[Illustration: FIG. 9. See legend for Fig. 10.]
[Illustration: FIG. 10. The relationship of size to age in _T. o. ornata_, based on studies of growth-rings in 115 specimens of known sex (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 sexual maturity one or more years before other turtles of the same age. A few turtles (chiefly males) attain adult size (and presumably become sexually 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.