Part 7

Eggs of T. ornata expand in the course of incubation, as do other reptilian eggs with flexible shells, owing to absorption of water. In the laboratory, 48 eggs increased by an average of approximately three grams in weight and three millimeters in width over the entire period of incubation; increase in width coincided with decrease in length. Cotton in incubation dishes was kept moist enough so that some water could be squeezed from it. When the cotton was constantly moist, eggs showed a fairly steady expansion from the first week of incubation until hatching. The process could be reversed by allowing the cotton to dry. Eggs that were allowed to dry for a day or more became grossly dented or collapsed. Eggs at the periphery of the incubation dish were ordinarily more seriously affected by drying than were those at the center or in the bottom of the dish. A generous re-wetting of desiccated eggs and cotton caused the eggs to swell to their original proportions within 24 hours. Recessions occurred, however, even in the clutches that received the most nearly even amount of moisture. Increases in weight and size seemed to reach a peak in the middle of the incubation period and again immediately before hatching. Infertile eggs expanded in the same manner as fertile eggs in the first week or two of incubation, but thereafter gradually regressed in bulk or failed to re-expand after temporary periods of dryness. Fertile eggs that were in good condition had a characteristically turgid, springy feel and could be bounced off a hard surface.

Temporary lack of moisture usually did not kill embryos; prolonged dryness, combined with high temperatures, probably could not be tolerated. Lynn and Ullrich (1950), by desiccating the eggs of Chrysemys picta and Chelydra serpentina, produced abnormalities in the young ranging from slight irregularities of the shell to eyeless monstrosities; eggs desiccated in the latter half of incubation produced a higher percentage of abnormal young than eggs that were desiccated earlier.

In 1956, three fertile eggs, from clutches that were at different stages of incubation, were immersed in water for 48 hours. The eggs rested on the bottom of the bowl in the same position in which they had been placed in the incubation dishes; when turned, they returned invariably to the original position. The embryos in two of the eggs (one and 27 days old at the time of immersion) were still living ten days after the eggs were removed from the water; the embryo in the remaining egg (21 days old at the time of immersion) was dead. Eggs immersed in water increased in size and weight at the same rate as eggs in incubation dishes, indicating that absorption of water probably operates on a threshold principle, the amount absorbed being no more than normal even under wet conditions.

Natural nests usually are in well-drained areas, but water probably stands in some nests for short periods after heavy rains. Provided the nest cavity itself is not damaged, water in the nest is probably more beneficial than harmful to the eggs; however, nests that are inundated during floods probably have little chance of survival.

Embryonic Development

Eggs were examined by transmitted light in the course of incubation. At the time of laying (or removal from oviducts) no embryonic structures were discernible even in eggs that had been retained in the oviducts of captive females some weeks past the normal time of laying; a colorless blastodisc could be seen if eggs were opened. Embryonic structures first became visible at eight to ten days of incubation; at this time vascularization of the blastodisc was evident and the eyes appeared as dark spots. Heart beats were observed in most embryos by the fifteenth day but were evident in a few as early as the tenth day. The pulse of a fifteen-day-old embryo averaged 72 beats per minute at a temperature of 30 degrees. Embryos at fifteen days, measured in a straight line from cephalic flexure to posteriormost portion of body, were approximately nine to ten millimeters long and at 22 days were 14 millimeters long. At approximately 35 days the eggs became dark red; embryonic structures were discernible thereafter only in eggs that had embryos situated at one end, close to the shell.

Incubation periods for 49 eggs (representing 12 clutches) kept in the laboratory ranged from 56 to 127 days, depending on the temperature of the air during the incubation period. In 1955, eggs were kept at my home in Lawrence where air temperatures were uncomfortably hot in summer and fluctuations of 20 degrees (Fahrenheit) or more in a 24-hour period were common. The following summer eggs were kept in my office at the Museum where temperatures were but slightly cooler than in my home and subject also to wide variation. In 1957 this part of the Museum was air-conditioned and kept at approximately 75 degrees. The greater lengths of incubation periods at lower temperatures are shown in Table 1. Risley (1933:698) found the incubation period of Sternotherus odoratus to be longer at lower temperatures; corresponding observations were made by Allard (1935:332) and Driver (1946:173) on the eggs of Terrapene carolina. Cagle (1950:40) and Cunningham (1939) found no distinct differences in length of incubation period for eggs of Pseudemys scripta and Malaclemys terrapin, respectively, at different temperatures within the range tolerated by the eggs.

Most nests observed in the field were in open situations where they would receive the direct rays of the sun for at least part of the day; the shorter average incubation periods (59 and 70 days, respectively), observed in 1955 and 1956, therefore, more nearly reflect the time of incubation under natural conditions than does the excessively long period (125 days at 75 degrees) observed in 1957 under cooler, more nearly even temperatures.

TABLE 1.--The Relationship of Temperature and Duration of Incubation Period as Determined from Laboratory Studies of 49 Eggs of T. ornata. =============+===================+=========+========+================= Average | Period of | Number | Number | daily | incubation (Days) | of | of | Remarks temperature |--------+----------|clutches | eggs | (Fahrenheit) | Mean | Range | | | -------------+--------+----------+---------+--------+----------------- | | | | | Wide daily 91 | 59 | 56-64 | 6 | 24 | fluctuations in | | | | | temperature -------------+--------+----------+---------+--------+----------------- | | | | | Wide daily 82 | 70 | 67-73 | 4 | 21 | fluctuations in | | | | | temperature -------------+--------+----------+---------+--------+----------------- | | | | | Temperature 75 | 125 | 124-127 | 2 | 4 | thermostatically | | | | | controlled -------------+--------+----------+---------+--------+-----------------

Sixty-five days seems to be a realistic estimate of a typical incubation 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.

A nest in an outdoor pen at the Reservation was discovered in early October. The cap had been recently perforated and the hatchlings had escaped. One of them, judged to be approximately two weeks old, was found in a burrow nearby. The cavity of the nest appeared to have been enlarged by the young. The eggs were probably laid in early July. Emergence of young from the nest had been delayed for a time after hatching, until rain softened the ground in late September and early October.

Fertility and Prenatal Mortality

Eggs were incubated in the laboratory at more nearly optimum temperature and humidity than were eggs in natural nests. Percentage of prenatal mortality probably was lower in laboratory-incubated eggs than in those under natural conditions.

Of sixty eggs studied in the laboratory, 45 (75 per cent) were fertile; 36 (80 per cent) of the fertile eggs (those in which the blastodisc was at some time discernible by transmitted light) hatched successfully. In six clutches all the eggs were fertile and five of these clutches hatched with 100 per cent success. One clutch contained eggs that were all infertile and another clutch had four infertile eggs and two fertile eggs that failed to hatch. Among nine fertile eggs that failed to survive, four casualties occurred in the late stages of incubation or after hatching had begun, indicating that these are probably critical periods.

Fertility of eggs was not correlated with size or age of female, with size of clutch, or with size of egg. Eggs laid in the laboratory had higher rates of infertility and prenatal mortality than did eggs dissected from oviducts. Handling of eggs in removing them from nests to incubation dishes, after embryonic development had begun, might have been responsible for reduced viability (Table 2).

TABLE 2.--Comparative Rates of Fertility and Prenatal Mortality for Eggs Dissected from Oviducts and for Eggs That Were Laid in the Laboratory and Subsequently Removed to Incubation Dishes. ===========================+==============+=============== | Eggs removed | Eggs dissected NUMBER OR PERCENTAGE | from nests | from oviducts ---------------------------+--------------+--------------- Number of eggs examined | 22 | 38 ---------------------------+--------------+--------------- Percentage of fertile eggs | 64 | 82 ---------------------------+--------------+--------------- Percentage of fertile | | eggs hatched | 50 | 94 ---------------------------+--------------+--------------- Percentage of eggs hatched | 32 | 76 ---------------------------+--------------+---------------

Reproductive Potential

Assuming that 4.7 eggs are laid per season, that all eggs are fertile and all hatch, that all young survive to maturity, that half the hatchlings are females, and that females first lay eggs in the eleventh year, the progeny of a single mature female would number 699 after twenty years. Considering that infertility and prenatal mortality eliminate approximately 40 per cent of eggs laid (according to laboratory findings) the average number of surviving young per clutch would be 2.8 and the total progeny, after 20 years, would be 270, provided that only one clutch of eggs was laid per year. But it is thought that, on the average, one third of the female population produces two clutches of eggs in a single season. If the second clutch contains 3.5 eggs (resulting in 2.1 surviving young when factors of infertility and prenatal mortality are considered), the progeny of a single female, after 20 years, would number approximately 380. Postnatal mortality reduces the progeny to a still smaller number.

The small number of eggs laid each year and the long period required to reach sexual maturity make the reproductive potential of T. ornata smaller than that of the other turtles of the Great Plains, and much smaller than nearly any of the non-chelonian reptiles of the same region.

Number of Reproductive Years