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 A+- 3.08[sigma]). 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).
[Illustration: 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.]