Fig. 22. The relationship of body temperature (Centigrade) and kind of activity in T. o. ornata, compiled from 355 field observations. Vertical and horizontal lines represent, respectively, the range and mean. Open and solid rectangles represent one standard deviation and two standard errors of the mean, respectively.
Toleration of Thermal Maxima and Minima
The foregoing remarks on basking indicate the approximate, normal, thermal tolerances of ornate box turtles. Many additional records of body temperature were taken from turtles that were found under cover. Turtles under cover in daylight were usually seeking protection from either below-optimum or above-optimum temperatures. In avoiding low temperatures, turtles usually chose more complete and permanent cover than in avoiding high temperatures.
Body temperatures of 64 box turtles that were seeking cover or that were under cover because of high temperatures ranged from 28.9 to 35.8 degrees (mean, 31.9 ± 1.55σ). Fifty-nine of these temperatures (92 per cent) were 30 degrees or higher. [Figure 22] shows this range to overlap broadly with the temperature range of active turtles and the means of the two groups are close to each other. Body temperatures below 30 degrees (5) were all recorded late in the morning on hot summer days when the air temperature was well above 30 degrees; they are somewhat misleading because they are from turtles that were under cover long enough to lower body temperature to the range of activity although the turtles remained under cover because of hazardous environmental temperatures.
The commonest retreats used by box turtles to escape heat were burrows of other animals and small dens under thick limestone rocks, where the air remained cool, even in late afternoon. Most of the burrows and dens on the Damm Farm were known to me and could be checked each day. Turtles seeking temporary refuge from high temperatures characteristically rested just inside the opening of a den or burrow. Less frequently, turtles burrowed into ravine banks or just under the sod on level ground. A number of individuals with above-optimum body temperatures were found in the shade of trees or high weeds in early afternoon on hot days. Mulberry trees provided ample shade for such activity and, in June and July, when ripe mulberries were abundant on the ground, turtles frequently fed on them at times of the day when temperatures were more hazardous in other areas.
Several turtles were found buried in mud or immersed in water at the edges of ponds in the hottest part of the day; they were discovered at first by accident and, on subsequent field trips by systematic probing. Ordinarily the turtles were covered with mud or muddy water and remained motionless, except for periodically raising the head to the surface to breath. There was little vegetation near the edges of ponds and by late morning on hot days the temperature of the shallowest water was as high as the air temperature or higher. Correspondingly, turtles found resting in mud and water had body temperatures much higher than turtles in dens, burrows, or forms at the same time of day. Box turtles that retreat to mud or shallow water cool themselves less efficiently than they would in drier, better protected microhabitats. I found no evidence that turtles went into deeper water to cool themselves.
The length of time spent under cover varied; most turtles had two daily periods of activity, the second beginning in late afternoon. Some turtles moved from shelter to shelter in the time between periods of activity. Several turtles were known to remain quiescent continuously for several days in the hottest part of the summer.
The maximum temperature that a reptile can tolerate physiologically is ordinarily higher than the maximum temperature tolerated voluntarily (Cowles and Bogert, 1944:277); but, the two maxima may be separated by only a few degrees. Most poikilothormous vertebrates neither tolerate nor long survive body temperatures exceeding 40 degrees (Cowles and Bogert, op. cit.:269).
It is evident ([Fig. 22]) that ornate box turtles do not often tolerate body temperatures above 33 degrees and that temperatures in excess of 35 degrees are probably never tolerated under natural conditions. At 9:15 A. M. on July 5, 1955, an adult female emerged from mud where she had spent the night (body temperature 28.4°, mud 28.4°, air 30°). After foraging for 40 minutes in bright sunlight on a grassy hillside she had moved approximately 100 feet and her temperature had reached 34.6 degrees (air 33.0°). At 9:56 A. M. she moved rapidly and directly to a den under a rock nearby; 15 minutes later her body temperature had not changed but after 65 minutes it had dropped to 33.4 degrees. The temperature of air in the den was 31 degrees. This female began her activities at nearly optimum body temperature relatively late in the morning and, by foraging intensively for less than one hour, probably was able nearly to satisfy her daily food requirements; by foraging near suitable cover she could remain active until her body temperature reached a critical threshold, and she thereby saved time otherwise required for finding cover or making a form.