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After the First Winter |
Even supporters of the old hypothesis realized that painted turtles can not survive freezing after their first winter of life (Storey and Storey 1992). Therefore, it is a universal belief that the painted turtles spend the other winters of their lives hibernating (Storey and Storey 1992, Packard and Packard 2001, etc.). They either rest on the sediment or burry in the mud at the bottom of the shallow parts of ponds, lakes, or rivers. Although burying in the mud prevents oxygen exchange with the water (see below), it also helps the turtles avoid fungus and hide from predators (Ultsch et. al. 1985).The turtles stay in the shallow water even though there is a greater risk of freezing and being found by predators in the shallow water. This is because this allows them to feel the warming water as soon as spring arrives, which is especially important in the north where temperatures are only warm enough for breeding for a short period during the year. If the water gets too close to freezing temperature, the turtles either move to deeper water or bury themselves further in the sediment (Storey and Storey 1992). As shown in Figure 1, painted turtles do not let their temperature drop below 0*C.
Figure 3: Mean body temperature of painted turtle for one year. Created by Kristine Grayson and Michael Dorcas, used with permission of Michael Dorcas.
Lack of Oxygen
The painted turtles remain submerged for months on end. This means they are unable to breathe for long time periods. The oxygen content of the blood drops to close to zero very quickly. Even so, the turtles are able to survive, and studies have shown they can live in deoxygenated water at 2.78*C for 150 days (Storey and Storey 1992). The turtles are able to survive because of many evolutionary adaptations.
- Painted turtles can exchange oxygen and carbon dioxide through their skin, especially since the oxygen content of near-freezing water is high. Numerous studies have shown that survival time is longer and metabolic stress is less when the turtles are kept in oxygenated water instead of deoxygenated water. However, this is not helpful if the oxygen concentration falls to zero or if the turtle is submerged (Storey and Storey 1992).
- The metabolic rate of hibernating painted turtles is only about 10% of normal. This means the animal can survive longer on the small amount of oxygen. The "metabolic arrest" is actually triggered by the drop in oxygen (Storey and Storey 1992).
- Painted turtles have adapted a way to break-up lactic acid. They buffer the acid with calcium and magnesium from their shell. This means they can use anaerobic means, like glycolysis, to generate energy. To prepare for this, turtles store glycogen, the reactant in glycolysis, in all their organs during the summer and fall (Storey and Storey 1992).
Although all painted turtles can survive hibernation and all use similar techniques, species that live further north, like the bellii, have a slower rate of lactate build-up and a smaller gap in ion levels than more southern dorsalis species (see map on the General Information page). This allows the bellii to survive brumation for longer periods of time (Ultsch et. al. 1985).