Box turtles of the genus _Terrapene_ are emyid turtles that are
specialized for terrestrial existence. Two of the seven species now
recognized--_T. ornata_ and _T. carolina_--occur in the United States.
_Terrapene carolina_ inhabits forested areas in the east whereas _T.
ornata_ is characteristic of open grassy areas in the west; the ranges
of the two species overlap in the broad belt of prairie-forest ecotone
in the central United States. _Terrapene ornata_ is considered to be
the most specialized of living box turtles.
The natural history of _T. o. ornata_ Agassiz was studied in the
period, 1953 to 1957. Intensive field studies were made in Douglas
County, northeastern Kansas, on a small area of prairie and on the
University of Kansas Natural History Reservation. Field observations
were made also in a number of other places in eastern Kansas.
Laboratory studies supplemented field studies.
Habitats occupied are chiefly open areas; they vary in regard to food
supply, temperature, moisture, and kind of soil. The grassy prairies
of Nebraska, Kansas, Oklahoma, and northern Texas seem to provide
optimum habitat for ornate box turtles; in these areas box turtles are
active on a majority of days from April to October. The subspecies
_luteola_ is adapted to the more rigorous and arid environment of the
southwestern United States, where activity may be possible for only a
few weeks in the year. The remainder of the year is spent in a state
of quiescence. Factors limiting the distribution of _T. ornata_ are:
1) the presence of a substrate too hard to permit digging of nests and
forms (altitudinal distribution in southwestern United States and
distribution at western edge of the range); 2) temperatures causing
the ground to freeze deep enough (approximately 30 inches) to kill
turtles in hibernacula (northern edge of range); and, 3) the lack of
one or more relatively wet periods in the course of the warm season,
preventing at least temporary emergence from quiescence (southwestern
part of range). The activities of man probably have affected
population density in local areas but limit the geographic range only
in the north (Blanchard, 1923:19-20, 24) where intensive cultivation
probably has excluded the species.
Preferred habitat in northeastern Kansas is open rolling grassland
grazed by cattle; populations are most dense near natural breaks in
the grassy vegetation such as fences, scattered rocks on hillsides,
ravines, and stream-beds.
Mating occurs most commonly in spring and autumn; courtship behavior
includes pushing and biting on the part of the male. In coitus the
hind legs of the male are held tightly by the female; the male falls
backward after coitus, still clasped by the female. A few sperm are
stored in the oviducts; fertilization without reinsemination can
occur. The spermatogenic cycle begins in May and reaches its peak in
September, when large numbers of sperm and spermatids are present in
the testes; the cycle is completed in October, when sperm pass into
the epididymides. The testes are smallest in spring and largest in
September. Females are inseminated with sperm produced in the
preceding year. The ovarian cycle begins in midsummer, soon after
ovulation, and continues up to the time of the next ovulation.
Follicular growth is rapid in the period from spring emergence to
ovulation. Large follicles remaining after ovulation represent, in
many instances, eggs that will be laid later in the same season.
Follicular atresia is never great enough to account for the
destruction of all large follicles remaining after ovulation. All
mature females lay at least one clutch of eggs per year. It is
estimated that one-third of the females produces two clutches of eggs
in a single season. Second clutches contain fewer eggs than first
clutches. An alternation of ovarian activity occurs, whereby one ovary
is more active than its partner in one season and less active in the
next season. Alternating activity of ovaries accounts in part for the
reduced number of eggs in young females, breeding for the first time,
and in older, nearly senile females. Extrauterine migration of ova
results usually in a more even distribution of eggs in the oviducts.
Corpora lutea constitute an accurate record of the number of eggs
produced by the ovary as well as the number of eggs laid.
Nesting occurs from May through July but is most common in mid-June;
some of the females nesting early in the season lay a second clutch of
eggs in July. Nests are dug in the earth by the female using her hind
legs. Preferred nesting sites are open, well-drained places with a
soft substrate. The nesting site is selected after a period of
wandering, in which the female tests the substrate at a number of
places; some females search for a nest site for more than a week. Nest
digging begins in the evening and is usually completed after dark.
Captive females dug a preliminary cavity in which the body rested
during the digging of the main nest cavity. The entire clutch of eggs
is laid in one nest. The average number of eggs in 23 clutches was 4.7
(range, 2 to 8). The average size of eggs tends to be inversely
proportional to the number of eggs in a clutch. Eggs increase in bulk
by absorption of water in the course of incubation. Immersion in water
for short periods does not harm eggs. The incubation period under
favorable environmental conditions is approximately 65 days; cool,
damp conditions prolong the incubation period and probably constitute
an important factor of prenatal mortality in certain years. Eggs that
do not hatch before winter probably do not survive. Emergence of
hatchlings from the nest may, however, be delayed until spring if the
soil is dry in autumn. Hatchlings can probably escape freezing by
burrowing into the walls of the nest. Infertility and prenatal
mortality account for at least 40 per cent of the eggs laid, according
to laboratory findings. Progeny of a single adult female (considering
factors of mortality, multiple layings, and average age of puberty)
would number approximately 300 after 20 years. Reproductive processes
probably continue throughout life, although possibly at a somewhat
reduced rate in later life.
Young box turtles are active soon after hatching but become quiescent
if allowed to burrow in soil or if they are covered with damp cotton.
Some captive hatchlings take live food in the first days of life but
others do not eat until the following spring; initiation of growth is
coincident with initiation of regular feeding. The yolk sac retracts
mainly during hatching; it sometimes ruptures after hatching. The
caruncle remains on the beak for a variable length of time, but never
is present in the spring following hatching.
Major growth-rings on the epidermal laminae are formed regularly, one
after each season of growth, in the first 10 to 14 years of life.
Minor growth-rings occur between major rings and are shallower. Growth
of epidermal laminae results from the formation, in spring, of a new
layer of epidermis beneath the existing scute. The peripheral
projection of the new layer is distinct in texture and color from the
older part of the scute and is separated from it by a major
growth-ring. Minor growth-rings form when growth slows or temporarily
stops during periods of quiescence; no new layer of epidermis is
formed. Growth-rings constitute an accurate record of growth that can
be studied at any time in the life of the turtle; they are accurate
indicators of age only as long as regular annual growth persists.
Growth in the season of hatching depends on early hatching and early
emergence from the nest. Turtles that remain in the nest until spring
probably do not grow. Slightly less than half of the free-living
individuals studied grew in the season of hatching. Precociousness in
early life often results in the attainment of sexual maturity at an
earlier than average age.
Growth is rapid at first (increments in plastral length average 68,
29, and 18 per cent, respectively, in the first three years) and then
slows gradually until puberty. Attainment of sexual maturity is more
closely correlated with size than with age. Males mature when smaller
(76 per cent were mature when plastron 100 to 109 mm. long) and
younger (average age, eight to nine years) than females (66 per cent
were mature when plastron 110 to 119 mm. long, average age at
maturity, ten to eleven years) but females grow larger than males. A
few individuals of each sex reach puberty three to four years sooner
than average.
The average number of growing days per season is approximately 160.
Amount of growth in any season depends on climatic factors that
influence food supply and foraging conditions. Growth rate is directly
correlated with precipitation, being highest when large populations of
grasshoppers and long periods of favorable weather occur in the same
year. Zones of epidermis formed in years when growth was especially
slow or especially fast constituted landmarks that were helpful in
interpreting growth-histories. Growth stops two to three years after
puberty. The total growing period is estimated to be not more than 15
to 20 years. Longevity is estimated to be approximately 50 years.
A number of changes in structure and appearance occur in the period
from hatching to puberty. Fontanelles of the bony shell close at or
before puberty. Movable parts of the plastron are not functional until
the fourth year. Markings on the carapace change from a series of dots
to distinct, straight-sided radiations, and a similar pattern develops
on the plastron. Markings on the heads of females resemble those of
juveniles but males have greenish heads. Males further differ from
females in having a red iris, more brightly colored antebrachial
scales, and a turned in first toe.
Analysis of some 500 body temperatures (Centigrade) obtained under
natural conditions revealed the following: the optimum temperature for
activity is near 30 degrees; box turtles emerge from cover usually
when body temperature is 24 degrees or higher, and almost never when
the body temperature is below 15 degrees; body temperature is raised
to optimum by basking in open areas although activity begins at
suboptimum temperatures if basking is impossible; cover of dens,
burrows, or forms is sought when the body temperature rises above 30
degrees; and, maximum and minimum body temperatures that would be
lethal to box turtles (for prolonged periods) are approximately 40 and
zero degrees, respectively. Laboratory experiments showed speed of
response to environmental temperature to be inversely proportional to
bulk; hatchlings could be chilled or warmed more than twice as fast as
adults and were active within a narrower range of temperature. Ornate
box turtles in general are subject to a narrower range of thermal
activity than are aquatic turtles that occur in the same areas.
Box turtles are dormant approximately five and one-half months of the
year--from late October to mid-April. Warm weather in November and
late March sometimes stimulates temporary activity but dormancy is
uninterrupted from mid-November to early March. Forms, dens, and
burrows are used as hibernacula. Depth of hibernacula is dependent on
severity of temperatures and amount of vegetational cover; hibernacula
in open grassland were seven to 18 inches deep whereas those in wooded
areas were six inches or shallower. Box turtles are ordinarily
solitary when hibernating. Injuries and deaths due to freezing
probably occur in the coldest part of the winter. The lowest body
temperature of a turtle that survived a winter was 2.7 degrees; an
individual, the temperature of which was nearly zero for several days,
subsequently died. Turtles burrow upward at the end of hibernation and
remain just below the surface for a week or two before emerging. The
primary stimulus for emergence seems to be a period of warm moist
weather.
Populations of _T. ornata_ observed under natural conditions were
chiefly carnivorous, although captives ate a variety of animal and
vegetable matter. Insects, consisting chiefly of beetles,
caterpillars, and one species of grasshopper, comprised approximately
89 per cent (by volume) of the food present in stomachs. Beetles
(chiefly scarabaeids and carabids) are obtained in or near dung and
seem to constitute the most important staple element of the diet.
Piles of dung, disturbed by turtles in the course of their foragings,
were characteristic "sign" of _T. ornata_ in the areas studied.
Insects form the bulk of the diet for most of the year, although
certain other foods, when especially abundant for short periods
(mulberries for example), are eaten in large quantity or eaten to the
exclusion of all other foods. Ornate box turtles occasionally eat the
eggs and young of ground-nesting birds and slightly damage vegetables,
but in no instance do these feeding habits significantly affect the
economy of man. Box turtles probably benefit man by destroying large
numbers of crop-damaging insects (locustids and noctuid caterpillars).
Box turtles were more numerous than most kinds of reptiles at the Damm
Farm and were the most conspicuous of any kind of reptile. One hundred
and ninety-four turtles were marked; one-fourth of these were
recaptured at least twice. Population density in certain areas of
favorable habitat ranged from 2.6 to 6.3 turtles per acre. The total
number of individuals on the study area was estimated to be 286. The
marked population consisted of 53 per cent adult or subadult females,
31 per cent adult males, and 16 per cent juveniles of undetermined
sex. Only six individuals had plastra shorter than 60 millimeters.
Small box turtles are not so rare as these samples indicate; they are
infrequently obtained because their smallness and ability to blend
with the substrate make them difficult to see. More females than males
were found in all months of the season of activity, excepting April
and August when more males were found; the preponderance of females
was greatest in the nesting season (June and July).
Ornate box turtles walk with the shell held off the substrate. They
are able to climb steep embankments or low barriers with some
facility. Swimming ability is sufficient to permit survival in water
and traversal of water-barriers but ornate box turtles almost never
swim voluntarily.
Daily activity consists of periods of basking, foraging, and rest, the
durations of which are influenced by temperature and humidity. There
is no activity after dark except that of nesting females. After
several days of activity there is a period of rest; rest periods
seemed not to be correlated with climatic conditions. The average
distance traveled per day in summer is 200 to 300 feet. Movements of
gravid females are more extensive (average, 363 feet per day) than
those of other members of the population; one individual traveled
approximately one-fourth of a mile in a single day. Turtles removed
from their normal home ranges traveled farther per day than any other
group. Movements in autumn are less extensive (average, 152 feet per
day) than at other times in the season of activity.
Individual box turtles tended to remain in small areas for long
periods; these areas were interpreted as home ranges. The estimated
average radius of 44 home ranges was 278 feet (average area, 5.6
acres). The average area of eight home ranges that were actually
measured was five acres. General suitability of habitat and certain
physical features of terrain (rock fences, ravines, barren fields)
that acted as barriers were thought to be the most important factors
governing size of home range. Of two turtles removed more than
one-fourth of a mile from their home ranges, one homed and one did
not. Home ranges of turtles of all ages and sexes overlap broadly.
There was no indication that territoriality or social hierarchy
existed in the population studied.
Box turtles are subject to injury from natural causes that include
fire, cold, molestation by predators, and trampling by cattle.
Automobiles and farm machinery now constitute major causes of
mortality and serious injury. Capacity to recover after serious injury
is great but there is increased chance for secondary injury,
infection, and predation in the period of recovery. Pits on the shell
from unknown causes ("shell pitting") are less common in ornate box
turtles than in other kinds of turtles.
Ectoparasites infesting _T. ornata_ are larvae of chigger mites (genus
_Trombicula_) and larvae of bot flies (_Sarcophaga cistudinis_).
Ectoparasites usually have little adverse effect on the turtles,
although heavy infestations cause occasional injury or death.
Few natural enemies other than man are known; however most wild
carnivores as well as opossums, large birds, and domestic dogs and
cats are suspect as predators. The incidence of predation on eggs and
small juveniles is far greater than on older juveniles and adults.
Adults of _T. ornata_ occasionally attack smaller individuals.
Ornate box turtles are able to detect the presence of intruders, by
sight, from a distance of several hundred feet in open country;
apparently, intruders are not detected until seen. Defensive behavior
is passive; the shell is closed tightly in response to painful stimuli
and, in some instances, at the sight of an intruder. Juveniles usually
void odoriferous fluid from the musk glands when handled but adults do
so only in response to pain or injury. The function of the musk glands
is unknown; possibly the odor of musk is a means of sexual
identification or stimulation. Although the musk is probably
distasteful to predators, repellent odor alone seems to be of doubtful
value as a defense mechanism.