Chrysemys picta, painted turtles, are small and belong to family Emydidae, order Testudinidae. They are found in shallow, slow-moving water in North America, but come out of the water to bask. They have winter dormancy. Hatchlings born late in the year overwinter in their nests, which are usually less than twelve centimetres below the soil surface and exposed to cold (Ernst et al., 1994.)
Packard et al. (1997) studied the effects of temperature on overwintering hatchlings. They wished to study conditions in the field, aiding research into the adaptations of Chrysemys picta for cold tolerance. Eggs were removed from nests in June and incubated in the lab. The hatchlings were acclimated by 1°C a day down to 2°C, and free water was prevented from contacting them so ice would not form. The hatchlings were returned to the nests, along with a thermistor hooked to a datalogger to record temperatures. They were recovered March 23, 1996. Temperature data was analyzed for December 1 to March 9, revealing that temperatures varied within and between nests. The results of the study were then combined with results from a study the previous year. The experimenters found that survival was positively correlated with higher minimum temperatures and negatively with temperatures continuously below -2°C and -4°C. By plotting the data on a probability scale, the experimenters predicted that all hatchlings would survive in a nest with temperatures higher than -5.1°C and all would die below -15.8°C. Because of the duration of low tempperatures in some nests, the experimenters concluded that the hatchlings could not have used freezing to survive. They concluded that Chrysemys picta hatchlings must use other mechanisms, such as supercooling or a combination of supercooling and freezing, to survive low temperatures.
Ernst, Carl H., Roger W. Barbour, and Jeffrey E. Lovich, 1994. Turtles of the United States and Canada. Smithsonian Institution Press, Washington, 276-296.
Packard, Gary C., Sarah L. Fasano, Marcus B. Attaway, Leslie D. Lohmiller, and Trina L. Lynch, 1997. Thermal environment for overwintering hatchlings of the painted turtle (Chrysemys picta). Canadian Journal of Zoology, 75:401-406.
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