Western Painted Turtles (Chrysemys picta bellii) are the most anoxia-tolerant tetrapods known, capable of surviving anaerobically at 3°C for nearly 5 mo. In the northernmost latitudes of their range, adult painted turtles can experience winters lasting 6–7 mo. During this time, the pond surface is covered with ice and snow, denying the turtles access to air and trapping them in water that can become severely hypoxic or anoxic. Previous studies have reported that turtles overwinter near the bottom of ponds, either buried in mud or on top of the substrate, where oxygen levels are hypoxic or anoxic. The oxygen is depleted by other respiring biomass within the mud, but why the turtles are found on or within it is not understood. To better understand the overwintering behavior of painted turtles, we conducted a series of laboratory experiments to determine how a turtle's buoyancy is affected by temperature, depth, and access to air. Acclimating turtles to 3°C resulted in a large variation in turtle buoyancy, but preventing access to air caused all turtles to become negatively buoyant. We suggest that painted turtles overwinter in the mud or on the pond bottom not necessarily because of any particular behavioral adaptation, but simply because they become negatively buoyant while overwintering without access to air.
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Vol. 70 • No. 4