Cowles and Bogert (1944:275-276) and Woodbury and Hardy (1948:177) emphasized the influence of soil temperatures on body temperatures. It is thought that air temperatures played a more important role than soil temperatures in influencing body tempera tures of T. ornata. Soil temperatures were taken in the present study only when the turtle was in a form, hibernaculum, or den.
Optimum Temperature
Cowles and Bogert (1944:277) determined optimum levels of body temperature of desert reptiles by averaging body temperatures falling within the range of normal activity; they defined this range as, "… extending from the resumption of ordinary routine [activity] … to … a point just below the level at which high temperatures drive the animal to shelter." Fitch (1956b:439) considered optimum body temperature in the several species that he studied to be near the temperature recorded most frequently for "active" individuals; he found (loc. cit.) that of body temperatures of 55 active T. ornata, 66 per cent were between 24 and 30 degrees, and that the temperatures 27 and 28 occurred most frequently. Fitch concluded (op. cit.:473) that the probable optimum body temperature of T. ornata was 28 degrees and that temperatures from 24 to 30 degrees were preferred. Although Fitch treated all non-torpid individuals that were abroad in daytime as "active" and did not consider the phenomenon of basking, his observations on optimum body temperature agree closely with my own.
Body temperatures of 153 box turtles that were known definitely to be active, ranged from 15.3 to 35.3 degrees. The mean body temperature for active turtles was 28.8 degrees (± 3.78σ) ([Fig. 22]). Ninety-two per cent of the temperatures were between 24 and 30 degrees and 50 per cent were between 28 and 32; temperatures of 29 and 30 degrees occurred most frequently (22 and 21 times, respectively). The ten body temperatures below 24 degrees all were recorded before 9 A. M. on overcast days when the air was cool and humid. It is noteworthy that two of these low temperatures (18.8° and 19.0°) were from a copulating pair of turtles; two others (21.8° and 22.0°) were from individuals that were eating. The highest temperature (35.3°) was from a large female that was feeding at mid-morning in a partly shaded area.
The mean body temperature for active individuals ([Fig. 22]) is probably somewhat below the ecological optimum, because a few temperatures were abnormally low. The large number of body temperatures in the range of 29 to 31 degrees indicates an optimum closer to 30 degrees. Optimum body temperatures may vary somewhat with the size, sex, or individual preference of the turtle concerned.
Basking
Although basking is common in terrestrial turtles, only a few authors have mentioned it. Woodbury and Hardy (1948:177-178) did not use the term in their account of thermal relationships in Gopherus agassizi; their discussion indicates, however, that the tortoises move alternately from sunny to shady areas to regulate body temperature. Desert tortoises removed from hibernacula and placed in the sun warmed to approximately 29.5 degrees before they became active, although a few did so at temperatures as low as 15 degrees. According to Cagle (1950:45), Sergeev (1939) studied body temperature and activity in the Asiatic tortoise, Testudo horsefieldi, and found that individuals basked for as much as two hours in the morning before beginning the first activity of the day (feeding), but that tortoises did not bask after a period of quiescense from late morning to late afternoon, during which body temperatures were seemingly maintained nearer the optimum than they were during nocturnal rest; body temperatures rose to approximately 30 degrees before the tortoises became active. Since body temperatures of 23 to 24 degrees were maintained at night, the basking range of Testudo horsefieldi may be considered to be approximately 23 to 32 degrees.
Ornate box turtles basked chiefly between sunrise and 10 or 11 A. M. Body temperatures of 60 basking turtles ranged from 17.3 to 31.4 degrees (mean, 25.5 ± 3.08σ). More than two-thirds (42) of these body temperatures were higher than the air temperature near the turtle, indicating probably that body temperature rises rapidly once basking is begun. In the instances where body temperature was below air temperature, the turtles had recently begun to bask (many were known to have just emerged from forms or other cover where they had spent the night) or were warming up more slowly because of reduced sunlight. On cloudy days basking began later than on clear days and body temperatures usually remained at a suboptimum level. Turtles that basked on days that were cloudy and windy, or cold and windy, did so in sheltered places, usually on the leeward sides of windbreaks such as limestone rocks, rock fences, or ravine banks. It was evident in these instances that the turtles either sought such shelter from the wind or remained ensconced in the more complete shelter of a form or burrow, not emerging at all.
Open areas of various kinds were used as basking sites. Level ground—such as on roads, cattle pathways, and bare areas surrounding farm ponds—having unobstructed morning sunlight, nearby dense vegetation, and choice opportunities for feeding (cow dung, mulberry trees) was preferred. Basking was frequently combined with feeding; in several instances box turtles were noted early in the morning at suboptimum body temperatures eating grasshoppers, berries, or dung insects. The predilection of box turtles for open areas is probably important in permitting extended activity at suboptimum temperatures. T. ornata probably carries on more nearly normal activity on cool days than do reptilian species with more sharply delimited thermal tolerances. Collared lizards (Crotaphytus collaris), for example, are chiefly inactive on days when the sky is overcast, although a few individuals having suboptimum body temperatures can be found in open situations (Fitch, 1956a:229 and 1956b:442).