Painted turtles in climates like ours hunker down for winter and don’t emerge until the sun is out and plants are sprouting again in the spring. On particularly cold, blustery winter days, I tend to think that isn’t a bad plan!

Painted turtles bask at Nature Boardwalk in this photo from April 2011.

Unlike humans, which need to keep a fairly consistent body temperature regardless of their surroundings, painted turtles are ectotherms, which means their body temperature depends on their environment. In the winter, as the pond begins to freeze, the painted turtles at Nature Boardwalk at Lincoln Park Zoo find a nice spot in the mud at the bottom, fairly close to shore and under the ice, where they will remain (more or less) for the rest of the winter.

Their body temperature drops to approximately that of the surrounding water. Their metabolism slows to a crawl, and they won’t come up for air until spring. Even though they abstain from breathing, they still have some minimal oxygen requirements, which they meet by taking up oxygen from the surrounding water through their skin. It’s therefore important that the dissolved oxygen content in the pond’s water is adequate all winter so the turtles can take in oxygen.

At Nature Boardwalk, aerators run all winter to keep patches of the pond from freezing over completely. This ensures that dissolved oxygen levels are sufficiently high. Taking up dissolved oxygen in water through the skin is obviously not as effective as breathing, but the turtles seem to make it work!

Last year the turtles we were tracking all overwintered in the vicinity of the island. This year it looks like a similar pattern is occurring even though we’re tracking different turtles this time. The turtles are slowing down, but still moving around a bit, as they settle in. This is a map of three of the turtles’ locations on December 14, 2011.

Perhaps the most interesting thing about all this is that even with their metabolisms practically shut down—their body temperatures nearly freezing—the turtles do in fact move around. Last year we tracked them through the winter using radio telemetry. While the turtles stuck to the edges around the island in the pond, they did mosey their way around, often making small, but perceptible, movements from week to week.

Urban Wildlife Institute Intern Mason Fidino uses radio telemetry equipment to pinpoint the turtles' locations in the pond as they ready themselves for winter.

We wouldn’t know this without radio telemetry. In fact, radio telemetry has helped scientists make many discoveries about the physiology of how turtles overwinter; without this technology we’d know a lot less about these amazing adaptations.

We don’t expect to see the painted turtles at Nature Boardwalk for several months now, but we’ll still be ‘keeping an eye’ on them via their radio transmitters!

Vicky Hunt

P.S. Learn more about what zoo reptiles do during winter with Curator Diane Mulkerin’s blog!