Toads, like all amphibians, are ectotherms; their body temperature and activity levels are directly tied to the temperature of their surroundings. As temperatures drop in temperate climates, toads cannot migrate or maintain internal warmth, forcing them to adopt a survival strategy. This physiological necessity drives them into a state of deep rest to survive the cold months when food is scarce and the risk of freezing is high.
The Biological State of Winter Rest
The winter dormancy of toads is more accurately described as brumation, a state distinct from the true hibernation seen in warm-blooded mammals. Brumation is a temporary shutdown of physical activity in response to cold temperatures, specifically affecting cold-blooded animals like amphibians and reptiles. During this time, the toad’s metabolic rate slows dramatically, sometimes dropping to less than 10% of its active rate, conserving stored energy reserves.
This reduced metabolism is accompanied by a significant decrease in heart rate and respiratory function, allowing the toad to survive for months on very little energy. Unlike mammals preparing for hibernation, toads do not “fatten up” beforehand. Their lowered metabolism means they can no longer efficiently digest food, making a pre-winter fast necessary.
Some anuran species, such as wood frogs, can survive having up to 65% of their body water freeze, a phenomenon known as freeze tolerance. Toads must protect their cells from the damage caused by ice crystal formation, though they are not generally freeze-tolerant to the same extent. They accomplish this by burying themselves deep enough to remain below the frost line, where the temperature is stable and above freezing. The use of glucose and urea in the body fluids of some amphibians acts as a natural cryoprotectant, helping to manage cell volume and integrity.
Selecting a Hibernation Site
Toads must secure a safe, subterranean location to enter brumation, a process that begins when temperatures start to fall, often between September and November. The primary goal is to find a site that will remain consistently above freezing and offer protection from predators. American Toads, for example, are terrestrial brumators and must burrow into the ground to survive the cold.
They are accomplished diggers, using hardened knobs on their hind feet to burrow backward into the soil. The depth of the burrow is paramount, as it must reach below the local frost line, requiring digging anywhere from one to two feet (30 to 60 cm) deep. Toads prefer sites with loose, sandy, or loamy soil, as this substrate is easier to dig through and provides insulation.
If suitable soil is not available, toads utilize existing natural structures to shelter themselves. This may include using abandoned mammal burrows, ant mounds, or seeking out crevices underneath large rocks or logs. Once the toad has backed itself into the burrow, the entrance typically collapses, sealing the amphibian inside its winter chamber until the spring thaw.
The Spring Emergence
The end of brumation is triggered by environmental cues that signal the return of favorable conditions for activity and reproduction. Rising soil temperatures are the most important signal, indicating that the threat of a deep freeze has passed and the ground is thawing. Toads will often begin to stir once the soil temperature is consistently warming, although the exact threshold varies by species and location.
After emerging from the soil, the toads’ first instinct is to migrate toward their ancestral breeding ponds. This movement is often stimulated by specific weather conditions, such as rising air temperatures and high humidity, with many species preferring to move on rainy nights. For Common Toads, a surge in migration may occur when evening temperatures reach 7 to 8 degrees Celsius.
Males typically emerge and migrate slightly earlier than females to secure the best breeding spots. This post-winter migration and subsequent reproductive activity are the immediate behaviors of the toad after its long period of underground rest. The successful completion of the brumation cycle allows the toad to re-enter its active life, hunt for food, and continue the species’ life cycle.