Thermal Environment of Overwintering Ornate Box Turtles, Terrapene Ornata Ornata, in Iowa
Bernstein, Neil P., Black, Robert W., The American Midland Naturalist
The carapace temperatures of ornate box turtles ( Terrapene ornata ornata) were monitored by affixing temperature dataloggers during two winters, 2001-2002 and 2003-2004. The dataloggers recorded temperature every 3 h to the nearest 0.5 C. Date of burrowing in the fall, emergence time in the spring and temperatures 48 h prior to emergence were analyzed along with the entire temperature profile of the turtles during the winter. In 2003-2004 soil temperatures from the surface to 0.75 m depth were also monitored.
Most turtles did not experience freezing temperatures during either winter, but two turtles were below freezing for up to 54 consecutive days during winter 2001-2002.
During 2001-2002, eight of nine turtles began burrowing within 7 d of each other in the fall, and all nine emerged within 7 d of each other in the spring. During 2003-2004, fall burrowing for 16 turtles occurred within 14-21 d, and emergence happened within 15 d. Overwintering periods varied between 172-201 d, which was intermediate between dates noted in studies to the north, south and west for this species. During 2003-2004, it was inferred that turtles burrowed to a minimum of 0.5-0.75 m by comparing turtle temperatures to those recorded at different depths in the soil, although we recorded a previous depth of up to 1.67 m.
Emergence onto the surface in the spring did not relate to soil temperature, and there was no support for emergence triggered by 48 h of soil temperatures above 7 C (Grobman, 1990). It is suggested that spring emergence may be correlated with a more complex set of factors and that Terrapene ornata ornata can withstand prolonged freezing temperatures.
The ornate box turtle (Terrapene ornata ornata) is primarily a prairie species, ranging from Indiana to the Rocky Mountains and from southern South Dakota, Minnesota and Wisconsin to Texas and Louisiana (Conant and Collins, 1991; Dodd, 2001). Within this geographic range, especially in Iowa, they are often found associated with sandy soils, which presumably facilitates burrowing (Dodd, 2001).
Litzgus et al. (1999) found that Clemmys guttata overwintered in thermally stable environments; however, Terrapene ornata ornata burrow to avoid wintertime freezing temperatures (Legler, 1960; Doroff and Keith, 1990). When underground, T. o. ornata body temperatures approximate soil temperatures (Legler, 1960). Terrapene ornata ornata burrow deeper in the northern portions of their range (Legler, 1960; Doroff and Keith, 1990) and have been reported to burrow progressively deeper into the soil as freezing temperatures descend (Legler, 1960). Progressive burrowing correlated with decreasing winter temperatures was also reported for the three-toed box turtle (T. Carolina triunguis) (Carpenter, 1957). Nevertheless, burrowed box turtles can freeze and die (Carpenter, 1957; Metcalf and Metcalf, 1979).
An increase in soil temperature has been suggested to trigger spring emergence. Grobman (1990) reported that individuals emerged when subsoil temperatures were consistently above 7 C and Curtin (1997) reported emergence correlated with an inversion of soil temperature that persisted for longer than 48 h. However, Dodd (2001) stated that spring emergence was probably related to a combination of soil temperature and moisture.
Based on these observations, we tested two hypotheses regarding the relationship between temperature and winter burrowing in ornate box turtles:
1) that successfully burrowing Terrapene ornata ornata maintain a position in the soil where the temperature is above freezing; and
2) that spring emergence is temperature sensitive.
To test these hypotheses, we continuously monitored the carapace temperatures of overwintering Terrapene ornata ornata near the northern limits of the range of this species. Measurements were made immediately prior to burrowing in September, throughout the winter and immediately following emergence in April. …