Table 1.—The Relationship of Temperature and Duration of Incubation Period as Determined from Laboratory Studies of 49 Eggs of T. ornata. | |||||
| Average daily temperature (Fahrenheit) | Period of incubation (Days) | Number of clutches | Number of eggs | Remarks | |
| Mean | Range | ||||
| 91 | 59 | 56-64 | 6 | 24 | Wide daily fluctuations in temperature |
| 82 | 70 | 67-73 | 4 | 21 | Wide daily fluctuations in temperature |
| 75 | 125 | 124-127 | 2 | 4 | Temperature thermostatically controlled |
Sixty-five days seems to be a realistic estimate of a typical incu bation period under natural conditions; eggs laid in mid-June would hatch by mid-August. Even in years when summer temperatures are much cooler than normal, eggs probably hatch by the end of October. Hatchlings or eggs would have a poor chance of surviving a winter in nests on exposed cut-banks or in other unprotected situations. Overwintering in the nest, hatchlings might survive more often than eggs, since hatchlings could burrow into the walls and floor of the nest cavity. Unsuitable environmental conditions that delay the nesting season and retard the rate of embryonic development may, in some years, be important limiting factors on populations of ornate box turtles.
In areas where T. ornata and T. carolina are sympatric (for example, in Illinois, Kansas, and Missouri) the two species occupy different habitats, ornata preferring open grassland and carolina wooded situations. Under natural conditions, the average incubation periods of these two species can be expected to differ, T. carolina having a somewhat longer period due to lower temperatures in nests that are shaded. In the light of these speculations, the remark of Cahn (1937:102)—that T. ornata nested later in the season (in Illinois) and compensated for this by having a shorter incubation period—is understandable.
The range of temperatures tolerated by developing eggs probably varies with the stage of embryonic development. When temperatures in the laboratory were 102 to 107 degrees Fahrenheit for approximately eight hours, due to a defect in a thermostat, the young in two eggs of T. ornata, that had begun to hatch on the previous day, were killed, as were the nearly full-term embryos in a number of eggs of T. carolina (southern Mississippi) kept in the same container. A five-day-old hatchling of T. ornata, kept in the same container, survived the high temperatures with no apparent ill effects. Cagle (1950:41) found that eggs of Pseudemys scripta could not withstand temperatures of 10 degrees for two weeks nor would they survive if incubated at 40 degrees. Cunningham (1939) reported that eggs of Malaclemys terrapin could not survive prolonged exposure to temperatures of 35 to 40.6 degrees but tolerated temporary exposure to temperatures as high as 46 degrees.
In the summer of 1955, a clutch of three eggs, all of which contained nearly full-term embryos, was placed in a refrigerator for 48 hours. The temperature in the refrigerator was maintained at approximately 4.5 degrees; maximum and minimum temperatures for the 48 hour period were 2.8 and 9.5 degrees, respectively. When the eggs were removed from the refrigerator they showed gains in weight and increases in size comparable to eggs, containing embryos of the same age, used as controls. The experimental eggs began to hatch two days after they were removed to normal temperatures—approximately 24 hours later than the controls.
In the late stages of incubation, the outer layer of the shell becomes brittle and is covered with a mosaic of fine cracks or is raised into small welts. Several days before hatching, movements of the embryo disturb the surface of the shell and cause the outer layer to crumble away, especially where the head and forequarters of the embryo lie against the shell. Some embryos could be seen spasmodically thrusting the head and neck dorsally against the shell.
The role of the caruncle in opening the shell seems to vary among different species of turtles. Cagle (1950:41) reported that it was used only occasionally by Pseudemys scripta; Allard (1935:332) thought that it was not used by Terrapene carolina; and, the observations of Booth (1958:262) and Grant (1936:228) indicate that embryos of Gopherus agassizi use the caruncle at least in the initial rupturing of the shell.
In the three instances in which hatching was closely observed in T. ornata, the caruncle made the initial opening in the shell; claws of the forefeet may have torn shells in other hatchings that were not so closely observed. In all observed instances, the shell was first opened at a point opposite the anterior end of the embryo. The initial opening had the appearance of a three-cornered tear. A quantity of albuminous fluid oozed from eggs as soon as the shells were punctured.
The initial tear is enlarged by lateral movements of the front feet, and later the hind feet reach forward and lengthen the tear farther posteriorly. In many instances a tear develops on each side and the egg has the appearance of being cleft longitudinally. The young turtle emerges from the anterior end of the shell or backs out of the shell through a lateral tear.
The process of hatching, from rupture of shell to completion of emergence, extended over three to four days in the laboratory. Many hatchlings from time to time crawled back into the shell over a period of several days after hatching was completed. In a clutch of eggs kept in a pail of earth, by William R. Brecheisen, eight days elapsed between onset of hatching and appearance of the first hatchling at the surface.