Garter snakes do not truly hibernate in the way that mammals do, but instead enter a state of dormancy known as brumation. This seasonal behavior allows them to survive across their wide distribution throughout North America, which includes regions with temperate climates and cold winters. They must undergo this period of inactivity to survive freezing temperatures and scarcity of food resources. Brumation allows these reptiles to conserve energy until conditions are favorable for activity and feeding again in the spring.
Brumation Versus True Hibernation
Brumation differs fundamentally from the hibernation seen in mammals due to the snake’s physiology as an ectotherm. Garter snakes depend entirely on external temperature to regulate their body functions. As the environmental temperature drops, the snake’s metabolism slows drastically to match it.
During this state, the snake’s oxygen consumption and cardiac activity are slowed. The snake remains semi-conscious, however, and is not in an unresponsive deep sleep like a true hibernator. This lighter state means the snake may move slightly or occasionally seek out water within the den.
Brumation is biologically required for the garter snake’s reproductive cycle. It helps regulate the timing of the annual cycle and stimulates mating behaviors and improves fertility upon emergence. The extended exposure to low temperatures followed by warming in the spring is the primary factor that triggers courtship activity in males.
Communal Denning Sites
Garter snakes must locate a suitable den site, known as a hibernaculum. Since they cannot dig their own burrows, they rely on pre-existing underground spaces like rock crevices, abandoned mammal tunnels, or deep root systems. The location must be below the frost line to ensure the temperature remains above freezing.
Garter snakes are well-known for communal denning, where they congregate in large numbers. A single hibernaculum may house hundreds or even tens of thousands of individuals, such as the massive dens found in Manitoba, Canada. This dense gathering provides collective thermoregulation, helping to buffer individual snakes from extreme temperature fluctuations.
Communal denning also offers a reproductive advantage realized immediately upon emergence in the spring. Having a large number of males and females emerge simultaneously maximizes the chances of successful mating. This concentration leads to the formation of mating balls, where multiple males compete for a single female, ensuring efficient reproduction before the snakes disperse.
Seasonal Timing and Environmental Signals
Brumation is dictated by a combination of internal and external environmental cues. The primary signals are the decreasing photoperiod and the sustained drop in ambient air temperature. Once temperatures consistently fall below approximately 10 degrees Celsius, the snakes begin migrating toward the communal den sites.
Before descending into the hibernaculum, garter snakes undergo a preparatory phase involving a cessation of feeding. These reptiles require warmth for their digestive enzymes to work efficiently, and consuming food when temperatures are dropping would be fatal. Undigested food remaining in the gut would putrefy in the cold.
Brumation can last from a few weeks in warmer climates to six or seven months in northern regions. The exit from the den in the spring is triggered by rising soil temperatures and increasing daylight hours. Males typically emerge first, followed by the females, with the population dispersing shortly after mating concludes.