Turtles are slow in any season, but when winter approaches they really throttle down their activity.
Some species, like eastern box turtles, simply burrow into the soil, retreat into their shells, and slip into a period of inactive torpor known as brumation, which they survive by burning fat reserves.
Painted turtles spend the winter submerged on the bottom of a water body, which protects them from freezing even if the surface ices over. Because these ectotherms' body temperatures match that of the surrounding water, the cold isn't a problem.
Normally, these reptiles breathe air, but during winter, they've evolved the ability to uptake oxygen from the water and discharge carbon dioxide into it. "When ectotherms are cold, they don't need much O2, so what they can pick up from the water column is generally enough to get them through the winter," says Jackie Litzgus, a biologist at Laurentian University in Ontario. (Learn why turtles are so vital to our planet.)
The turtles exchange gases via specialized blood vessels near the skin's surface, the mouth lining, and even the cloaca -- a cavity that also serves as both a waste discharge and reproductive opening.
When oxygen becomes extremely scarce, painted turtles and snapping turtles can even enter a metabolic mode that requires none. This aerobic respiration produces a dangerous buildup of lactic acid, but the turtles can cannibalize calcium from their shells to neutralize the acid buildup.