Natural History of the Ornate Box Turtle, Terrapene ornata ornata Agassiz Part 6

=1953= [85.6]: This was the second poorest growing year and the driest year in the period considered. Intermittently cold weather in spring delayed emergence until the last week in April when nearly an inch of rain fell. Temperatures were higher than normal from June to October.

The period from September to the end of October was dry and the small amount of precipitation that occurred was concentrated chiefly at the beginning and end of that period. Temperatures in late October and early November were lower than normal. Gra.s.shopper populations were low (2.2).

=1952= [88.3]: Environmental conditions were poor for growth and much like the conditions described for 1953. In both years growth was much less than normal in turtles of all ages except for one group (adults that were 10 and 11 years old in 1952 and 1953, respectively) that was slightly below normal in 1952 and slightly above normal in 1953.

The small number of records for 1955 were not considered in Figure 12. Warm weather in the last half of March lengthened the growing season, and environmental conditions, as in 1954, were more or less favorable throughout the rest of the summer; 1955 probably ranks with 1954 as an exceptionally good year for growth of box turtles.

Although the number of records available for turtles hatched in the period from 1950 to 1954 is small, a few records are available for all these years except 1951. In general, small samples of turtles hatched in these years reflect only the difficulty of collecting hatchlings and juveniles. In 1951, conditions for incubation and hatching were poor and the lack of records for that year actually represents a high rate of prenatal and postnatal mortality. Rainfall in the nesting season was two to three times normal and temperatures were below normal. Flooding occurred in low areas of Douglas County and many eggs may have been destroyed when nests were inundated. Cold weather probably increased the time of incubation for surviving eggs so that only a few turtles could hatch before winter. Flooding and cold, wet weather in the season of growth and reproduction, affecting primarily eggs and hatchlings, may act as checks on populations of _T. ornata_ in certain years.

[Ill.u.s.tration: FIG. 12. The relation of growth rate in _Terrapene o. ornata_ (solid line) to precipitation (dotted line) in eastern Kansas. "Normal" rate of growth was determined by averaging records of increase in length of plastron for turtles in each age group. The growth index is expressed as a percentage of normal growth and is the mean departure from normal of all age groups in each calendar year. Precipitation is for the period, April to September (inclusive), at Lawrence, Douglas Co., Kansas. The means for precipitation (4.3) and growth index (100) are indicated by horizontal lines at the right of the graph.]

The environmental factors governing activity of terrestrial turtles seem to differ at least in respect to threshold, from the factors influencing the activity of aquatic turtles. A single month that was drier or cooler than normal probably would not noticeably affect growth and activity of aquatic emyids in northeast Kansas, but might greatly curtail growth of box turtles.

Cagle (1948:202) found that growth of slider turtles (_Pseudemys scripta_) in Illinois paralleled the growth of ba.s.s and bluegills in the same lake; in the two years in which the fish grew rapidly, the turtles did also, owing, he thought to "lessened total population pressure" and "reduced compet.i.tion for food." Growth of five-lined skinks (_Eumeces fasciatus_) on the Natural History Reservation paralleled growth of box turtles, probably because at least some of the same environmental factors influence the growth of both species.

Calculations of departure from normal growth in _E. fasciatus_, made by me from Fitch's graph (1954:84, Fig. 13), show that relative success of growth in the period he considered can be ranked by year, in descending order, as: 1951, 1949, 1948, 1950, 1952. This corresponds closely to the sequence, 1951, 1948, 1949, 1950, 1952, for _T. ornata_.

Number of Growing Years

Growth almost stops after the thirteenth year in females and after the eleventh or twelfth year in males, approximately three years, on the average, after s.e.xual maturity is attained. The oldest individuals in which plastral length had increased measurably in the season of capture were females 14 (2 specimens) and 15 (1) years old. The age of the oldest growing male was 13 years.

The germinal layer of the epidermis probably remains semiactive throughout life but functions chiefly as a repair mechanism in adults that are no longer growing. Growth-rings continue to form irregularly in some older adults. Growth-rings formed after the period of regular growth are so closely approximated that they are unmeasurable and frequently indistinguishable to the unaided eye. If the continued formation of growth-rings is not accompanied by wear at the edges of the laminae, the laminae meeting at an interlaminal seam descend, like steps, into the seam (Pl. 22, Fig. 2). Interlaminal seams of the plastron deepen with advancing age in most individuals.

Some individuals that are well past the age of regular growth show measurable increments in years when conditions are especially favorable. The three oldest growing females were collected in 1954--an exceptionally good year for growth. Allowing some lat.i.tude for irregular periods of growth in favorable years subsequent to the period of regular, more or less steady growth, 15 to 20 years is a tenable estimate of the total growing period.

Longevity

Practically nothing is known about longevity in _T. ornata_ or in other species of _Terrapene_ although the several plausible records of ages of 80 to more than 100 years for _T. carolina_ (Oliver, 1955:295-6) would indicate that box turtles, as a group, are long-lived. There is no known way to determine accurately the age of an adult turtle after it has stopped growing. It was possible occasionally to determine ages of 20 to 30 years with fair accuracy by counting all growth-rings (including those crowded into the interabdominal seam) of specimens having unworn sh.e.l.ls. Without the presence of newly formed epidermis as a landmark, however, it was never certain how many years had pa.s.sed since the last ring was formed.

[Ill.u.s.tration: FIG. 13. The relations.h.i.+p of s.e.xual maturity to size in 164 specimens (94 females and 70 males) of _Terrapene o. ornata_, expressed as the percentage of mature individuals in each of five groups arranged according to plastral length.

s.e.xual maturity was determined by examination of gonads. Solid bars are for males and open bars for females. The bar for males in the largest group is based on a.s.sumption since no males in the sample were so long as 130 mm. Males mature at a smaller size and lesser age (see also Figs. 9 and 10) than females.

Plastral lengths of the smallest s.e.xually mature male and female in the sample were, respectively, 99 and 107 mm.]

Mattox (1936) studied annual rings in the long bones of painted turtles (_Chrysemys picta_) and found fewer rings in younger than in older individuals but, beyond this, reached no important conclusion.

In the present study, thin sections were ground from the humeri and femurs of box turtles of various ages and sizes; the results of this investigation were negative. Distinct rings were present in the compact bony tissue but it appeared that, after the first year or two, the rings were destroyed by encroachment of the marrow cavity at about the same rate at which they were formed peripherally.

The only methods that I know of to determine successfully the longevity of long-lived reptiles would be to keep individuals under observation for long periods of time or to study populations of marked individuals. Both methods have the obvious disadvantage of requiring somewhat more than a human lifetime to carry them to completion.

Restudy, after one or more decades, of the populations of turtles marked by Fitch and myself may provide valuable data on the average and maximum age reached by _T. ornata_.

Ornate box turtles probably live at least twice as long as the total period of growing years. An estimated longevity of 50 years would seem to agree with present scant information on age. Considering environmental hazards, it would be unusual for an individual to survive as long as 100 years in the wild.

Weight

Weights of ornate box turtles varied so much that no attempt was made to correlate weight with size. Absolute weights have little significance since weight is affected to a large extent by the amount of fluid in the body. Turtles that had recently imbibed were naturally heavier than those that had not; turtles brought to the laboratory and kept there for several days lost weight by evaporation and by voiding water. Weights of 22 adult females (53 records) and 10 adult males (22 records) averaged 391 and 353 grams respectively, in the period from September, 1954, to October, 1956. Females characteristically gained weight in spring and early summer and were lighter after nesting.

Turtles of both s.e.xes gained weight in September and October.

Bony Sh.e.l.l

_Fontanelles_

At the time of hatching, fontanelles remain where bones of the sh.e.l.l have not yet articulated with their neighbors. In general, the fontanelles of the sh.e.l.l are closed by the time s.e.xual maturity is attained, but some remain open a year or two longer.

The fontanelles of the sh.e.l.l are cla.s.sified as follows (see Figs. 14 to 16 and 18 to 19):

_Plastron_

1.) _Anteromedian._ Rhomboidal; limited anteriorly by hyoplastral bones and posteriorly by hypoplastral bones; posterior tip of entoplastral bone may project into this fontanelle.

2.) _Posteromedian._ Limited anteriorly by hypoplastral bones and posteriorly by xiphyplastral bones (since hypoplastral bones do not articulate medially in hatchlings, anteromedian and posteromedian fontanelles form a single, more or less dumbbell-shaped opening).

[Ill.u.s.tration: FIG. 14. Extent of closure of the costoperipheral fontanelles in relation to length of plastron in 17 skeletons of _T. o. ornata_ from eastern Kansas. Extent of closure is expressed as an estimated percentage of total closure of all the costoperipheral fontanelles, even though some of them close sooner than others. Closure is usually complete by the time s.e.xual maturity is attained.]

_Carapace_

1.) _Costoperipheral._ Openings between the free ends of developing ribs, between nuchal bone and first rib, and, between pygal bone and last rib; limited laterally by peripheral bones; variable in shape.

2.) _Costoneural._ Triangular openings on either side of middorsal line between proximal ends of costal plates and developing neural plates.

The costoneural fontanelles are nearly closed in individuals of the 70 millimeter (plastron length) cla.s.s and seldom remain open after a length of 80 millimeters is attained (Fig. 14). Of the costoperipheral fontanelles, the anterior one (between first rib and nuchal bone) closes first and the posterior one (between last rib and pygal bone) last. It remains open in some turtles in which the plastron is longer than 100 millimeters. The remaining costoperipheral fontanelles close in varying sequence but those in the area of the bridge (nos. 2 to 5), where there is presumably greater stress on the sh.e.l.l, close sooner than the others.

The plastral fontanelles are closed in most specimens of the 90 millimeter (plastron length) cla.s.s; the anteromedian fontanelle closes first.

The meager covering of the fontanelles makes juvenal turtles more susceptible than adults to many kinds of injuries and to predation.

_Movable Parts of the Sh.e.l.l_

Parts of the sh.e.l.l that are more or less movable upon one another and that function in closing the sh.e.l.l are found in several families of Recent turtles. African side-necked terrapins of the genus _Pelusios_ have a movable forelobe on the plastron. Kinosternids have one or two flexible transverse hinges on the plastron. In the Testudinidae the African _Kinixys_ has a movable hinge on the posterior part of the carapace and _Pyxis arachnoides_ of Madagascar has a short, hinged, anterior plastral lobe. Certain trionychid turtles, such as _Lissemys_, utilize the flexible flaps of the carapace (the flaps of some species are reinforced with peripheral bones) to close the sh.e.l.l.

Movable sh.e.l.l-parts of turtles are, in general, protective in function; they cover parts of the soft anatomy that would otherwise be exposed.

A hinged plastron, capable of wholly or partly closing the sh.e.l.l, occurs in six genera of the family Emyidae (see introduction). In these emyids the plastron is divided into two lobes, which are joined to each other by ligamentous tissue at the junction of the hyoplastral and hypoplastral bones; externally, the hinge occurs along the seam between the pectoral and abdominal laminae. This junction forms a more or less freely movable hinge in adults. The plastron is attached to the carapace by ligamentous tissue. Both lobes of the plastron or only the b.u.t.tresses of the hind lobe may articulate with the carapace. The former condition obtains in _Emys_ and _Emydoidea_; the latter more specialized condition is found in _Terrapene_.

In generalized emyid turtles such as _Clemmys_ there are no movable sh.e.l.l parts. The plastron is joined to the carapace by the sutures of the bridge. A long stout process, the axillary b.u.t.tress, arises on each side from the hyoplastron and articulates with the tip of the first costal. A similar process, the inguinal b.u.t.tress, arises from the anterior part of each of the hypoplastral elements and meets the sixth costal on each side. The b.u.t.tresses form the anterior and posterior margins of the bridge. It is clear that movement of the plastron in many emyids is mechanically impossible because of the bracing effect of the b.u.t.tresses.

In _Terrapene_ the movable articulations of the sh.e.l.l are neither structurally nor functionally developed in juveniles. Adults of _T.

ornata_ have highly modified bony b.u.t.tresses on the plastron that are h.o.m.ologous with those in more generalized emyids. The inguinal b.u.t.tresses are low and wide, and have a sheer lateral surface forming a sliding articulation with the fifth and sixth peripheral bones of the carapace. The axillary b.u.t.tresses are reduced to mere bony points near the posterolateral corners of the forelobe and do not articulate directly with the carapace (Figs. 15 and 16).

The fifth peripheral bone, const.i.tuting the lowest point of the carapace, has a medial projection that acts as a pivoting point for both lobes of the plastron; the roughened anterior corners of the hind lobe articulate with these processes. The roughened posterior corners of the forelobe of the plastron likewise articulate with these processes. The posterior process or "tail" of the entoplastron extends to, or nearly to, the bony transverse hinge.

In juveniles that have been cleared and stained, the h.o.m.ologues of the parts that are movable in adults are easily identifiable; the proportions of these parts and their relations to one another are, however, much different.

In juveniles (Figs. 18 and 19) the b.u.t.tresses are relatively longer and narrower, and are distinct--more nearly like those of generalized emyids than those of adult _T. ornata_. The b.u.t.tresses enclose a large open s.p.a.ce, which in adults is filled by the fifth peripheral. The hyoplastral and hypoplastral bones are in contact only laterally. They are firmly joined by bony processes; the interdigitating nature of this articulation contrasts with its h.o.m.ologue in the adult, the point where the roughened corners of the forelobes and hind lobes meet. The fifth peripheral in juveniles (Fig. 19) lies dorsal to this articulation. The position of the future transverse hinge corresponds to a line pa.s.sing through the articulations of the hyoplastra and hypoplastra. The tail of the entoplastron ordinarily extends posterior to this line in juveniles.

[Ill.u.s.tration: FIG. 15. Lateral view of adult sh.e.l.l (? ), showing movable parts with anterior portion at left.

(Abbreviations are as follows: ab, axillary b.u.t.tress; hp, hypoplastron; hy, hyoplastron; ib, inguinal b.u.t.tress; p5, fifth peripheral bone; th, transverse hinge).]

[Ill.u.s.tration: FIG. 16. Medial view of adult sh.e.l.l (? ), showing movable parts with anterior portion at left.