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 sex); they give a general, over-all picture of growth rate but
do not reflect the changes that occur in growth rate at puberty (as
shown in Figs. 9 and 10).
Rate of growth and, ultimately, size are influenced by the attainment
of sexual maturity. Adult females grow larger than adult males. Males,
nevertheless, grow faster than females and become sexually 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 sexual maturity, it is possible, by examination of
growth-rings, to estimate the age of puberty in mature specimens.
[Illustration: 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 sexual 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
sexual 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 sexually
mature at the end of the sixth year but most of them mature in the
tenth and eleventh years.