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 shells of ornate box turtles, particularly those of the subspecies luteola, have only a thin layer of epidermis through which the bones of the shell 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 shell 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 constitute 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 agassizi. 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 sexual maturity but are formed irregularly after puberty.
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 relationship 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 Relationship 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 Carapace | Number of Specimens | Length of abdominal as a percentage of length of plastron | Individuals having plastron longer than carapace | ||
| Mean ± σm | Extremes | Number | Percentage | ||
| Less than 50 mm. (Juveniles) | 23 | 18.3±.498 | 13.7-20.3 | 7 | 38.5 |
| 50 to 69 mm. (Juveniles) | 20 | 17.8±.303 | 15.2-20.2 | 8 | 40.0 |
| 70 to 100 mm. (Subadults) | 20 | 17.9±.445 | 14.3-20.6 | 15 | 75.0 |
| More than 100 mm. (Adult males) | 20 | 17.8±.236 | 16.4-20.6 | 13 | 65.0 |
| More than 100 mm. (Adult females) | 20 | 18.8±.510 | 15.1-25.7 | 19 | 95.0 |
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 proportion,
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