American bullfrogs, known for their distinctive calls and large size, are common inhabitants of freshwater environments across North America, including lakes, ponds, and marshes. As ectothermic animals, their body temperature is regulated by their external surroundings. When colder temperatures arrive, these amphibians must adapt their behavior to survive the winter months. Understanding these adaptations is key to appreciating their resilience.
Winter Dormancy
As temperatures drop, American bullfrogs enter a state of dormancy known as brumation, which differs from the deep hibernation seen in some mammals. They typically seek refuge at the bottom of larger, permanent bodies of water such as lakes, ponds, or slow-moving streams where temperatures remain above freezing. During this period, bullfrogs often rest on top of or are partially submerged in the mud and leaf litter at the water’s bed, rather than fully burying themselves deep within the substrate. This specific positioning is important because they need access to oxygen from the surrounding water.
While in brumation, their activity levels decrease, and they may occasionally move slowly along the pond bottom. Their choice of deeper water bodies helps ensure the water does not freeze solid, which would cut off their oxygen supply. Unlike some other amphibian species, bullfrogs are not freeze-tolerant, meaning their bodies cannot withstand freezing solid.
Physiological Adaptations for Survival
During brumation, a bullfrog’s body undergoes several physiological adjustments to conserve energy and endure the cold. Their metabolic rate slows considerably, reducing the amount of energy they consume. This allows them to subsist for extended periods without needing to feed, relying on stored reserves. Their heart rate and breathing also slow down, reflecting their reduced energy demands.
A crucial adaptation for overwintering underwater is cutaneous respiration, where bullfrogs absorb oxygen directly through their permeable skin from the surrounding water. This highly efficient method of oxygen uptake is sufficient when their metabolic needs are low, eliminating the necessity to surface for air. While some amphibians can produce a natural “antifreeze” to survive freezing, bullfrogs do not possess this capability and rely on remaining in unfrozen water.
Emergence and Spring Activity
As winter recedes and environmental conditions become more favorable, bullfrogs begin to emerge from their brumation. Rising water temperatures and increasing daylight hours serve as key cues that trigger their return to activity. This reawakening typically occurs in the spring, with adult bullfrogs observed initiating post-brumation activities as early as late April in some regions.
Following emergence, their metabolic rates increase, and they resume their normal behaviors, including foraging for food. The return to warmer conditions also marks the beginning of their breeding season, which can extend from May to July in many areas, and even longer in southern states. The distinct, loud “jug-o-rum” call of male bullfrogs becomes a prominent sound around ponds and wetlands as they seek mates.