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.
[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.