Natural History of the Ornate Box Turtle, Terrapene ornata ornata Agassiz Part 7

(Abbreviations as in fig. 15).]

[Ill.u.s.tration: FIG. 17. Lateral view of adult sh.e.l.l (? ), showing scutellation of movable parts with anterior portion at left. (Abbreviations are as follows: ap, apical scale; ax, axillary scale; m5, fifth marginal scale; pl, pectoral lamina.)]

The external scutellation of the plastral hinge in adults also differs from that in juveniles. In adults (Fig. 17 and Pl. 8) the transverse hinge is marked by ligamentous tissue between the pectoral and abdominal laminae; the forelobe of the plastron is distinctly narrower than the hind lobe. Two small scales lie near the corner of the hinge on each side. The larger and more anterior of these scales is the axillary; it is present in box turtles of all ages. The smaller scale (Fig. 17), to my knowledge, has never been named or mentioned in the literature; it is herein termed the apical scale. It is a constant feature in adults but is always lacking in hatchlings and small juveniles. Other scales, much smaller than the axillary and apical, occur on the ligamentous tissue of the hinge of some adults.

[Ill.u.s.tration: FIG. 18. Plastron of hatchling (? 2), cleared and stained to show bony structure. (Abbreviations not listed in legend for Fig. 15 are as follows: af, anteromedian fontanelle; ep, epiplastron; pf, posteromedian fontanelle.)]

[Ill.u.s.tration: FIG. 19. Carapace of hatchling (? 1), cleared and stained to show bony structure; lateral view; anterior end at left. (Abbreviations as in Fig. 15.)]

[Ill.u.s.tration: FIG. 20. Lateral view of hatchling (? 1); note the lateral process of the pectoral lamina (pl) extending posterior to the axillary scale (ax) in a position corresponding to the apical scale of adults. There is no external indication of the transverse hinge in young individuals. The yolk sac of this individual has been retracted but the umbilicus (umb) has not yet closed.]

In juveniles (Fig. 20) the pectoroabdominal seam contains no ligamentous tissue and is like the other interlaminal seams of the plastron. A lateral apex of the pectoral lamina projects upward behind the axillary scale on each side, in the position occupied by the apical scale of adults. Examination of a large series of specimens revealed that the apical scale of adults becomes separated from the lateral apex of the pectoral lamina at approximately the time when the hinge becomes functional as such.

Ontogenetic changes in the sh.e.l.l can be summarized as follows: 1) b.u.t.tresses become less distinct in the first two years of life (plastral lengths of 40 to 55 mm.); 2) Interdigitating processes of the forelobes and hind lobes become relatively shorter and wider, the entoplastron no longer projects posterior to the hinge, the lateral apex of the pectoral lamina becomes creased, and some movement of the plastron can take place between the second and third years (plastral lengths of 55 to 65 mm.); 3) Plastral lobes become freely movable upon one another and upon the carapace by the end of the fourth year (plastral length approximately 70 mm.) in most individuals.

The plastron of a juvenal box turtle is not completely immovable. The bones of the sh.e.l.l are flexible for a time after hatching and allow some movement of the plastron; but the relatively greater bulk of the body in young box turtles would prevent complete closure of the sh.e.l.l even if a functional hinge were present. Hatchlings can withdraw the head and forelegs only to a line running between the anterior edges of the sh.e.l.l. To do so the rear half of the sh.e.l.l is opened and the hind legs are extended. When the head and forelegs are retracted to the maximum, the elbow-joints are pressed against the tympanic region or behind the head; the fore-limbs cannot be drawn part way across the snout, as in adults. Hatchlings can elevate the plastron to an angle of approximately nine degrees; the plastron of an adult, with sh.e.l.l closed, is elevated about 50 degrees. Hatchlings flex the plastron chiefly in the region of the humeropectoral seam, rather than at the anlage of the transverse hinge.

Adult box turtles, when walking, characteristically carry the forelobe of the plastron slightly flexed. This flexion of the plastron, combined with its naturally up-turned anterior edge, cause it to function in the manner of a sled runner when the turtle is moving forward. A movable plastron, therefore, in addition to its primarily protective function, seems to aid the turtle in traveling through tall gra.s.s or over uneven ground. The gular scutes, on the anterior edge of the forelobe, become worn long before other plastral laminae do.

An adult female from Richland County, Illinois, had an abnormal but functional hinge on the humeropectoral seam in addition to a normal hinge on the pectoroabdominal seam. The abnormal hinge resulted from a transverse break in which ligamentous tissue later developed. The muscles closing the plastron moved the more anterior of the two hinges; the normal hinge was not functional.

Color and Markings

The markings of the sh.e.l.l change first when postnatal growth begins and again when s.e.xual maturity is attained. They are modified gradually thereafter as the sh.e.l.l becomes worn.

In hatchlings the ground color ordinarily is dark brown but in some individuals is paler brown or tan. Markings on the dark background are pale yellow. Markings on the central and lateral scutes vary from a regularly arranged series of well defined spots and a middorsal stripe to a general scattering of small flecks. In some specimens the pale markings of the carapace are faint or wanting. Lateral parts of marginal scutes are always pale yellow and form a border around the carapace.

Close examination of the carapace of any hatchling shows the following basic arrangement of markings: each lateral scute has a centrally placed pale spot and four to seven smaller pale marks arranged around the edge of the scute; each central scute has a central, longitudinal mark and several (usually two, four, or six) smaller pale marks arranged around the edge of the scute, chiefly the lateral edges (Pl.

23). Variations in pattern result when some or all of the markings divide into two or more parts.

By the end of the first full season of growth, the markings have a radial pattern. At this stage, the markings of the areola, with the exception of the central spot, are obscure. The radial marks, sharply defined and straight-sided, appear only on the newly formed parts of the epidermal laminae. Each radial mark originates opposite one of the peripheral marks of the areola. Other radial marks are developed later by bifurcation of the original radiations.

The ground color of the plastron of hatchlings is cream-yellow, or less often, bright yellow. The solid, dark brown markings on the medial part of each lamina form a central dark area that contrasts sharply with the pale background (Pl. 24). The soft tissue of the navel is pale yellow or cream; when the navel closes, the dark central mark of the plastron is unbroken except for thin, pale lines along the interlaminal seams.

When growth begins, the areas of newly formed epidermal tissue on the anterior and medial borders of each areolar scute are pale. Wide, dark radial marks, usually three per scute, appear on the newly formed tissue. Subsequently, finer dark radiations appear between the three original radiations. The wide radiations later bifurcate. By the time adult or subadult size is reached, the plastron appears to have a pattern of pale radiations on a _dark_ background. In general, the markings of the plastron are less sharply defined than the markings of the carapace (Pl. 24).

There is a tendency for the dark markings of the plastron to encroach on the lighter markings, if no wear on the sh.e.l.l occurs. However, as the plastron becomes worn, the pale areas become more extensive and the dark markings become broken and rounded. Severely worn plastra of some old individuals lack dark markings. Wear on the carapace produces the same general effect; but markings of the carapace, although they may become blotched, are never obliterated in _Terrapene o. ornata_.

The top of the head in most hatchlings is dark brown, approximately the same shade as the ground color of the carapace; the part anterior to the eyes is usually unmarked but a few individuals have a semicircle of small pale spots over each eye or similar spots on much of the head. The posterior part of the head is ordinarily flecked with yellow. The skin on the top of the head, particularly between the eyes, is roughened. The granular skin of the neck is grayish brown to cream-yellow. There are one or two large pale spots behind the eye and another pale spot at the corner of the mouth. Smaller, irregularly arranged pale markings on the necks of some specimens form, with the post-orbital and post-rictal spots, one or two short, ragged stripes.

The gular region is pale.

In juveniles, the yellow markings of the head and neck are larger and contrast more sharply with the dark ground color than in hatchlings.

Markings above the eyes, if present, fuse to form two pale, semicircular stripes. In some older juveniles yellow marks on top of the head blend with the dark background to produce an amber color. The top of the neck darkens or develops blotches of darker color that produce a mottled effect. Spots and stripes on the side of the neck remain well defined. The skin on top of the head becomes smooth and s.h.i.+ny.

Adult females tend to retain the color and pattern of juveniles on the head and neck although slight general darkening occurs with age. Many adult females have the top of the head marked with bright yellow spots. In adult males, the top and sides of the head, anterior to the tympanum, are uniformly grayish green or bluish green; the mandibular and maxillary beaks are brighter, yellowish green. Markings on the head and neck of most adult males are obscure (Pl. 25) but the sides of the neck remain mottled in some individuals.

The antebrachium has large imbricated scales and _is_ distinctly set off from the proximal part of the foreleg which is covered with granular skin. The antebrachial scales of hatchlings are pale yellow; each scale is bordered with darker color. General darkening of the antebrachium occurs at p.u.b.erty. In adult females each scale on the anterior surface of the antebrachium is dark brown and has a contrasting yellow, amber, or pale orange center. The anterior antebrachial scales of adult males are dark brown to nearly black and have bright orange or red centers. Old males have thickened antebrachial scales.

The iris of hatchlings and juveniles is flecked with yellow and brown; the blending of these colors makes the eye appear yellow, golden, or light brown when viewed without magnification. Adult females retain the juvenal coloration of the eye; the iris of adult males is bright orange or red. The work of Evans (1952) on _T. carolina_ suggests that eye color in box turtles is under hormonal control.

Wear

Presence or absence of areolae on laminae of the sh.e.l.l indicated degree and sequence of wear. The anterior edges of carapace and plastron, and the slightly elevated middorsal line (Pl. 23) wear smooth in some individuals before the first period of hibernation.

Subsequent wear on the carapace proceeds posteriorly. For example, turtles that retained the areola of the third central lamina, retained also the areolae of the fourth and fifth centrals; when only one central areola remained, it was the fifth. Lateral laminae wear in the same general sequence. The areola of the fifth central lamina, because of its protected position, persists in adult turtles that are well past the age of regular growth. Areolae that are retained in some older turtles are shed along with the epidermal layers formed in the first year or two of life. Wear on the sh.e.l.l is probably correlated with the habits of the individual turtle; smoothly-worn specimens varied in size and age but were usually larger, older individuals. No smoothly worn individual was still growing.

Wear on the plastron is more evenly distributed than wear on the carapace; wear is greatest on the lowest points of the plastron (the gular laminae, the anterior portions of the a.n.a.l laminae, and the lateral edge of the tranverse hinge).

The claws and the h.o.r.n.y covering of the jaws are subject to greater wear than any other part of the epidermis; presumably they continue to grow throughout life. The occasional examples of hypertrophied beaks and claws that were observed, chiefly in juveniles, were thought to result from a continuous diet of soft food or prolonged activity on a soft substrate. Ditmars (1934:44, Fig. 41) ill.u.s.trated a specimen of _T. carolina_, with hypertrophied maxillary beak and abnormally elongate claws, that had been kept in a house for 27 years.

The conformation of the maxillary beak in all species of _Terrapene_ is influenced to a large extent by wear and is of limited value as a taxonomic character. The beak of _T. ornata_ is slightly notched in most individuals at the time of hatching and remains so throughout life. The underlying premaxillary bone is always notched or bicuspidate. The sides of the beak are more heavily developed than the relatively thin central part. Normal wear on the beak maintains the notch (or deepens it) in the form of an inverted U or V, much in the manner of the bicrenate cutting edge on the grooved incisors of certain rodents. In a series of 34 specimens of _T. ornata_ from Kansas, selected at random from the K. U. collections, 92 per cent had beaks that were "notched" to varying degrees, four per cent had hooked (unnotched) beaks, and four per cent had beaks that were flat at the tip (neither hooked nor notched).

[Ill.u.s.tration: FIG. 21. Plantar views of right hind foot (male at left, female at right) of _T. o. ornata_ (? 1), showing s.e.xual dimorphism in the shape and position of the first toe.

The widened, thickened, and inturned terminal phalanx on the first toe of the male is used to grasp the female before and during coitus.]

s.e.xUAL DIMORPHISM

Differences between adult males and females of _T. ornata_ have been mentioned in several places in the preceding discussion of growth and development. Several s.e.xual characteristics--greater prea.n.a.l length, thickened base of the tail, slightly concave plastron, and smaller bulk--are found also in males of many other kinds of emyid turtles.

From females, males of _T. ornata_ are most easily distinguished by the bright colors of their eyes, heads, and antebrachial scales. An additional, distinctive characteristic of males is the highly modified hind foot. The first toe is greatly thickened and widened; when the foot is extended, the first toe is held in a horizontal plane nearly at right angles to the medial edge of the plantar surface (Fig. 21).

The hind foot of females is unmodified in this respect. Males tend to have heavier, more muscular hind legs than females.

The bright colors of males are maintained throughout the year and do not become more intense in the breeding season. Males of _T. o.

luteola_ become melanistic in old age whereas males of the subspecies _ornata_ do not. In old males of _luteola_ the skin becomes dark gray, bluish, or nearly black and much of the bright orange or red of the antebrachial scales and the green of the head is obliterated; the iris may also darken but in most specimens it retains some red. Females of _luteola_ tend also to darken somewhat in old age but not so much as males; females of _ornata_ do not. Table 4 summarizes the more important secondary s.e.xual characters of _T. ornata_.

TABLE 4.--A Summary of s.e.xual Dimorphism in _Terrapene ornata_.

============+============================+============================== CHARACTER | MALES | FEMALES ------------+----------------------------+------------------------------ Head | Snout truncate in lateral | Snout relatively round in | profile, top of head and | lateral profile; front of | front of maxilliary beak | maxillary beak not forming | forming an angle of nearly | right angle with top of head; | 90; head yellowish green | head dark brown, distinct | to bluish green; markings | pale markings on head and | on head and neck reduced; | neck; head commonly spotted | head never spotted dorsally| dorsally (Pl. 25, Figs. 5 | (Pl. 11, Figs. 7 and 8). | and 6).

------------+----------------------------+------------------------------ Iris | Red | Yellowish brown ------------+----------------------------+------------------------------ Hind legs | Heavy and muscular; first | Not especially heavy or | toe turned in, thickened, | muscular; first toe, if | and widened (Fig. 21). | turned in, never thickened | | or widened (Fig. 21).

------------+----------------------------+----------------------------- Forelegs | Centers of antebrachial | Centers of antebrachial | scales bright orange or | scales yellow, pale orange, | red. | or brown.

------------+----------------------------+----------------------------- Carapace | Relatively lower, length | Relatively higher, length | contained in height (48 | contained in height (94 | specimens) .58 times | specimens) .50 times | ( .005[sigma]m, range, | ( .005[sigma]m, | .50 to .69). | range .44 to .60).

------------+----------------------------+------------------------------ Plastron | Ordinarily slightly | Flat or convex, never (hind lobe)| concave. | concave.

TEMPERATURE RELATIONs.h.i.+PS

Tolerances to environmental temperatures, and reactions to thermal stimuli influence the behavior of ectothermal animals to a large extent. _Terrapene ornata_, like other terrestrial reptiles inhabitating open gra.s.sland, is especially subject to the vicissitudes of environmental temperature. Other species of turtles living in the same area are more nearly aquatic and therefore live in a microhabitat that is more stable as regards temperature.

Approximately 500 temperature readings in the field and many others in the laboratory were obtained from enough individuals to permit interpretation of reactions involved in basking, in seeking cover, and in emerging from temporary periods of quiescence at various times of the day.

Box turtles commonly used open places such as cow paths, ravines, and wallows, for basking as well as for feeding and as routes of travel.