opened. The stomach of one (OU 27593) was full of naiads of the
burrowing mayfly _Hexagenia_; that of the other female (OU 27594)
contained exoskeletal remains of crawfish. The two specimens were
drowned in gill nets between the hours of 11 a. m. and 7 p. m., on
July 10, 1954; the intact condition of the mayfly naiads indicated
recent feeding.
The species of American softshells are mainly carnivorous. The presence
of vegetable matter (mentioned in previous paragraphs) in the digestive
tracts of many specimens and True's statement (1893:152) that
soft-shelled turtles include a variety of vegetable matter in their food
indicates omnivorous habits. Duellman and Schwartz (1958:272) stated
that _ferox_ is omnivorous and Carr (1952:430) made a similar statement
for _spinifer_. The diet seems to be determined by the food supply
available, which may vary seasonally or with adverse conditions such as
flooding; under normal environmental conditions, however, vegetable
matter probably is ingested incidentally to other food. There is no
indication of a preference in food habits according to species and
subspecies. Most of the food seems to be obtained by active foraging
that is triggered primarily by movement of the prey; the sense of smell
is probably secondary.
Reproduction
_Size of males at Sexual Maturity_
Elsewhere (1956:121) I have shown that males of _spinifer_ from Lake
Texoma, Oklahoma, and scattered localities in Texas and Louisiana
are sexually mature when they reach a plastral length of 9.0-10.0
centimeters. Adult males have distinct, convoluted, non-pigmented vasa
deferentia and elongate testes, the maximal measurements of which are
about 10 by 30 millimeters. Testes of hatchlings are approximately 4.0
by 0.4 millimeters (TU 13698.12, plastral length 3.2 cm., measured with
ocular micrometer). I am not aware of seasonal changes in size of the
testes.
In reading the discussion that follows, it is well to remember that
males having the cloaca extending beyond the posterior edge of the
carapace are regarded as sexually mature. As an indication of geographic
variation in _spinifer_, I have listed the measurements of the 10
smallest males for only those subspecies of which there are numerous
records (Table 7). Corresponding data for _T. muticus muticus_ are also
listed for comparison.
TABLE 7. Size at Sexual Maturity of the 10 Smallest Males of T. m.
muticus and Selected Subspecies of T. spinifer. The Extremes
Precede the Mean (in Parentheses).
========================+=======================
SPECIES AND SUBSPECIES | Plastral length (cm.)
------------------------+-----------------------
_T. s. spinifer_ | 8.8-10.3 (9.6)
_T. s. hartwegi_ | 9.6-10.5 (10.2)
_T. s. pallidus_ | 9.1-11.2 (10.5)
_T. s. guadalupensis_ | 9.3-10.8 (10.1)
_T. s. emoryi_ | 8.2-9.0 (8.8)
_T. m. muticus_ | 8.2-9.2 (8.7)
------------------------+-----------------------
The data indicate that the size at which sexual maturity is attained in
_emoryi_ (about 8.0-9.0 cm.) is less than in any other subspecies of _T.
spinifer_ (about 9.0-10.0 cm.), and, more importantly, corresponds to
that of _T. m. muticus_. Although the mean for _T. s. spinifer_ is
slightly less than in the remaining subspecies, I doubt that there is
any significant difference according to subspecies in the size at which
sexual maturity is attained in the subspecies _spinifer_, _hartwegi_,
_asper_, _pallidus_ and _guadalupensis_. The corresponding size in _T.
m. muticus_ and _T. s. emoryi_ heightens the morphological resemblance
between these forms. The only sexually mature male of _T. ater_, which
morphologically resembles _emoryi_ and _muticus_, is 9.5 centimeters in
plastral length. I do not know the size at which _T. ferox_ attains
sexual maturity. The smallest sexually mature individual examined by me
was 12.0 centimeters; probably _ferox_ attains sexual maturity at a
larger size than _spinifer_ or _muticus_. The relative size of
attainment of sexual maturity in _ferox_, _spinifer_, and _muticus_
corresponds to the maximum size of the three species; _ferox_ is the
largest species and _muticus_ is the smallest (Table 2).
_Size of Females at Sexual Maturity_
Breckenridge (1955:6) wrote that the development of the mottled pattern
"undoubtedly indicates a stage in the attainment of sexual maturity"; I
have mentioned (1956:121) that the mottled pattern is apparent on
females before sexual maturity is attained. To my knowledge females have
no external characters which appear at the time of attainment of sexual
maturity.
Sexually mature individuals of _ferox_ have been described in various
terms: 31-1/4 pounds (Goff and Goff, 1935:156); six pounds, lengths of
carapace 10-1/2 and 13 inches (Hamilton, 1947:209); greatest width of
head 3-1/2 inches (Wright and Funkhouser, 1915:120). A 10-1/2 inch
carapace presumably represents the smallest turtle and corresponds to a
plastron approximately 22.0 centimeters in length. There is no other
information available concerning size at sexual maturity in _T. ferox_.
There is little published information concerning the size at sexual
maturity in _T. spinifer_. Cahn (1937:193) wrote that _spinifer_ in
Illinois "must attain a carapace length of about 24 centimeters
[plastral length approximately 18.0 cm.] before the females become
sexually mature"; this statement is the basis for Smith's mentioning a
length of 9-1/2 inches (1956:162). Evermann and Clark (1920:595)
recorded the lengths of carapace of some females (presumably all adult)
from Lake Maxinkuckee, Indiana, as 11, 11-3/4, 12-1/2, and 13 inches;
the smallest measurement corresponds to a plastral length of
approximately 21.0 centimeters.
The data concerning reproduction presented in succeeding paragraphs is
based principally upon examinations of turtles in the TU collections; I
am indebted to Dr. Fred R. Cagle for permission to dissect these
turtles. Females are regarded as sexually mature when they have oviducal
eggs or corpora lutea or ovarian follicles exceeding 15 millimeters
in diameter. Hatchlings of _spinifer_ have ovaries that measure
approximately 6.0 × 0.3 millimeters, and straight oviducts 0.2
millimeters in width (TU 5988, plastral length 3.5 cm. measured with
ocular micrometer). In the size at which sexual maturity is attained
there seems to be much individual variation as well as geographic
variation.
Females of _T. s. emoryi_ from the Río Grande in the Big Bend region
of Texas are sexually mature when the plastron is approximately 16.0
centimeters (16.2 cm., KU 51960), and are the smallest adult females
of _spinifer_ that I have seen; these females are representative of
the population from which the smallest adult males of _spinifer_ are
known and which is unique in showing sexual differences in coloration.
A female (TU 3697), having a plastral length of 16.0 centimeters,
which was obtained in the Río Grande near Eagle Pass, Texas, in
mid-July, is immature; the ovaries are compact having the largest
follicles 2.5 millimeters in diameter, and the oviduct is wrinkled and
convoluted but only six millimeters wide. Of three females of _emoryi_
from the Pecos River, Terrell County, Texas, having plastrons 17.4,
18.3 and 18.8 centimeters in length and obtained on June 11, the
largest and smallest are immature, and internally resemble TU 3697. TU
14453.2 (18.3 cm.) is sexually mature having large corpora lutea and
enlarged ovarian follicles. KU 53754, from the Río Salado in central
Coahuila, México, having corpora lutea and a plastral length of 20.3
centimeters, is sexually mature.
Females of _T. s. guadalupensis_, measuring 14.5, 15.7, 16.3, 16.5,
16.8, 17.0, 19.0, and 20.0 centimeters in plastral length and obtained
from June to September, are immature. The female measuring 19.0
centimeters indicates the approach of sexual maturity in having
swollen and convoluted oviducts seven to ten millimeters in width, but
compact ovaries having the largest follicles 4.0 millimeters. The
other _guadalupensis_ whose measurements are given above have oviducts
that do not exceed four millimeters in width, and ovarian follicles
that do not exceed two millimeters in diameter. TU 10187, obtained in
July, plastral length 19.5 centimeters, is sexually mature having
corpora lutea and enlarged follicles. Two other _guadalupensis_, 21.5
and 22.0 centimeters (Pl. 12, top), having ovaries with enlarged
ovarian follicles (the largest in one, only 11 mm.) are considered
sexually mature.
Concerning the subspecies _pallidus_, females (all collected in June
or July) measuring 15.7, 16.3, 17.3, 17.5, 18.7, 19.5, 20.8 and 21.3
centimeters in plastral length are immature having solid, compact
ovaries with the largest follicles not exceeding two millimeters in
diameter; oviducts are straight not exceeding three millimeters in
greatest width, except those turtles measuring 17.3 and 21.3
centimeters in which the oviducts are swollen and convoluted and,
respectively, five and eight millimeters in greatest width. The
smallest sexually mature _pallidus_ is 19.8 centimeters in length;
recorded lengths of other adult females are 23.5, 26.8 and 30.5
centimeters.
Of especial interest are three large female _pallidus_, measuring
24.8, 27.5, and 28.0 centimeters, which appear to be immature; two of
these (TU 13303-04) are from the Sabine River, collected in July, and
the other specimen is without data (presumably from the Sabine River).
The oviducts are large, swollen and convoluted, resembling those in
sexually mature individuals. The ovaries, however, are relatively
solid and compact having approximate measurements of 125 × 6
millimeters (TU 13303) and 85 × 10 millimeters (TU 13304), and
follicles not exceeding five millimeters in diameter.
Females of _spinifer_ from the lower Mississippi Valley of Louisiana
having plastral lengths of 15.0, 15.5, 16.7, 17.5, 18.0, 19.5, 20.0,
20.4, and 20.8 centimeters are considered immature; the ovaries are
compact and solid having follicles not exceeding three millimeters in
diameter, and the oviducts, swollen and convoluted in the larger
individuals, do not exceed six millimeters in width. The ovaries of
the specimen 19.5 centimeters in length mentioned immediately above
had been removed prior to my examination; the oviducts, however, were
relatively straight and only five millimeters in width. Three females
23.0, 25.5, and 25.8 centimeters in length are sexually mature. TU
5518, measuring 21.5 centimeters in length and obtained in June,
indicates the onset of sexual maturity in having large convoluted
oviducts, but the ovaries are solid, compact, measuring 85 × 13
millimeters, and the largest follicles are only 4.5 millimeters. A
larger turtle (TU 13080), 24.5 centimeters, obtained in July, has
juvenal ovaries (largest follicles five mm.); the oviducts are
enlarged and convoluted as in adult females.
Of two _T. s. asper_ collected from the Escambia River in June and
July, one 18.0 centimeters in plastral length is immature, whereas the
other, 27.0 centimeters long, is adult. A female _T. s. hartwegi_,
measuring 20.7 centimeters, is adult having enlarged follicles and
corpora lutea (TTC 719, Pl. 36, bottom).
In summary, females of all subspecies of _spinifer_, except some
_emoryi_, may be sexually mature when the plastron is 18.0 to 20.0
centimeters in length; probably all physiologically normal females are
adult when 22.0 centimeters long. In general, females are sexually
mature at a plastral length of approximately 20.0 centimeters, a
measurement that corresponds to a length of carapace of approximately
28.0 centimeters or about 11 inches. Females representative of that
population of _emoryi_ inhabiting the Río Conchos and the Río Grande
in the Big Bend region of Texas are adult when the plastron is
approximately 16.0 centimeters in length, and are thus the smallest
sexually mature females of the species _spinifer_. Oviducts are large
(at least eight mm. in width, undistended), swollen and convoluted prior
to the first ovulation.
Of interest are the large females (for example, TU 13303, plastral
length 28.0 cm.) that seemingly have immature, relatively small, ovaries
(the oviducts are large and convoluted as in adult females). Possibly
such ovaries represent a regression and are in senile turtles, but I am
inclined to believe that the development of these ovaries has been
arrested probably owing to hormonal unbalance, and that they have never
been functional.
The size of adult females of _T. ater_ is unknown but probably
approximates that of _T. spinifer_ or is slightly less. Females of
_ater_ 15.5 and 17.2 centimeters in length are immature; the largest
female, the holotype, is 18.3 centimeters in length, and was not
dissected.
Females of _T. muticus_ are sexually mature when smaller than _T.
spinifer_. Two turtles, 13.8 and 14.0 centimeters in length, have large
convoluted oviducts about 10 millimeters in width and ovarian follicles
nine to twelve millimeters in diameter, and seem to be near sexual
maturity. The smallest sexually mature female (subspecies _muticus_) is
TU 14436, measuring 14.4 centimeters in plastral length and having
oviducal eggs. Recorded lengths of other adult females are 16.3, 16.5,
17.2 (subspecies _muticus_), and 18.0 centimeters (subspecies
_calvatus_). Two females having plastral lengths of 17.5 (subspecies
_muticus_) and 16.0 centimeters (subspecies _calvatus_) seem sexually
immature. These turtles collected in April and May have ovarian
follicles not exceeding three millimeters in diameter.
_Sexual Activity_
Observations by Mitsukuri (1905:263), Conant (1951:160) and Legler
(1955:98), constitute the extent of our knowledge concerning courtship
and copulation. Legler observed a male _spinifer_ and a female _muticus_
in captivity; the male was the aggressor, following at the rear or above
the female, and at times nipping at the anterior part of her carapace.
During these movements, the posterior edge of the female's carapace was
turned up slightly whereas that of the male was turned down; the turtles
frequently surfaced to breathe. Occasionally the female followed the
male. On the bottom the male crawled onto the female's carapace from the
rear, remaining in a somewhat posterior position as described by Conant
(_loc. cit._), and seemingly not clasping the female with his feet.
Copulation probably occurs in this position; Mitsukuri (_loc. cit._)
mentioned that copulation in _Trionyx sinensis_ occurs at the surface of
the water. The male remains in the coital position for approximately 15
seconds and then slowly drifts to one side and swims away. Legler
observed five coital unions in one-half hour, each preceded by courting
movements.
Legler's observations indicate that the courtship patterns of _muticus_
and _spinifer_ are similar, and that interspecific matings are possible.
I have not noted any hybrid.
Risley (1933:689) mentioned differential movements of the sexes of
_Sternothaerus odoratus_ in conjunction with the breeding cycle. Such
movements are revealed by trapping procedures that yield deviations from
the expected 1:1 sex ratio. That differential sexual movements probably
occur in _Trionyx_ is indicated by my trapping 17 males in a group of 19
_spinifer_ in hoop-nets in Lake Texoma in the period June 14-July 12,
1954. On June 24-26, 1959, a field party from the University of Kansas
collected 15 softshells in hoop-nets at the mouth of the Río San Pedro,
near Meoquí, Chihuahua; all turtles were males. On June 17-18, 1959, the
same expedition trapped 11 males in a group of 13 turtles in the Río
Conchos, near Ojinaga, Chihuahua. Earlier, June 12-14, 1959, 39
softshells were trapped in the Río Grande near Lajitas, Brewster County,
Texas. Of these turtles, however, 19 were adult males and 20 were
females; eight females were adult (sexually mature) all having oviducal
eggs (Fig. 23). One of the two females from Ojinaga, KU 51174, is
sexually mature (plastral length, 16.5 cm.) having oviducal eggs; the
other is immature (plastral length, 8.0 cm.). The only softshell taken
on June 21, 1959, 8 mi. N and 16 mi. W Ojinaga, KU 51173 (plastral
length, 16.3 cm.) is a female having oviducal eggs. The two females from
Lake Texoma are immature (plastral lengths, 9.8 and 12.4 cm.).
The results of trapping may indicate that females frequent shallow water
for a short time before the period of deposition of eggs, but disperse
to deep water after such periods or between them. The movements of
immature females probably approximate those of adult males; the absence
of immature females in the Meoquí series, and near absence (only one) in
the Ojinaga series perhaps is due to fortuitous collecting methods or to
slightly different diurnal movements between adult males and immature
females. Females approaching sexual maturity and those sexually mature
but not having oviducal eggs ready for deposition possibly remain
relatively sedentary in deep water; such females possibly represent
those absent in the 13.0-15.9 size group (Fig. 23). Certainly, factors
other than those pertaining to egg deposition may cause mature egg-laden
females to live in shallow water, or explain the deviations from the
expected 1:1 ratio.
[Illustration: FIG. 23. Size distribution of 39 _Trionyx spinifer
emoryi_ (19 males and 20 females) collected in the period June 12
through June 14, 1959, from the Río Grande, near Lajitas, Brewster
County, Texas. Solid squares represent sexually mature specimens.
Females approaching sexual maturity or those not ready for egg
deposition (13.0-15.9 cm. size group) are possibly sedentary in
deep water.]
One of the immature softshells (KU 51979, plastral length, 9.7 cm.) of
the series from Lajitas is considered to be a female. It combines
characteristics of both sexes. It resembles a male in having a carapace
gritty to the touch, in having prominent white dots posteriorly and in
not having a faint mottled and blotched pattern as do females of the
same size. The postocular and postlabial markings are mostly yellow
(female), but a small patch of the postocular stripe near the junction
with the pale ventral coloration laterally is tinted with orange (male);
the morphological characters and secondary sexual difference in
coloration of this series of softshells has been mentioned on page 512.
The tail is short and pyramidal resembling that of a female. Internally,
there are a pair of ovaries and oviducts; KU 51979 is functionally a
female. An over-production of androgens probably is responsible for the
external masculine characteristics (orange color, gritty carapace and
absence of mottling on carapace).
_Deposition of Eggs_
Concerning _T. ferox_, Wright and Funkhouser (1915:122-23) wrote that
deposition of eggs occurred in June and July in the Okefinokee Swamp
on the sandy parts of the islands or in sandy fields in places exposed
to the direct rays of the sun. The same authors recorded a gravid
female taken on June 22 (_op. cit._:120), and a nest with eggs on June
26. Harper (1926:415) reported egg-laying in June in the Okefinokee
Swamp. Goff and Goff (1935:156) found a female in search of a nesting
site crawling toward a cleared area within a hammock at 11 a. m. on
May 19, about 25 yards from the western shore of Lake Griffin,
Florida. Carr (1940:107) stated that eggs in Florida "are laid from
March to July 10. One individual laid her eggs on a block of ice which
we had buried in the sand." Hamilton (1947:209) observed deposition of
eggs near Fort Myers, Florida, in "a sandy roadbed slightly above the
cypress swamp and ditch levels on either side of the road." ... either
in ... "the ruts formed by cars or the slope of the roadbed"; dates of
deposition of eggs recorded are March 30 at 11 a. m. in bright sun,
and March 31 (from context, the date given as March 21 is considered
an error) at 5 p. m. following a heavy rain. The daily temperatures at
the time of Hamilton's observations "averaged 85° F., the first really
warm spell of the season."
Eigenmann (1896:262) reported egg-laying of _spinifer_ in sand and
gravel in June and July at Turkey Lake (= Lake Wawasee), Indiana. A
turtle was seen digging a nest on June 26, and fresh nests of eggs
were found on June 27 and July 9. Hedrick and Holmes (1956:126) wrote
that a clutch of eggs of _spinifer_ in Minnesota was found about ten
inches deep in sand about one foot from the river; a steep gravel bank
was also cited as a favorite nesting site. Surface (1908:123) stated
that eggs of _spinifer_ in Pennsylvania are laid in May, and the young
hatch in August. Gehlbach and Collette (1959:142) found eggs of
_spinifer_ on June 19 on a sand bank 15 feet from the edge of the
Platte River in Nebraska. Breckenridge (1944:187) wrote that
_spinifer_ in Minnesota nests on sandy beaches from June 14 to July 6.
Cahn (1937:193) stated that deposition of eggs in Illinois occurs in
"June or early July: earlier in the southern part of the state, later
in the northern portion." Force (1930:38) mentioned a gravid female
from Oklahoma obtained on May 20. Evermann and Clark (1920:593) were
of the opinion that _spinifer_ began laying about mid-June and
continued until perhaps late July at Lake Maxinkuckee, Indiana; a
female opened on June 14 had oviducal eggs, and the first nest was
found on June 18. Nests were usually at the edge of an abrupt ascent
in sand; one nest was found in black, mucky soil (_op. cit._:595).
Newman (1906:128) wrote that _spinifer_ in the same lake nests later
than the other species of turtles, as a rule not earlier than the
middle of June (but as early as June 10, _op. cit._:132), and rarely
later than the middle of July; he observed deposition of eggs on June
22. Sites of deposition of eggs were mostly in soft sand not more than
six feet from water; other sites found by Newman (_op. cit._:132-33)
were a sandy, abandoned road bed separated from the shore by a strip
of tall grass, a rock pile (the eggs being dropped into crevices and
sand packed around them), among roots of a tree (the eggs being
deposited between the roots and under them in a very irregular
fashion), and in clay "so hard packed that one could scarcely break it
with the fingers." Natural nest sites in hard clay and a rock pile
seem incongruous with nesting habits of softshells. I note that
Newman's study was not begun until 1902 (_op. cit._:127), and it was
that year that the water level of the lake was high, flooding the
surrounding lowlands (Evermann and Clark, 1920:49-53). Perhaps some of
the nests found by Newman were old and not natural because of
conditions resulting from the receding water level.
Newman (_op. cit._:134-35) mentioned that in small sandy areas nests
were frequently in contact and overlapped; he found one nest
containing nine small eggs contiguous with 23 large eggs. Breckenridge
(1944:187) reported a nest of 56 eggs of two slightly differing sizes,
and probably from two females. Evermann and Clark (1920:594)
discovered "probably 10 or 12 nests in a distance of a few yards" and
mentioned one nest containing 25 eggs "that evidently belonged to two
different sets ... In the bottom were 10 eggs that looked old ... and
... separated from them by sand, were 15 other eggs."