Part 14

Box turtles were never active at body temperatures below 15 degrees and were seldom active at temperatures below 24 degrees. The two lowest temperatures (15.3A deg. and 16.3A deg.) were taken from individuals crossing roads on overcast days in early May.

In 78 box turtles that were under cover because their environmental temperatures were low, the body temperatures ranged from 2.7 to 30.6 degrees (mean 19.8 A+- 6.38[sigma]). The range of body temperatures in this group is greater than in the other groups shown in Figure 22 because low body temperatures were studied over a wide range of conditions, including hibernation.

Box turtles actually seek cover because of low temperatures only in fall and spring and on occasional unseasonable days in summer when temperatures drop rapidly. Retreat to cover, in the normal cycle of daily activity, is governed usually by high temperatures at mid-day or by darkness at the end of the day. Turtles in dens, burrows, and grass forms, tended to burrow if temperatures remained low for more than a few hours.

Box turtles under cover where they cannot bask have little control over the lower range of body temperatures. The freezing temperatures of winter can be escaped by burrowing deeper into the ground. Temperatures approaching the lethal minimum, however, seldom occur during the season of normal activity. By remaining hidden in a burrow or den therefore, box turtles are fairly well protected from predators but are at a thermal disadvantage.

A number of turtles that had wet mud on their shells were found basking in early morning near ditches, ponds, and marshy areas; several others were partly buried in mud, shortly after daybreak, and another was at the edge of a pond after dark.

Eight adults, located just as they emerged from cover in early morning on sunny days, had body temperatures of 19.7, 21.9, 24.2, 24.5, 25.8, 26.6, 28.7, and 29.5 degrees. In five emerging from earth forms, body temperatures were at least a degree or two below the temperature of the air; the other three came from mud or shallow water and had body temperatures higher than the air temperature.

Temperature is probably the primary stimulus governing emergence after temporary periods of quiescence. Turtles in earthen forms are usually completely covered or are head downward with only the hind quarters exposed. Obviously, the more thoroughly a turtle protects itself (beneath the insulating cover of a form, burrow, or den) against unfavorable temperatures, the longer it will take for favorable temperatures to bring about normal activity again. Turtles in forms and deep burrows have a minimum of contact with the outer environment; but in dens beneath rocks and in shallow burrows light and air can enter freely. Turtles might be influenced in their activities to some extent by the intensity of light at the opening of a burrow or den; they are surely stimulated by changes in the temperature and humidity of air coming through the opening. Shallow retreats that a turtle can enter and leave with the least effort therefore seem most efficient for purposes of thermocontrol, especially when they provide earthen surfaces into which the turtles can burrow more deeply if more severe environmental conditions develop.

In October, 1955, nine T. ornata of various sizes, collected in Douglas County, Kansas, were brought to the laboratory for observation under conditions of controlled temperature. They were kept at room temperature for several days and were fed regularly, with the exception of one hatchling that was fed nothing in this period. On October 22 the turtles were placed in a room where the temperature was maintained constantly at zero degrees. One of the nine turtles, an adult female, was killed with chloroform immediately prior to its removal to the cold room. A list of the turtles used in this experiment is given below.

Age Carapace Weight class length in mm. in grams

1) Hatchling 33.1 8.4 2) Hatchling 29.9 6.7 3) Juvenile 52.5 29.3 4) Juvenile 50.2 26.1 5) Adult 125 376 6) Adult [Female] 118 400 7) Adult 119 386 8) Adult [Female] 110 325 9) Adult [Female] 115 ----

Starved.

Turtles were kept in the cold room for periods of 100 minutes (hatchlings and juveniles) and 200 minutes (adults). The entire experiment, including the time in which the turtles were allowed to warm after they were taken from the cold room, covered a period of nearly six hours (375 minutes) during which the turtles were under constant observation. Individual body temperatures were taken continuously in this period (39 for each juvenile and 24 for each adult) in the order that the turtles were numbered; gaps between records of the body temperature of a given individual therefore represent the time required to record temperatures for the rest of the turtles in the group. The rates of rise and fall of temperature for each of the nine turtles considered are shown as a graph in Figure 23. Rate of temperature change was inversely proportional to bulk; hatchlings, for example, cooled and warmed a little more than twice as rapidly as did adults. Rate of temperature change was intermediate in juveniles but was more nearly like that of adults in the warming phase and closer to that of hatchlings in the cooling phase (Table 5).

Considering that hatchling no. 2 was smaller than no. 1, the rate of change in its temperature did not seem to be significantly altered by starvation. The adult males showed a tendency to change temperature faster than adult females even though both males were larger than any of the females. The slight difference in rate of temperature change between the sexes (Fig. 23) may have been fortuitous.

One hatchling (No. 1), when its temperature dropped below one degree, fully extended all four limbs and the body was elevated and only the anterior edge of the plastron was in contact with the confining glass dish. Raising the body from an uncomfortably cold or hot substrate is a well known phenomenon in many lizards and in crocodilians, but to my knowledge has not been reported for turtles.

TABLE 5.--Average Rate of Change in Temperature (Expressed in Degrees per minute) for four Groups of Turtles Subjected to Temperature of Zero Degrees and then Allowed to Warm at 27 Degrees (Centigrade). ==================+========+=========+============= | | Cooling | Warming GROUP | Number | phase | phase | | | (to 25A deg.) ------------------+--------+---------+------------- Hatchlings | 2 | .282 | .310 | | | Juveniles | 2 | .264 | .180 | | | Adult | 2 | .122 | .152 | | | Adult [Female] | 3 | .119 | .130 | | | Adult (all) | 5 | .120 | .138 ------------------+--------+---------+-------------

None of the females reached a temperature of 25A deg. before the experiment was terminated.

Hibernating turtles and those experimentally chilled were usually comatose but were almost never completely incapacitated even at temperatures at or near zero degrees. Experimental pinching, probing, and pulling revealed that muscles operating the neck, the limbs, and the lobes of the plastron could be controlled by the turtle at low temperatures; hissing, resulting from rapid expulsion of air through the mouth and nostrils (when the head and limbs are drawn in reflexively) occurred at all body temperatures but was sometimes barely audible in the coldest turtles. Of all living turtles observed, only two (hatchlings 1 and 2 in coldroom experiment) were completely immobile at low temperatures, failing to respond even to pinpricks at body temperatures of 0.8 and 1.7 degrees, respectively, although other turtles, under the same experimental conditions, consistently gave at least some response to the same stimulation.

Turtles chilled experimentally continued to move about voluntarily, albeit sluggishly, at temperatures much lower (2.5A deg. for each of four adults; 10.0A deg. and 6.2A deg. for two juveniles) than those at which locomotion was resumed in the warming phase (13A deg. for the adults, 21.7A deg. and 20.1A deg. for the juveniles). Hatchlings chilled so rapidly that it was difficult to ascertain accurately the temperature at which inactivity was induced. Juveniles became active gradually, moving slowly about when the body temperature reached approximately 20 degrees but not attempting more strenuous activities such as climbing the walls of enclosures, until body temperatures of 22 to 25 degrees were attained. Adults, on the other hand, exhibited "normal" activity as soon as they became voluntarily active.

The ability of ornate box turtles to move about when the body temperature is near the lethal minimum probably enables those caught in the open by a sudden drop in environmental temperature to find cover that keeps them from freezing to death. Prolonged chilling, on the other hand, seems to create a physiologically different situation; the temperature at which activity is resumed is higher and subject to less variation.

Juveniles were more rapidly affected by environmental temperatures, were subject to different thresholds, and were inactive over a wider range than were the adults. Indeed, the rate of chilling, rather than absolute body temperature alone, might in large measure influence the reactions of turtles to environmental temperatures. If this be so, smaller turtles, having a narrower thermal range of normal activity, must lose at least some of the advantages gained by their ability to warm up more rapidly.

Hatchlings and juveniles at the Damm Farm were always active on days when at least some adults were also active. Fitch (1956b:466) found that, in northeastern Kansas, species of small reptiles and amphibians are active earlier in the season than larger species and that the young of certain species become active earlier than adults. Fitch stated, "... small size confers a distinct advantage in permitting rapid rise in body temperature by contact with warmed soil, rock or air, until the threshold of activity is attained"; he pointed out also that young animals, if able to emerge earlier than adults, would benefit from a longer growing season. Hatchlings and juveniles of T. ornata would benefit greatly from an extra period of activity of say, one or two weeks in spring and a similar period in autumn, especially if food were plentiful. The extra growth realized from such a "bonus" period of feeding would significantly increase the chance of the individual turtle to survive in the following season of growth and activity.

Ornate box turtles are active within a narrower range of temperatures than are aquatic turtles in nearby ponds and streams of the same region. Observations by William R. Brecheisen and myself on winter activity of aquatic turtles indicate that, in Anderson County, Kansas, the commoner species (Chelydra serpentina, Chrysemys picta, and Pseudemys scripta) are more or less active throughout the year; although they usually do not eat in winter, they are able to swim about slowly and in some instances (P. scripta) even to carry on sexual activity at body temperatures only one or two degrees above freezing. But, ornate box turtles hibernating in the ground a few yards away are incapable of purposeful movement at such low body temperatures.

HIBERNATION

In northeastern Kansas ornate box turtles are dormant from late October to mid-April--approximately five and one half months of the year. Individuals may be intermittently active for short periods at the beginning and end of the season, however. Once a permanent hibernaculum is selected dormancy continues until spring; unseasonably warm weather between mid-November and March does not stimulate temporary emergence. There is little movement during dormancy except for the deepening or horizontal extension of the hibernaculum.

Woodbury and Hardy (1948:171) found desert tortoises (Gopherus agassizi) in dormancy from mid-October to mid-April in southwestern Utah; some tortoises became temporarily active on warm days in winter. Cahn (1937:102) was able to compare hibernation in several individuals each of T. ornata and T. carolina, kept under the same conditions in Illinois. Individuals of T. ornata burrowed into the ground in October, two weeks before those of T. carolina did, and continued to burrow to a maximum depth of 22A1/2 inches. Some individuals of T. carolina spent the entire winter in the mud bottom of a puddle and became semiactive on warm winter days. Other individuals of T. carolina burrowed nearly as deeply as did T. ornata. Individuals of T. ornata emerged from hibernation one or two weeks later in the spring than did those of T. carolina. There are some indications that populations of T. carolina in eastern Kansas are dormant for a shorter period of time than those of T. ornata but comparative studies are needed to verify this. Richard B. Loomis gave me a large female of T. carolina that he found active beside a highway in Johnson County, Kansas, on November 23, 1954; on that date most individuals of T. ornata under my observation had already begun permanent hibernation but a few at the Reservation were still semiactive.

Fitch (1956b:438) listed earliest and latest dates on which box turtles were active at the Reservation in the years 1950 to 1954; in the five year period box turtles were active an average of 162 days per year (range, 140-187) or approximately 5.3 months of the year. It is significant that 1954, having the most days of activity was, according to my studies of growth-rings, an exceptionally good year for growth. Fitch's data indicate the approximate season of growth and reproduction but not of total activity, since he did not take into account the sporadic movements of box turtles in late fall and early spring.

Activity in autumn is characterized by movement into ravines and low areas; many turtles move into wooded strips along the edges of fields or small streams. Sites protected from wind, providing places for basking and for burrowing, are sought. Burrows of other animals, along the banks of ravines, were often used for temporary shelter; overhanging sod at the lips of ravine-banks provided cover beneath which turtles could easily burrow. After mid-October progressively fewer box turtles were found in open places and activity was restricted to a few hours in the warmest part of the day.

Low air temperature probably is the primary stimulus for hibernation. Autumn rains are usually followed by a decrease in general activity. Rain probably hastens burrowing by softening the ground.