Most North American freshwater turtle neonates typically hatch in late summer or early fall and move from their subterranean nests to nearby lakes and ponds, avoiding winter's subzero temperatures in the water's depths. However, painted turtle (Chrysemys picta) hatchlings remain entombed in their shallow subterranean nest throughout their first winter, frequently experiencing ice and body temperatures as low as -10°C. Nevertheless, the hatchlings emerge unscathed when the ground thaws in the spring.

Although two mechanisms could explain the painted turtle hatchling's remarkable cold tolerance (a capacity for supercooling and an ability to tolerate freezing), supercooling is thought to play a significant role in the neonate's ability to overwinter. However, the importance of freeze tolerance in survival continues to be debated. Gary and Mary Packard of Colorado State wondered why hatchling painted turtles can only recover from a few days of freezing under relatively mild conditions (-2 to -2.5°C) and decided to investigate whether lactate accumulation in the hatchling's tissues affects the youngster's freeze tolerance.

The Packards explain that when hatchlings freeze, much of the extracellular water forms ice, preventing the circulatory system from functioning and the delivery of oxygen for mitochondrial ATP production. Cells are instead forced to meet their ATP demands through anaerobic glycolysis, which causes an increase in tissue lactic acid levels, known as anoxic lactic acidosis. Normally, unfrozen turtles counteract the dangerous drop in tissue pH that accompanies lactate accumulation by the release of carbonate buffers into the blood from the shell, and lactate is also transported to and sequestered in the shell itself. However, these buffering processes may not be available to frozen hatchlings due to their arrested circulation.

Assessing lactate's role in freeze tolerance, the Packards exposed recently hatched painted turtles to freezing conditions at -2°C for periods between zero and eight days. Every second day, the Packards thawed a group of animals and recorded the mortality percentage. They also measured whole-body lactate levels from another group that was not allowed to thaw. The team found that the frozen hatchlings' mortality levels were correlated with the amount of whole-body lactate. They also found that the frozen turtles' whole-body lactate levels were greater than those previously measured from comparable supercooled turtles, which maintain a functional circulatory system. Thus, freezing compromises the hatchling's ability to effectively deal with the anoxic lactic acidosis that accompanies anaerobic metabolism, a phenomenon the Packards suspect is due to the cessation of the circulation that occurs with freezing.

The Packards hypothesize that without a functional circulatory system, frozen turtles cannot shuttle lactate from individual tissues to the shell to be sequestered or mobilize the shell's buffers. As a result, individual organs may accumulate lactate to lethal levels sooner than if the circulatory system remained operational. As follows, the Packards suggest that a cold tolerance strategy based on freezing is more stressful than a strategy based on supercooling and argue that the inability to deal with lactic acidosis may be one of the reasons why hatchling painted turtles are unable to tolerate freezing for prolonged time periods.