Natural History of the Ornate Box Turtle, Terrapene ornata ornata Agassiz Part 4

The role of the caruncle in opening the sh.e.l.l 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 aga.s.sizi_ use the caruncle at least in the initial rupturing of the sh.e.l.l.

In the three instances in which hatching was closely observed in _T.

ornata_, the caruncle made the initial opening in the sh.e.l.l; claws of the forefeet may have torn sh.e.l.ls in other hatchings that were not so closely observed. In all observed instances, the sh.e.l.l 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 quant.i.ty of alb.u.minous fluid oozed from eggs as soon as the sh.e.l.ls 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 sh.e.l.l or backs out of the sh.e.l.l through a lateral tear.

The process of hatching, from rupture of sh.e.l.l to completion of emergence, extended over three to four days in the laboratory. Many hatchlings from time to time crawled back into the sh.e.l.l 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

a.s.suming 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 s.e.xual 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

The total span of reproductive years is difficult to determine; I am unable to ascertain the age of a turtle that has stopped growing. No clearly defined external characteristics of senility were discovered in the populations studied. A male that I examined had one atrophied testis. In another male both testes were shrunken and discolored and appeared to be encased by fibrous tissue. Both males were large, well past the age of regular growth, and had smoothly worn sh.e.l.ls. Several old females had seemingly inactive ovaries. Reproductive processes probably continue throughout life in most members of the population, although possibly at a somewhat reduced rate in later life.

GROWTH AND DEVELOPMENT

Initiation of Growth

Young box turtles became active and alert as soon as they hatched, and remained so until low temperatures induced quiescence. If sand or soil was available, hatchlings soon burrowed into it and became inactive.

Covering containers with damp cotton also induced inactivity; the hatchlings usually made no attempt to burrow through the confining layer. Desire to feed varied in hatchlings of the same brood and seemed not to be correlated with retraction of the yolk sac or retention of the caruncle. Some hatchlings actively pursued mealworms; on subsequent feedings they learned to a.s.sociate my presence with food and eagerly took mealworms from forceps or from my hand. Meat, vegetables, and most other motionless but edible objects were ignored by hatchlings but some individuals learned to eat meat after several forced feedings. Hatchlings that regularly took food in the first month of life ordinarily grew faster than hatchlings that did not eat.

Many of the hatchlings in the laboratory showed no areas of new epidermal growth on the sh.e.l.l in the time between hatching and first (induced) hibernation.

Size and Appearance at Hatching

The proportions of the sh.e.l.l change somewhat in the first few weeks of life. At hatching the sh.e.l.l may be misshapen as a result of confinement in the egg. Early changes in proportions of the sh.e.l.l result from expansion--widening and, to a lesser degree, lengthening of the carapace--immediately after hatching. Subsequent retraction or rupture of the yolk sac and closure of the navel are accompanied by a decrease in height of sh.e.l.l and slight, further widening of the carapace.

The yolk sac retracts mainly between the time when the egg sh.e.l.l is first punctured and the time when the turtle actually emerges from the sh.e.l.l. When hatching is completed, the yolk sac usually protrudes no more than two millimeters, but in some individuals it is large and retracts slowly over a period of several days.

One individual began hatching on November 11 and was completely out of the egg sh.e.l.l next day; the yolk sac was 15 millimeters in diameter, protruded six millimeters from the umbilical opening, and hindered the hatchling's movements. The sac broke two days later, smearing the bottom of the turtle's dish with semifluid yolk. The hatchling then became more active. Twenty-six days later the turtle was still in good condition and its navel was nearly closed. A turtle that hatched with a large yolk sac in a natural nest possibly would benefit, through increased ease of mobility, if the yolk sac ruptured.

A recently hatched turtle was found at the Reservation in October, 1954, and was kept in a moist terrarium in the laboratory where it died the following May. The turtle was sluggish and ate only five or six mealworms while in captivity; no growth was detectable on the laminae of the sh.e.l.l. Autopsy revealed a vestige of the retracted yolk sac, approximately one millimeter in diameter, on the small intestine.

The navel ("umbilical scar") of captive hatchlings ordinarily closed by the end of the second month but in three instances remained open more than 99 days. The position of the navel is marked by a crescent-shaped crease, on the abdominal lamina, that persists until the plastron is worn down in later years (Pl. 24, Fig. 1).

[Ill.u.s.tration: FIG. 7. A hatchling of _T. o. ornata_ (? 2) that still retains the caruncle ("egg tooth"). A distinct boss will remain on the maxillary beak after the caruncle is shed.]

The caruncle ("egg tooth") (Fig. 7) remains attached to the h.o.r.n.y maxillary beak for a variable length of time; 93 per cent of the live hatchlings kept in the laboratory retained the caruncle on the tenth day, 71 per cent on the twentieth day, and only 10 per cent on the thirtieth day of life. Few individuals retained the caruncle when they entered hibernation late in November, and none retained it upon emergence from hibernation. Activities in the first few days or weeks of life influence the length of time that the caruncle is retained; turtles that begin feeding soon after hatching probably lose the caruncle more quickly than do those that remain quiescent. The caruncles of some laboratory specimens became worn before finally dropping off. Almost every caruncle present after 50 days could be flicked off easily with a probe or fingernail. The initiation of growth of the h.o.r.n.y maxillary beak probably causes some loosening of the caruncle. The caruncle may aid hatchlings in escaping from the nest.

After the caruncle falls off, a distinct boss remains, marking its former place on the h.o.r.n.y beak (Pl. 25, Fig. 1); this boss is gradually obliterated over a period of weeks by wear and by differential growth, and is seldom visible in turtles that have begun their first full year of growth. The "first full year of growth" is here considered to be the period of growth beginning in the spring after hatching.

Growth of Epidermal Laminae

Growth of ornate box turtles was studied by measuring recaptured turtles in the field, by periodically measuring captive hatchlings and juveniles, and by measuring growth-rings on the epidermal laminae of preserved specimens. Studies of growth-rings provided by far the greatest volume of information on growth, not only for the years in which field work was done, but for the entire life of each specimen examined.

It was necessary to determine the physical nature of growth-rings and the manner in which they were formed before growth could be a.n.a.lyzed.

Examination of epidermal laminae on the sh.e.l.l of a box turtle reveals that each has a series of grooves--growth-rings--on its surface. The deeper grooves are major growth-rings; they occur at varying distances from one another and run parallel to the growing borders of the lamina. Major growth-rings vary in number from one to 14 or more, depending on the age of the turtle (Pl. 22). In juvenal turtles and in young adults, major growth-rings are distinct and deep. Other grooves on the sh.e.l.l--minor growth-rings--have the same relations.h.i.+p to the borders of the laminae but are shallower and less distinct than major growth-rings. One to several minor growth-rings usually occur on each smooth area of epidermis between major growth-rings. As the sh.e.l.l of an adult turtle becomes worn, the minor growth-rings disappear and the major rings become less distinct. Both sets of rings may be completely obliterated in old turtles but the major rings usually remain visible until several years after p.u.b.erty.

In cross section, major growth-rings are V- or U-shaped. The inner wall of each groove is the peripheral edge of the part of the scute last formed whereas the outer wall represents the inner edge of the next new area of epidermal growth. The gap produced on the surface of the lamina (the open part of the groove) results from cessation of growth at the onset of hibernation. Minor growth-rings are shallow and barely discernible in cross-section (Fig. 8). It may therefore be understood that growth-rings are compound in origin; each ring is formed in part at the beginning of hibernation and in part at the beginning of the following growing season.

The few publications discussing growth in turtles express conflicting views as to the exact mode of growth of epidermal laminae. Carr (1952:22) briefly discussed growth of turtle scutes in general and stated that eccentric growth results from an entirely new laminal layer forming beneath, and projecting past the edges of the existing lamina. Ewing (1939) found the scutes of _T. carolina_ to be the thickest at the areola and successively thinner in the following eight annual zones of growth; parts of scutes formed subsequent to the ninth year varied irregularly in thickness. He stated that epidermal growth took place at the margins of the laminae rather than over their entire under-surfaces.

It is evident that the mode of scutular growth described by Carr (_loc. cit._) applies to emyid turtles that shed the epidermal laminae more or less regularly (for example, _Chrysemys_ and _Pseudemys_). In these aquatic emyids a layer of the scute, the older portion, periodically becomes loose and exfoliates usually in one thin, micalike piece; since the loosened portion of the scute corresponds in size to the scute below, it must be concluded that a layer of epidermis is shed from the entire upper surface of the scute, including the area of new epidermal growth. Box turtles ordinarily do not shed the older parts of their scutes; the areola and successively younger portions of the lamina remain attached to the sh.e.l.l until worn off. The appearance of a single unworn scute, especially one of the centrals or the posterior laterals, closely resembles a low, lopsided pyramid.

Examination of parasagittal sections of scutes revealed that they were composed of layers, the number of layers varying with the age of the scute. A scute from a hatchling consists of one layer. A scute that shows a single season of growth has two layers; a new layer is added in each subsequent season of growth. Stratification is most evident in the part of the scute that was formed in the first three or four seasons and becomes increasingly less distinct in newer parts of the scute. It may further be understood that scutes grow in the manner described by Carr (_loc. cit._).

When the epidermal laminae are removed, a sheet of tough, pale grayish tissue remains firmly attached to the bones of the sh.e.l.l beneath. This layer probably includes, or consists of, germinal epithelium.

Contrasting pale and dark areas of the germinal layer correspond to the pattern of markings on the scute removed.

[Ill.u.s.tration: FIG. 8. The second central scute from a juvenal _T. o. ornata_ (KU 16133) in its third full season of growth.

A) Entire scute from above (? 2); dashed line shows portion removed in parasagittal section. B) Diagonal view of section removed from scute in "A" (? 4-3/8, thickness greatly exaggerated) showing layers of epidermis formed in successive seasons of growth. Each layer ends at a major growth-ring (M 1-3) that was formed during hibernation; minor growth-rings (m), formed in the course of the growing season, do not result from the formation of a new layer of epidermis. Note the granular texture of the areola (a); the smooth zone between the areola and M1 shows amount of growth in the season of hatching.]

Growth of epidermal laminae is presumably stimulated by growth of the bony sh.e.l.l. As the bone grows, the germinal layer of the epidermis grows with it. When growth ceases at the beginning of hibernation, the thin edges of the scutes are slightly down-turned where they enter the interlaminal seams (Fig. 8). When growth is resumed in spring, the germinal layer of the epidermis, rather than continuing to add to the edge of the existing scute, forms an entirely new layer of epidermis.

The new layer is thin and indistinct under the oldest part of the scute but becomes more distinct toward its periphery. Immediately proximal to the edge of the scute, the new layer becomes greatly thickened, and, where it pa.s.ses under the edge, it bulges upward, recurving the free edge of the scute above. At this time the formation of a major growth-ring is completed. The newly-formed epidermis, projecting from under the edges of the scute, is paler and softer than the older parts of the scute; the presence or absence of areas of newly formed epidermis enables one to determine quickly whether a turtle is growing in the season in which it is captured. There is little actual increase in thickness of the scute after the first three or four years of growth. The epidermal laminae are therefore like low pyramids only in appearance. This appearance of thickness is enhanced by the contours of bony sh.e.l.l which correspond to the contours of the scutes.

Minor growth-rings differ from major growth-rings in appearance and in origin. Ewing (_op. cit._: 91) recognized the difference in appearance and referred to minor growth-rings as "pseudoannual growth zones."

Minor growth-rings result from temporary cessations of growth that occur in the course of the growing season, not at the onset of hibernation. They are mere dips or depressions in the surface of the scute. The occurrence of minor growth-rings indicates that interruptions in growth of short duration do not result in the formation of a new layer of epidermis. Slowing of growth or its temporary cessation may be caused by injuries, periods of quiescence due to dry, hot, or cold weather, lack of food, and possibly by physiological stress, especially in females, in the season of reproduction. Minor growth-rings that lie immediately proximal to major growth-rings (Pl. 22, Fig. 2), are the result of temporary dormancy in a period of cold weather at the end of a growing season, followed by nearly normal activity in a warmer period before winter-long hibernation is begun. Cagle (1946:699) stated that sliders (_Pseudemys scripta elegans_) remaining several weeks in a pond that had become barren of food would stop growing and develop a growth-ring on the epidermal laminae; he did not indicate, however, whether these growth-rings differ from those formed during hibernation.

In species that periodically shed scutes a zone of fracture develops between the old and new layers of the scute as each new layer of epidermis is formed, and the old layer is shed. Considering reptiles as a group, skin shedding is of general occurrence; the process in _Pseudemys_ and _Chrysemys_ differs in no basic respect from that in most reptiles. Retention of scutes in terrestrial emyids and in testudinids is one of many specializations for existence on land.

Retention of scutes protects the sh.e.l.l of terrestrial chelonians against wear. Some box turtles were observed to have several scutes of the carapace in the process of exfoliation but no exfoliation was observed on the plastron. Exfoliation ordinarily occurred on the scutes of the carapace that were the least worn; the exfoliating portion included the areola and the three or four oldest (first formed) layers of the scute. The layer of scute exposed was smooth and had yellow markings that were only slightly less distinct than those on the portion that was exfoliating.