The Tiger Salamander, Ambystoma tigrinum, is the largest terrestrial salamander in North America, found across a broad geographical range. When facing the extreme cold of winter, the Tiger Salamander does not enter the deep sleep known as hibernation. Instead, like many other amphibians and reptiles, this species enters a state of cold-induced dormancy known as brumation. This physiological slowdown allows the salamander to endure periods when external conditions make activity and foraging impossible.
Brumation: Why Amphibian Dormancy is Unique
Brumation is an energy-saving state distinct from the hibernation seen in mammals. The difference lies in the salamander’s ectothermic nature, meaning its body temperature is regulated by the surrounding environment. Because they cannot generate their own heat, their metabolic rate slows dramatically as the external temperature drops.
During this dormancy, the salamander remains semi-conscious and can be roused, unlike a deeply hibernating mammal. The animal’s body temperature simply matches that of its shelter, with no internal regulatory mechanisms to control it. This slows all bodily functions, including heart rate and respiration, to the minimum required for survival.
Tiger Salamanders do not rely on large fat reserves like hibernating mammals. Instead, a major concern during brumation is hydration. The salamander may occasionally wake up and move within its burrow to absorb moisture through its skin, a process vital for survival throughout the cold season.
Triggers and Timing of Winter Dormancy
The onset of brumation is primarily governed by environmental cues, specifically a sustained drop in temperature and the shortening of daylight hours, known as photoperiod. As the days grow shorter and the ground begins to cool, the salamander seeks a secure, subterranean refuge. This movement often coincides with or immediately follows the autumn breeding migration to nearby aquatic habitats.
To avoid freezing, salamanders must burrow deep enough to remain below the frost line, typically descending 0.6 to 1.5 meters (about two to five feet) beneath the surface. While they can dig their own tunnels, they frequently utilize and enlarge burrows created by small mammals, such as gophers, prairie dogs, or ground squirrels. These underground networks offer stable temperature and humidity levels.
The duration of brumation is highly variable and depends entirely on the local climate and geographical location. In the northern parts of their range, dormancy can last for several months, from late autumn well into spring. Conversely, salamanders in warmer, southern regions may experience a much shorter, or even intermittent, brumation period, only retreating underground during the coldest snaps.
Surviving Summer Heat: The Role of Aestivation
The Tiger Salamander employs a related state of dormancy called aestivation to survive hot, dry conditions. Aestivation is a period of summer dormancy triggered by extreme heat and a lack of environmental moisture or drought. This is important for populations living in arid or semi-arid regions.
The goal of aestivation is to prevent desiccation, or drying out, which is a major threat to amphibians with their permeable skin. The salamander retreats into its burrow to escape high surface temperatures and retain body moisture. By significantly reducing activity, the salamander minimizes water loss through evaporation and metabolic processes.
During this summertime retreat, the salamander can tolerate high concentrations of urea in its blood, a physiological adaptation that helps conserve water. Aestivation allows the species to remain dormant until rainfall returns, signaling favorable conditions and allowing the salamander to emerge and resume normal feeding and movement.