Zones of epidermis formed in some years are wider or narrower than the
zones bordering them (Pl. 22). Zones notably narrower or wider than
the average, formed in certain years, constituted distinct landmarks
in the growth-histories of nearly all specimens; for example, turtles
of all ages grew faster than average in 1954 and zones of epidermis
formed in this year were always wider than those formed in 1953 and
1955.
An index to the relative success of growth in each calendar year was
derived. Records of growth for all specimens in each age group were
averaged; the figure obtained was used to represent "normal" or
average growth rate in each year of life (Fig. 12). The over-all
averages for the various age groups were then compared with records of
growth attained by individuals of corresponding age in each calendar
year, growth in a particular year being expressed as a percentage of
the over-all average. The percentages of average growth for all ages
in each calendar year were then averaged; the mean expressed the
departure from normal rate of growth for all turtles growing in a
particular calendar year. For example, the over-all average increment
in plastral length in the fifth year of life was 12.1 per cent, the
increment in the sixth year was 10 per cent, and so on (Fig. 11). In
1953, turtles in their fifth and sixth years increased in plastral
length by 11.4 and 9.1 per cent, or grew at 94.2 and 91.0 per cent of
the normal rate, respectively. The percentages of normal growth rate
for these age groups averaged with percentages of the other age groups
in 1953 revealed that turtles grew at approximately 86 per cent of the
normal rate in 1953.
Growth rates were computed for the twelve-year period, 1943-1954,
because of the concentration of records in these years. Scattered
records also were available for many of the years from 1901-1942.
Records for individuals in the season of hatching and the first full
season of growth were not considered.
Direct correlation exists between growth rate and average monthly
precipitation in the season of growth (April to September) (Fig. 12).
In nine of eleven years, the curve for growth rate followed the trend
of the curve for precipitation; but because other climatic conditions
also influenced growth, the fluctuations in the two curves were not
proportional to one another.
Grasshoppers form an important element in the diet of box turtles.
Smith (1954) traced the relative abundance of grasshoppers over a
period of 100 years in Kansas, and this information is of significance
for comparison with data concerning growth of box turtles. In general,
the growth index was higher when favorable weather and large
populations of grasshoppers occurred in the same year.
In the following summary, the numbers (1 to 5) used to express the
relative abundance of grasshoppers are from Smith (_op. cit._). Maxima
and minima refer to the twelve-year period, 1943-1954. The growth
index for each year (shown as a graph in Fig. 12) appears in brackets
and indicates the percentage of normal growth attained by all turtles
in that year.
_Years Favorable for Growth_
=1954= [126.3]: Growth was better than average for turtles of all
ages. Grasshopper populations were highest (4+) since 1948.
Continuously warm weather, beginning in the last few days of March,
permitted emergence in the first week of April; thereafter conditions
were more or less continuously favorable for activity until late
October. Although there was less than an inch of precipitation in
September, precipitation in August and October was approximately twice
normal and more or less evenly distributed. Warm weather in early
November permitted an additional two weeks of activity.
=1945= [125.5]: This was the second most favorable year for growth and
the second wettest year. Records of growth are all from young turtles
(one to four years old), all of which grew more than average. Daily
maximum temperatures higher than 60 degrees Fahrenheit on 18 of the
last 19 days of March, combined with twice the normal amount of
precipitation in the same period, stimulated early emergence. August
and October were both dry (each with less than one inch of
precipitation) but diurnal temperatures remained warm through the
first week in November and probably prolonged activity of box turtles
at least until then. Grasshoppers were more abundant (3.7) than
normal.
_Years Unfavorable for Growth_
=1944= [83.1]: This was the poorest growing year for the period
considered. The lack of a continuously warm, wet period in early
spring probably delayed emergence until the last week in April.
Temperatures remained warm enough for activity until early November,
but dry weather in September and October probably curtailed activity
for inducing long periods of quiescence; most of the precipitation
that occurred in the latter two months fell in a one-week period
beginning in the last few days of September. Grasshopper populations
were higher (4.0) than normal.
=1953= [85.6]: This was the second poorest growing year and the driest
year in the period considered. Intermittently cold weather in spring
delayed emergence until the last week in April when nearly an inch of
rain fell. Temperatures were higher than normal from June to October.
The period from September to the end of October was dry and the small
amount of precipitation that occurred was concentrated chiefly at the
beginning and end of that period. Temperatures in late October and
early November were lower than normal. Grasshopper populations were
low (2.2).
=1952= [88.3]: Environmental conditions were poor for growth and much
like the conditions described for 1953. In both years growth was much
less than normal in turtles of all ages except for one group (adults
that were 10 and 11 years old in 1952 and 1953, respectively) that was
slightly below normal in 1952 and slightly above normal in 1953.
The small number of records for 1955 were not considered in Figure
12. Warm weather in the last half of March lengthened the growing
season, and environmental conditions, as in 1954, were more or less
favorable throughout the rest of the summer; 1955 probably ranks with
1954 as an exceptionally good year for growth of box turtles.
Although the number of records available for turtles hatched in the
period from 1950 to 1954 is small, a few records are available for all
these years except 1951. In general, small samples of turtles hatched
in these years reflect only the difficulty of collecting hatchlings
and juveniles. In 1951, conditions for incubation and hatching were
poor and the lack of records for that year actually represents a high
rate of prenatal and postnatal mortality. Rainfall in the nesting
season was two to three times normal and temperatures were below
normal. Flooding occurred in low areas of Douglas County and many eggs
may have been destroyed when nests were inundated. Cold weather
probably increased the time of incubation for surviving eggs so that
only a few turtles could hatch before winter. Flooding and cold, wet
weather in the season of growth and reproduction, affecting primarily
eggs and hatchlings, may act as checks on populations of _T. ornata_
in certain years.
[Illustration: FIG. 12. The relation of growth rate in
_Terrapene o. ornata_ (solid line) to precipitation (dotted
line) in eastern Kansas. "Normal" rate of growth was determined
by averaging records of increase in length of plastron for
turtles in each age group. The growth index is expressed as a
percentage of normal growth and is the mean departure from
normal of all age groups in each calendar year. Precipitation
is for the period, April to September (inclusive), at Lawrence,
Douglas Co., Kansas. The means for precipitation (4.3) and
growth index (100) are indicated by horizontal lines at the
right of the graph.]
The environmental factors governing activity of terrestrial turtles
seem to differ at least in respect to threshold, from the factors
influencing the activity of aquatic turtles. A single month that was
drier or cooler than normal probably would not noticeably affect
growth and activity of aquatic emyids in northeast Kansas, but might
greatly curtail growth of box turtles.
Cagle (1948:202) found that growth of slider turtles (_Pseudemys
scripta_) in Illinois paralleled the growth of bass and bluegills in
the same lake; in the two years in which the fish grew rapidly, the
turtles did also, owing, he thought to "lessened total population
pressure" and "reduced competition for food." Growth of five-lined
skinks (_Eumeces fasciatus_) on the Natural History Reservation
paralleled growth of box turtles, probably because at least some of
the same environmental factors influence the growth of both species.
Calculations of departure from normal growth in _E. fasciatus_, made
by me from Fitch's graph (1954:84, Fig. 13), show that relative
success of growth in the period he considered can be ranked by year,
in descending order, as: 1951, 1949, 1948, 1950, 1952. This
corresponds closely to the sequence, 1951, 1948, 1949, 1950, 1952, for
_T. ornata_.