Part 20

The relatively low, flattened shell of T. ornata is an adaptation associated with the tendency to seek shelter in the limited space of earthen forms, burrows, or small natural cavities in the course of the warm season and to burrow more deeply into the ground in winter. Terrapene ornata is, in fact, the only species of the genus that may be considered an habitual burrower. Individuals of T. carolina tend to seek shelter in the warm season by making forms in dense vegetation or by digging into yielding substrata such as mud or humus, although they may burrow deeply into the earth in winter. Extreme weakness or absence of the middorsal keel of T. ornata seems to be a modification associated with burrowing habits and general adaptation to terrestrial life; the keel is similarly reduced in testudinids.

Retention of epidermal laminae (as opposed to regular exfoliation of the older parts of scutes) occurs in all box turtles, in several other groups of terrestrial emyids, and in testudinids. The phenomenon is here considered to be a specialization of scute shedding--developed in terrestrial and semiterrestrial chelonians--that provides additional protection to the shell against wear and minor injuries.

General shortening of digits--the result of reduction in number of phalanges as well as in their length, and to a lesser degree the shortening of metapodial elements--has occurred in several groups of chelonians with terrestrial tendencies (the opposite--lengthening of phalanges and metapodials, and hyperphalangy--has occurred in certain groups that are highly aquatic). The pes of box turtles has remained relatively unchanged in this respect; a few phalanges on the lateral digit have been lost (especially in three-toed forms), but little reduction in length has occurred. The chief modification of the pes is a general narrowing brought about by the tendency of the digits to be crowded together, one on top of the other, rather than spread in a horizontal plane. Considerably more modification is seen in the manus of Terrapene. Phalangeal formulae (expressing the number of phalanges from the first digit outward) range from 2-3-3-3-2 (primitive in Terrapene) to 2-3-3-2-2 in the races of carolina and have the same range in the species of eastern Mexico. Extreme reduction in number (2-2-2-2-2) as well as general shortening of phalanges occurs in T. ornata. The formula is the same in the one specimen of T. klauberi that has been skeletonized. This modification of the forelimb in T. ornata has produced a more rigid, stronger manus that is well adapted to the requirements of burrowing and to locomotion over unyielding substrata. Shortening of the manus (and, to a lesser extent, the pes) has been accompanied by reduction and loss of interdigital webbing. It is noteworthy that T. ornata has achieved the same reduction in number of phalanges as Gopherus, which displays the extreme of specialization in this respect among North American turtles. The manus in T. ornata is not shortened so much as in Gopherus.

The first toe in males of T. ornata is uniquely widened, thickened, and inturned. Males of some other species of Terrapene have greatly enlarged rear claws, some of which turn slightly inward, but none has the flexed first toe hooklike as it is in ornata (a modified first toe, resembling that described for T. ornata, has been observed in a live male of T. klauberi [now KU 51430] since the preparation of this manuscript). In males of T. ornata the penultimate phalanx of the first toe has a normal, vertical articular surface on its proximal end. However, the distal articular surface (when viewed from the distal end of the phalanx) has its axis rotated away from the vertical plane approximately 45 degrees in a counterclockwise direction. As the foot is pronated and extended, and as the digits are flexed, there is a concomitant inward rotation of the first metatarsal at its proximal joint; this rotation, combined with the divergent planes of the articulating surfaces on the penultimate phalanx, cause the ungual phalanx to be flexed at right angles to the inner side of foot, in a plane perpendicular to that of the other toes (Fig. 21).

The precise function of the modified first toe of males is unknown, although it is reasonably safe to assume that the modification is closely associated with clasping during coition. In the matings that I observed, the inturned first claw of the male secured a hold on the female's rump or just beneath her legs, whereas the remaining three toes gripped the edge of her plastron. The combined hold, on shell and skin, clearly affords the male a more secure position during coitus (whether the female clasps his legs with hers or not) than would a hold on skin or shell alone. Possibly intromission can be maintained in this position even when the female is attempting to escape. In males the plastron is less concave in T. ornata than in T. carolina. Furthermore, males of T. ornata are, on the average, smaller than females, whereas the reverse is true in T. carolina. Possibly the ability of the male to secure an especially firm grip on the female enhances the probability of small males mounting and inseminating larger females, whereas successful matings might otherwise be limited to pairs in which the male was the larger member.

It is worthy of note that turtles of the genus Terrapene are seemingly the only North American emyids that carry out the entire process of mating on land; other, semiterrestrial emyids (for example, Clemmys insculpta and Emydoidea blandingi) return to water for actual coition, although the precoital behavior sometimes occurs on land.

Nearly all gradations from a fully developed zygomatic arch to a greatly reduced arch can be observed in skulls of the various species of Terrapene (Fig. 2) (Taylor, 1895:586, Figs. 2-7). The highest degree of reduction is achieved in T. ornata and T. klauberi, both of which lack the quadratojugal bone and have no zygomatic arch whatever (except for an occasional, poorly defined anterior vestige formed by the postfrontal, the jugal, or both). Reduction of the zygoma clearly represents modification of a more generalized, complete arch. As yet there is no clear evidence that reduction of the zygomatic arch is of adaptive value. It is noteworthy, however, that similar reduction of the arch has occurred independently in a number of emyid and testudinid groups, nearly all of which have terrestrial or semiterrestrial habits. Although discussion of phyletic lines in Terrapene is beyond the scope of this report, I tentatively suggest that reduced zygomatic arches have arisen independently in more than one group of Terrapene and that similar reduction of the arch in two species of the genus does not necessarily indicate an especially close relationship of such species.

In a recent survey of cloacal bursae in chelonians, Smith and James (1958:88) reported T. ornata and T. mexicana to be among the few emyids that lacked these structures; in the opinion of the authors (op. cit.:94) cloacal bursae evolved in chelonians that required an accessory respiratory organ for long periods of quiescence (hibernation or aestivation) under water, and were secondarily lost in terrestrial forms that hibernated on land. The assumption is a reasonable one, at least in regard to emyids and testudinids. Lack of cloacal bursae in T. ornata and in all testudinids, can be correlated with the completely terrestrial habits of those turtles. Cloacal bursae seem to be vestigial in the species of Terrapene possessing them and to be of little or no use as respiratory structures (except perhaps in T. coahuila).

In most of the species of Terrapene the carapace has a pattern of pale markings on a darker background; however, unicolored individuals are the rule in certain populations (for example, at the western edge of the range of T. carolina and in T. ornata luteola) and occur as occasional variations in other populations (in T. yucatana, T. mexicana, and, throughout the range of T. carolina, albeit more commonly in the southeastern part of the range). Personal observation of interspecific and ontogenetic variation of color patterns of box turtles has convinced me that a basic pattern of more or less linear radiations is the one from which all other patterns (including spots, blotches, rosettes, and the unicolored condition) can be derived, and, that the radial pattern is generalized and primitive for Terrapene (possibly for all emyids and testudinids as well). In the light of this conclusion, the radial pattern of T. ornata may be considered generalized. I suspect, however, that the pattern of a living species most closely approaching that of the primitive ancestral stock of Terrapene is the pattern of fine, wavy, dark radiations (on a paler background) present in young examples of T. coahuila.

Box turtles in general have lower reproductive potentials (as indicated by fewer eggs and longer prepuberal period) than do most aquatic emyids. This low potential seems to be compensated for by a lower rate of postnatal mortality (especially in adults) due to the protection afforded by the closable shell and the ability to recover from serious injury. Terrapene o. ornata and T. c. carolina are the only box turtles the life histories of which are known well enough to permit significant comparison. The reproductive potentials of T. o. ornata and T. c. carolina seem to be much the same.

Aerial photograph of Damm Farm (July, 1954).

Numbers and letters on photograph denote the following: 1. Main pasture with subdivisions a to c, respectively, northwest corner area, house pond area, and southern ravine area; 2. Wooded area; and, 3. Cultivated area.

FIG. 1. A water-filled ravine in the northern part of the pasture at the Damm Farm (June 28, 1958). The subdivision of the pasture referred to in text as "northwest corner area" can be seen sloping into the ravine from the west (left background).

FIG. 2. A cow path leading southward away from a ravine, at the Damm Farm (June 28, 1958). Ornate box turtles used such paths as routes of travel in the course of their daily activities.

FIG. 1. Grassland on crest of hill at Damm Farm with northeastern corner of main pasture in background (June 29, 1958).

FIG. 2. A bare area along the rock fence at northern edge of pasture at Damm Farm. Ornate box turtles could nearly always be found foraging in cow dung here and in similar areas along other fences (June 28, 1958).

FIG. 1. A ravine in the southern part of the pasture at the Damm Farm (June 28, 1958). Small springs at the heads of such ravines produced marshy conditions at their bottoms and provided drinking water, in the form of shallow pools, for box turtles for at least part of the year. Banks of ravines provided suitable sites for the construction of nests and forms.

FIG. 2. A mulberry tree on the bank of a ravine near northern edge of Damm pasture (June 28, 1958). Box turtles frequented the area beneath the tree when fruit fell to the ground in June and July. The ravine shown here filled with water after being dammed in June, 1956.

Representative stages in the spermatogenic cycle of T. o. ornata (all specimens obtained in Douglas County, Kansas, 1955).

FIGS. 1 to 5, respectively, are sections of seminiferous tubules obtained on May 17, June 14, July 15, Aug. 31, and Oct. 4. FIG. 6: seminiferous tubule of immature male (plastral length, 88 mm.), six years old, obtained on June 30. FIG. 7: section of epididymis from mature male obtained on April 17, three days after turtle emerged from hibernation; mature sperm form a continuous dark mass in center of epididymis. FIG. 8: sperm in uterine portion of oviduct of female obtained on April 18, 1954.

Figs. 1 to 6 and 8 were photographed A-- 430, and were enlarged 1.4 times. Fig. 7 was photographed A-- 35, and was enlarged 1.4 times.

FIG. 1. Left ovary of mature female, prior to ovulation, May 15, 1956 (A-- 1). FIG. 2. Fresh corpus luteum, June 2, 1956 (A-- 4A1/2). FIG. 3. Testes of mature male, August 31, 1955 (A-- 1). FIG. 4. Testes of mature male, April 14, 1956 (A-- 2). FIG. 5. Left ovary of subadult female (seven years old, plastral length, 114 mm.) that would have matured in approximately one year (A-- 1A1/2). FIG. 6. Left ovary of juvenal female (11 years old, plastral length, 95 mm., A-- 1A1/2).

FIG. 1. A trial nest cavity excavated by a gravid T. o. ornata at the Damm Farm on June 8, 1956. The cavity was situated at the edge of a grassy area on the upper rim of a ravine embankment. Twelve-inch ruler shows scale.

FIG. 2. A depression, resulting from an old post-hole, showing the openings made by three box turtles as they left their hibernacula in April, 1956 (photographed May 15, 1956). Twelve-inch ruler shows scale.

FIG. 1. Right abdominal lamina (A-- 2A1/2) of a four-year-old juvenal male showing method of measuring growth-rings. The last growth-ring (4) was formed at the end of the 1954 growing season. The first growth-ring (H) marks the end of the season of hatching (1950). The umbilical scar (U) is faintly visible. The growth-zone for 1955 (specimen captured June 23) is just beginning to show in interlaminal seam.

FIG. 2. Left--Right abdominal lamina (A-- 2) of subadult female, eight years old. The last growth-zone was formed in 1954. Note the relatively small growth increments in 1952 and 1953. The growth-zone for 1955 (date of capture, May 8) is not yet visible. This specimen grew more in the season of hatching (1946) than the specimen shown above in Fig. 1. Right--Interpectoral seam (A-- 3) of adult male showing slowness of growth in later life. The widest growth-zone seen here was formed in the tenth year and is followed by four zones too narrow to measure accurately. It is uncertain whether this specimen was still growing in the year it was captured (1923).

Ontogenetic change in color and markings of carapace. Radial markings begin to develop at the onset of epidermal growth. Markings are sharply defined in juveniles and young adults but may be obscured in later life by the encroachment of dark ground color or by wear on the shell. Figures are as follows: Upper left--Hatchling (A-- 1A1/2); Upper right--Juvenile (A-- 1), one year old; Lower left--Juvenile (A-- 1), one year old; Lower left--Female (A-- 7/16) showing typical adult markings; Lower right--Adult male (A-- A1/2showing blotched pattern resulting from wear on shell.

Ontogenetic change in color and markings of plastron. Dark markings on plastra of hatchlings are unbroken. Dark radiations appear when epidermal growth begins. Figures are as follows: Upper left--Hatchling (A-- 1A1/2); Upper right--Juvenile (A-- 1); Lower left--Female (A-- 7/16) showing typical adult markings; Lower right--Adult male (A-- A1/2) showing the effect of wear on markings. Plastra of old individuals are sometimes solid yellow. Note the break in the plastron that has healed and filled with ligamentous tissue.