Garter snakes (Thamnophis sirtalis) species is immensely successful throughout the Nearctic, having diversified into a plethora of subspecies and color forms. They are widely distributed, spanning from the southern tip of Florida and Southeast Texas to the Northwest Territories of Canada— though there is a big ‘hole’ in their range in the southwestern U.S.. This qualifies garter snakes the most northern ranging reptile in North America. Only a few reptiles in the Old World reach higher latitudes, such as the European adder (Vipera berus) and the lizard (Zooteca vivipara).
It is astounding that northern populations of garter snakes are capable of surviving subzero temperatures. Thamnophis sirtalis can endure several hours of freezing and anoxia due to the reduction of blood and oxygen transport to tissues. As in many frogs, garter snakes exhibit elevated levels of glucose in the liver & muscle tissues and lactase in the liver as cryprotectants to save critical tissue function. However, generally among freeze-resistant squamates, cryoprotectant concentrations are much lower than in amphibians, making them poorly equipped to deal with long-term exposure to freezing body temperatures.
During early winter, garter snakes take refuge in underground dens to avoid freezing mortality. As temperatures drop, they move around within dens to locate warmer and deeper microsites. If an optimal location is found, snakes will experience subzero body temperatures only during overnight frosts, successfully avoiding prolonged freezing exposures. Cryoprotectants allow the body to ‘supercool’, which means that body temperatures drop below the freezing point of water without ice crystal formation. In a study of red-sided garter snakes (T. sirtalis parietalis), the supercooling point varied from -5.5°C in late autumn to -0.8°C midwinter. Remarkably, their survival rate is not negatively affected after up to 40% of their body water content becoming crystallized!
A final but interesting note about overwintering in garter snakes is the other side of the coin— the physiological change from a freezing to a thawed state. As ice crystals melt in the body, the reperfusion of tissues with oxygenated blood initiates aerobic metabolism. This sudden transition from ischemia creates oxidative stress which, can result in cell injury. To cope with this problem, antioxidant enzyme production increases prior to thawing, reducing oxidative damage. Even though their physiology is not able to tolerate long-term freezing exposure, garter snakes still prove themselves as very capable poikilotherms (animals that maintain bodily functions over a wide range of internal temperatures).
