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 latitude 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 shells. Without the presence of newly formed epidermis as a landmark, however, it was never certain how many years had passed since the last ring was formed.
Fig. 13. The relationship of sexual 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. Sexual 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 assumption 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 sexually 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