-blooded animals, or ectotherms, whose body temperature is directly dependent on the surrounding environment. The onset of cooling water temperatures is the primary trigger for the bullfrog to initiate this period of inactivity.
Unlike mammalian hibernation, brumation is a slowing of processes in response to cold. The frog’s metabolic rate and heart rate drop drastically to conserve energy, relying on fat reserves built up during the warmer months. This physiological slowdown minimizes the need for oxygen, allowing the bullfrog to survive for months without food or surfacing for air.
Choosing the Winter Refuge
To prevent their body tissues from freezing, bullfrogs must seek out a stable, aquatic winter refuge. They exclusively choose permanent water sources deep enough to ensure the bottom layer remains liquid and above freezing, typically around 4°C (39°F). Ponds, lakes, or deep bends of streams provide the necessary thermal protection beneath the insulating layer of surface ice.
The bullfrog will settle lightly onto the soft substrate at the bottom, preferring areas of mud or silt. They do not burrow deeply into the anoxic (oxygen-deprived) mud, which is a common misconception. Instead, they remain partially exposed or just lightly covered, maintaining contact with the surrounding water. This specific positioning is mandatory for their unique method of winter respiration.
Physiological Survival Underwater
The bullfrog’s winter strategy involves its ability to breathe entirely through its skin, a process known as cutaneous respiration. During brumation, the lungs are rendered useless, and oxygen is obtained by absorbing it directly from the water. The bullfrog’s highly permeable skin is thin and richly supplied with a dense network of blood capillaries near the surface.
With its metabolic demands reduced to a fraction of its summer rate, the small amount of oxygen dissolved in the cold water is sufficient. Oxygen molecules diffuse across the skin barrier and into the bloodstream, accounting for nearly 100% of the animal’s oxygen uptake during the dormant period. The skin is also the main route for expelling carbon dioxide waste from the body.
The success of this survival mechanism relies entirely on the water remaining cold and adequately oxygenated. If a pond freezes too deeply or is covered by thick ice for too long, decaying organic matter can deplete the dissolved oxygen. In such anoxic conditions, the bullfrog cannot obtain the oxygen it needs through its skin and will suffocate. This makes cold, flowing water or large bodies of water with constant oxygen levels the safest overwintering locations.
The Spring Awakening
Rising water temperatures signal the bullfrog’s emergence from brumation. As the spring thaw occurs and the water warms, typically reaching temperatures above 15°C (60°F), the frog’s metabolism begins to increase. This rise in temperature acts as a biological switch, signaling that the environment is suitable for activity.
The bullfrog slowly surfaces from the lake or pond bottom, transitioning back to its active life cycle. It immediately resumes the use of its lungs and begins feeding actively to replenish the energy stores depleted over the winter months. Soon after emerging, the bullfrogs will enter their breeding cycle, filling the wetlands with their characteristic, deep calls.