Seasonal Thermal Ecology of Bog Turtles (Glyptemys muhlenbergii) in Southwestern Virginia
Aquatic and semiaquatic turtles use wetlands and thermally buffered soils to thermoregulate during the active season and to sustain body temperatures above freezing during overwintering. We deployed temperature data loggers during wintertime and summertime in wetlands occupied by Bog Turtles (Glyptemys muhlenbergii) in southwestern Virginia, United States, to contrast the temperatures of the wetland environment to turtle carapace temperatures over two field seasons. Temperature signatures were used to evaluate probable hibernation depth and the timing and cues of spring emergence. We measured soil temperature profiles during winter in saturated hibernacula and similar but unsaturated structures to evaluate the dependence of temperature on water-table depth. Mean daily turtle temperature (n = 16) during the coldest portion of two winters was 2.5°C (SE = 0.29), with one turtle experiencing 14 continuous days at temperatures between −1°C and 0°C when ambient temperatures dipped below −10°C. Water tables remained within 15 cm below the soil surface throughout both winters, preventing freezing temperatures for shallow-hibernating turtles. The date of first emergence differed by 15 d between the two study years, with both soil temperature and ambient temperature potentially serving as emergence cues. During summer activity, turtles alternated between periods of thermoregulation and thermoconformity by basking and accessing deep, thermally buffered saturated soil. The dependence of Bog Turtle body temperature on soil saturation and depth is important for understanding how hydrologic alteration in wetlands can affect Bog Turtle ecology.Abstract
Typical wintertime mean daily temperatures of ambient conditions, Bog Turtle carapaces, and surrounding soil measured at the GDF wetland in January and February, 2009.
Mean daily maximum temperatures of ambient conditions, soil at 10-cm depth, and carapaces of Bog Turtles (n = 8) before and during the time of emergence from winter hibernacula in 2008 (top) and 2009 (bottom). Error bars on average turtle temperatures show the standard deviation among the turtles. Dotted line on both graphs is at 10.3°C, the mean daily maximum temperature of soil at 10 cm at time of emergence calculated from both study winters.
Mean daily ambient and soil temperatures of true and simulated hibernacula during an 8-d consistently cold period in January 2009. Simulated hibernacula (n = 4) were excavated in randomly selected areas of drier soil located within an approximate 50-m distance to the true hibernacula (n = 4). Thermochrons were placed at the soil surface (data not shown), 10 cm, 20 cm, and 30 cm (not shown) in each replicate.
Depth to the water table at the location of multiple Bog Turtle hibernacula over two winters in Southwestern Virginia. Water-table values were averaged over 7-d periods. The hibernacula were approximately 30 cm deep and the water table remained above this level during winters 2007–2008 and 2008–2009. The drawdown of the water table in the summer of 2008 resulted from a 2-yr drought (Feaga et al., 2012).
Mean daily temperatures between 15 June 2008 and 31 July 2008 at four wetlands in southwestern Virginia for ambient conditions, soil at several depths, and Bog Turtle carapaces (n = 8). Soil at 10- and 25-cm depths were cooler than ambient conditions during the day and warmer than ambient conditions at night, providing a thermal buffer available to moderate turtle temperature.
Relationship between Bog Turtle carapace temperatures and soil surface temperatures from 15 June to 30 July 2008 at four wetlands in southwestern Virginia. Points represent each unique temperature pair measured at each 90-min interval.
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