Natural History of the Ornate Box Turtle, Terrapene ornata ornata Agassiz Part 5

Wear on the sh.e.l.l of a box turtle reduces the thickness of scutes, as does the shedding of scutes in the aquatic emyids mentioned. It is noteworthy that any of the layers in the scute of a box turtle can form the cornified surface of the scute when the layers above it wear away or are shed.

It is uncertain whether turtles that have ceased to grow at a measurable rate continue to elaborate a new layer of epidermis at the beginning of each season. Greatly worn sh.e.l.ls of ornate box turtles, particularly those of the subspecies _luteola_, have only a thin layer of epidermis through which the bones of the sh.e.l.l and the sutures between the bones are visible. I suspect that, in these old individuals, the germinal layer of the epidermis does not become active each year but retains the capacity to elaborate new epidermis if the sh.e.l.l becomes worn thin enough to expose and endanger the bone beneath it. The germinal layer of old turtles loses the capacity to produce color.

Major growth-rings const.i.tute a valuable and accurate history of growth that can be studied at any time in the life of the turtle if they have not been obliterated. They are accurate indicators of age only as long as regular growth continues but may be used to study early years of growth even in turtles that are no longer growing.

Minor growth-rings, if properly interpreted, provide additional information on growing conditions in the course of each growing season.

Nichols (1939a: 16-17) found that the number of growth-rings formed in marked individuals of _T. carolina_ did not correspond to the number of growing seasons elapsed; he concluded that growth-rings were unreliable as indicators of age and that box turtles frequently skipped seasons of growth. Woodbury and Hardy (1948:166-167) and Miller (1955:114) came to approximately the same conclusion concerning _Gopherus aga.s.sizi_. It is significant that these workers were studying turtles of all sizes and ages, some of which were past the age of regular, annual growth. Cagle's review of the literature concerning growth-rings in turtles (1946) suggests that, in most of the species studied, growth-rings are formed regularly in individuals that have not attained s.e.xual maturity but are formed irregularly after p.u.b.erty.

Cagle's (_op. cit._) careful studies of free-living populations of _Pseudemys scripta_ showed that growth-rings, once formed, did not change in size, that the area between any two major growth-rings represented one season of growth, and that growth-rings were reliable indicators of age as long as the impression of the areola remained on the scutes studied. Cagle noted decreasing distinctness of growth-rings after each molt.

The relative lengths of the abdominal lamina and the plastron remain approximately the same throughout life in _T. ornata_. Measurements were made of the plastron, carapace, and abdominal lamina in 103 specimens of _T. o. ornata_ from Kansas and neighboring states. The series of specimens was divided into five nearly equal groups according to length of carapace. Table 3 summarizes the relations.h.i.+p of abdominal length to plastral length, and of carapace length to plastral length. The mathematical mean of the ratio, abdominal length/plastral length, in each of the four groups of larger-sized turtles, was not significantly different from the same ratio in the hatchling group. The relative lengths of carapace and plastron are not so constant; the carapace is usually longer than the plastron in hatchlings and juveniles, but shorter than the plastron in adults, especially adult females.

TABLE 3.--The Relations.h.i.+p of Length of Abdominal Scute to Plastral Length, and of Plastral Length to Length of Carapace, in 103 Specimens of _T. o. ornata_ Arranged in Five Groups According to Length of Carapace. The Relative Lengths of Abdominal Scute and Plastron are not Significantly Different in the Five Groups. The Plastron Tends to be Longer than the Carapace in Specimens of Adult or Nearly Adult Size.

===============+=========+===========================+================== | | Length of abdominal |Individuals having | | as a percentage of | plastron longer LENGTH | Number | length of plastron | than carapace OF CARAPACE | of | | |specimens|----------------+----------+------+----------- | |Mean [sigma]m | Extremes |Number|Percentage ---------------+---------+----------------+----------+------+----------- Less than | | | | | 50 mm. | 23 | 18.3.498 |13.7-20.3 | 7 | 38.5 (Juveniles) | | | | | ---------------+---------+----------------+----------+------+----------- 50 to 69 mm. | 20 | 17.8.303 |15.2-20.2 | 8 | 40.0 (Juveniles) | | | | | ---------------+---------+----------------+----------+------+----------- 70 to 100 mm. | 20 | 17.9.445 |14.3-20.6 | 15 | 75.0 (Subadults) | | | | | ---------------+---------+----------------+----------+------+----------- More than | | | | | 100 mm. | 20 | 17.8.236 |16.4-20.6 | 13 | 65.0 (Adult males) | | | | | ---------------+---------+----------------+----------+------+----------- More than | | | | | 100 mm. | 20 | 18.8.510 |15.1-25.7 | 19 | 95.0 (Adult females)| | | | | ---------------+---------+----------------+----------+------+-----------

The length of any growth-ring on the abdominal lamina can be used to determine the approximate length of the plastron at the time the growth-ring was formed. Actual and relative increases in length of the plastron can be determined in a like manner. For example, a seven-year-old juvenile (KU 3283) with a plastron 74.0 millimeters long had abdominal growth-rings (beginning with areola and ending with the actual length of the abdominal) 5.9, 7.8, 9.5, 10.7, 12.0, 12.5, 14.3, and 14.9 millimeters long. Using the

[AB AB]

proportion, [-- = -- ], where AB is the abdominal length, PL the [PL X ]

plastral length, AB the length of any given growth-ring, and X the plastral length at the time growth-ring AB was formed, the plastral length of this individual was 29.3 millimeters at hatching, 38.8 at the end of the first full season of growth, and 47.2, 53.2, 59.6, 62.1, and 71.0 millimeters at the end of the first, second, third, fourth, fifth, and sixth seasons of growth, respectively. The present length of the abdominal (14.9 mm.) indicates an increment of three millimeters in plastral length in the seventh season, up to the time the turtle was killed (June 25). This method of studying growth in turtles was first used by Sergeev (1937) and later more extensively used by Cagle (1946 and 1948) in his researches on _Pseudemys scripta_. Because the plastron is curved, no straight-line measurement of it or its parts can express true length. Cagle (1946 and 1948) minimized error by expressing plastral length as the sum of the laminal (or growth-ring) lengths. This method was not possible in the present study because growth-rings on parts of one or more laminae (chiefly the gulars and a.n.a.ls) were usually obliterated by wear, even in young specimens. It was necessary to express plastral length as the sum of the lengths of forelobe and hind lobe.

The abdominal lamina was selected for study because of its length (second longest lamina of plastron), greater symmetry, and flattened form. Although the abdominal is probably subject to greater, over-all wear than any other lamina of the sh.e.l.l, wear is even, not localized as it is on the gulars and a.n.a.ls.

In instances where some of the growth-rings on an abdominal lamina were worn but other rings remained distinct, reference to other, less worn lamina permitted a correct interpretation of indistinct rings.

Abdominal laminae were measured at the interlaminal seam; since the laminae frequently did not meet perfectly along the midline (and were of unequal length), the right abdominal was measured in all specimens.

Growth-rings on the abdominal laminae were measured in the manner shown in Plate 22.

Data were obtained for an aggregate of 1272 seasons of growth in 154 specimens (67 females, 48 males, and 39 of undetermined s.e.x, chiefly juveniles). Averages of calculated plastral length were computed in each year of growth for specimens of known s.e.x (Figs. 9 and 10) and again for all specimens examined. Annual increment in plastral length was expressed as a percentage of plastral length at the end of the previous growing season (Fig. 11). Increment in plastral length for the first season of growth was expressed as a percentage of original plastral length because of variability of growth in the season of hatching; growth increments in the season following hatching are, therefore, not so great as indicated in Figure 11.

Growth of Juveniles

Areas of new laminal growth were discernible on laboratory hatchlings soon after they ate regularly. Hatchlings that refused to eat or that were experimentally starved did not grow. The first zone of epidermis was separated from the areola by an indistinct growth-ring (resembling a minor growth-ring) in most hatchlings, but in a few specimens the new epidermis appeared to be a continuation of the areola. Major growth-rings never formed before the onset of the first hibernation.

Growth in the season of hatching seems to depend on early hatching and early emergence from the nest. Under favorable conditions hatchlings would be able to feed and grow eight weeks or more before hibernation. Hatchlings that emerge in late autumn or that remain in the nest until spring are probably unable to find enough food to sustain growth.

Sixty-four (42 per cent) of the 154 specimens examined showed measurable growth in the season of hatching. The amount of increment was determined in 36 specimens having a first growth-ring and an areola that could be measured accurately. The average increment of plastral length was 17.5 per cent (extremes, 1.8-66.0 per cent) of the original plastral length. Ten individuals showed an increment of more than 20 per cent; the majority of these individuals (8) were hatched in the years 1947-50, inclusive.

[Ill.u.s.tration: FIG. 9. See legend for Fig. 10.]

[Ill.u.s.tration: FIG. 10. The relations.h.i.+p of size to age in _T. o. ornata_, based on studies of growth-rings in 115 specimens of known s.e.x (67 females and 48 males) from eastern Kansas. Size is expressed as plastral length at the end of each growing season (excluding the year of hatching) through the twelfth and thirteenth years (for males and females, respectively) of life. Vertical and horizontal lines represent, respectively, the range and mean. Open and solid rectangles represent one standard deviation and two standard errors of the mean, respectively. Age is expressed in years.]

Some hatchlings that grow rapidly before the first winter are as large as one- or two-year-old turtles, or even larger, by the following summer. Individuals that grew rapidly in the season of hatching tended also to grow more rapidly than usual in subsequent seasons; 80 per cent of the individuals that increased in plastral length by 20 per cent or more in the season of hatching, grew faster than average in the two seasons following hatching. Early hatching and precocious development presumably confer an advantage on the individual, since turtles that grow rapidly are able better to compete with smaller individuals of the same age. Theoretically, turtles growing more rapidly than usual in the first two or three years of life, even if they grew subsequently at an average rate, would attain adult size and s.e.xual maturity one or more years before other turtles of the same age. A few turtles (chiefly males) attain adult size (and presumably become s.e.xually mature) by the end of the fifth full season of growth (Figs. 9 and 10). These individuals, reaching adult size some three to four years sooner than the average age, were precocious also in the earlier stages of postnatal development.

Young box turtles reared in the laboratory grew more slowly than turtles of comparable ages under natural conditions; this was especially evident in hatchlings and one-year-old specimens. Slower growth of captives was caused probably by the unnatural environment of the laboratory. Captive juveniles showed a steady increase in weight (average, .52 grams per ten days) as they grew whereas captive hatchlings tended to lose weight whether they grew or not.

Growth in Later Life

After the first year growth is variable and size is of little value as an indicator of age. Although in the turtles sampled variation in size was great in those of the same age, average size was successively greater in each year up to the twelfth and thirteenth years (for males and females, respectively), after which the samples were too small to consider mathematically.

Increments in plastral length averaged 68.1 per cent in the year after hatching, 28.6 per cent in the second year and 18.1 per cent in the third year. From the fourth to the fourteenth year the growth-rate slowed gradually from 13.3 to about three per cent (Fig. 11). These averages are based on all the specimens examined (with no distinction as to s.e.x); they give a general, over-all picture of growth rate but do not reflect the changes that occur in growth rate at p.u.b.erty (as shown in Figs. 9 and 10).

Rate of growth and, ultimately, size are influenced by the attainment of s.e.xual maturity. Adult females grow larger than adult males. Males, nevertheless, grow faster than females and become s.e.xually mature when smaller and younger. Examination of gonads showed 17 per cent of the males to be mature at plastral lengths of 90 to 99 millimeters, 76 per cent at 100 to 109 millimeters, and 100 per cent at 110 millimeters, whereas the corresponding percentages of mature females in the same size groups were: zero per cent, 47 per cent, and 66 per cent. Of the females, 97 per cent were mature at 120 to 129 millimeters and all were mature at 130 millimeters (Fig. 13). Because growth slows perceptibly at s.e.xual maturity, it is possible, by examination of growth-rings, to estimate the age of p.u.b.erty in mature specimens.

[Ill.u.s.tration: FIG. 11. Average increment in plastral length (expressed as a percentage of plastral length at the end of the previous season of growth) in the season of hatching (H) and in each of the following 14 years of life, based on 1073 growth-rings. The number of specimens examined for each year of growth is shown in parentheses. Records for males and females are combined.]

Attainment of s.e.xual maturity, in the population studied, was more closely correlated with size than with age. For example, nearly all males were mature when the plastron was 100 to 110 millimeters long, regardless of the age at which this size was attained. The smallest mature male had a plastral length of 99 millimeters; according to the data presented in Figures 9 and 10, therefore, a few males reach s.e.xual maturity in the fourth year, and increasingly larger portions of the population become mature in the fifth, sixth, and seventh years. The majority become mature in the eighth and ninth years.

Likewise, females (smallest mature specimen, 107 mm.) may be s.e.xually mature at the end of the sixth year but most of them mature in the tenth and eleventh years.

Annual Period of Growth

In growing individuals, narrow zones of new epidermis form on the laminae in spring. Nearly all the growing individuals collected in May of 1954 and 1955 had zones of new epidermis on the sh.e.l.l but those collected in April did not. Activity in the first week or two after spring emergence is sporadic and regular feeding may not begin until early May. Once begun, growth is more or less continuous as long as environmental conditions permit foraging. The formation of minor growth-rings and adjacent growth-zones in autumn, provides evidence that growth commonly continues up to the time of hibernation. The number of growing days per year varies, of course, with the favorableness of environmental conditions. The length of time (162 days) given by Fitch (1956b:438) as the average annual period of activity for _T. ornata_ is a good estimate of the number of growing days per season.

Environmental Factors Influencing Growth

Zones of epidermis formed in some years are wider or narrower than the zones bordering them (Pl. 22). Zones notably narrower or wider than the average, formed in certain years, const.i.tuted distinct landmarks in the growth-histories of nearly all specimens; for example, turtles of all ages grew faster than average in 1954 and zones of epidermis formed in this year were always wider than those formed in 1953 and 1955.

An index to the relative success of growth in each calendar year was derived. Records of growth for all specimens in each age group were averaged; the figure obtained was used to represent "normal" or average growth rate in each year of life (Fig. 12). The over-all averages for the various age groups were then compared with records of growth attained by individuals of corresponding age in each calendar year, growth in a particular year being expressed as a percentage of the over-all average. The percentages of average growth for all ages in each calendar year were then averaged; the mean expressed the departure from normal rate of growth for all turtles growing in a particular calendar year. For example, the over-all average increment in plastral length in the fifth year of life was 12.1 per cent, the increment in the sixth year was 10 per cent, and so on (Fig. 11). In 1953, turtles in their fifth and sixth years increased in plastral length by 11.4 and 9.1 per cent, or grew at 94.2 and 91.0 per cent of the normal rate, respectively. The percentages of normal growth rate for these age groups averaged with percentages of the other age groups in 1953 revealed that turtles grew at approximately 86 per cent of the normal rate in 1953.

Growth rates were computed for the twelve-year period, 1943-1954, because of the concentration of records in these years. Scattered records also were available for many of the years from 1901-1942.

Records for individuals in the season of hatching and the first full season of growth were not considered.

Direct correlation exists between growth rate and average monthly precipitation in the season of growth (April to September) (Fig. 12).

In nine of eleven years, the curve for growth rate followed the trend of the curve for precipitation; but because other climatic conditions also influenced growth, the fluctuations in the two curves were not proportional to one another.

Gra.s.shoppers form an important element in the diet of box turtles.

Smith (1954) traced the relative abundance of gra.s.shoppers over a period of 100 years in Kansas, and this information is of significance for comparison with data concerning growth of box turtles. In general, the growth index was higher when favorable weather and large populations of gra.s.shoppers occurred in the same year.

In the following summary, the numbers (1 to 5) used to express the relative abundance of gra.s.shoppers are from Smith (_op. cit._). Maxima and minima refer to the twelve-year period, 1943-1954. The growth index for each year (shown as a graph in Fig. 12) appears in brackets and indicates the percentage of normal growth attained by all turtles in that year.

_Years Favorable for Growth_

=1954= [126.3]: Growth was better than average for turtles of all ages. Gra.s.shopper populations were highest (4+) since 1948.

Continuously warm weather, beginning in the last few days of March, permitted emergence in the first week of April; thereafter conditions were more or less continuously favorable for activity until late October. Although there was less than an inch of precipitation in September, precipitation in August and October was approximately twice normal and more or less evenly distributed. Warm weather in early November permitted an additional two weeks of activity.

=1945= [125.5]: This was the second most favorable year for growth and the second wettest year. Records of growth are all from young turtles (one to four years old), all of which grew more than average. Daily maximum temperatures higher than 60 degrees Fahrenheit on 18 of the last 19 days of March, combined with twice the normal amount of precipitation in the same period, stimulated early emergence. August and October were both dry (each with less than one inch of precipitation) but diurnal temperatures remained warm through the first week in November and probably prolonged activity of box turtles at least until then. Gra.s.shoppers were more abundant (3.7) than normal.

_Years Unfavorable for Growth_

=1944= [83.1]: This was the poorest growing year for the period considered. The lack of a continuously warm, wet period in early spring probably delayed emergence until the last week in April.

Temperatures remained warm enough for activity until early November, but dry weather in September and October probably curtailed activity for inducing long periods of quiescence; most of the precipitation that occurred in the latter two months fell in a one-week period beginning in the last few days of September. Gra.s.shopper populations were higher (4.0) than normal.