the length of the carapace averages 2.7 centimeters (hatchlings of
_sinensis_ seem to average smaller than any American species), and
that the average length of carapace (cm.) at the end of the first year
is 4.5, second year 10.5, third year 12.5, fourth year 16.0, and end
of fifth year 17.5; he stated also that females of _sinensis_ are
sexually mature in their sixth year. Breckenridge (1955:7-9) computed
a growth curve based on 11 recaptures of females of _spinifer_ in
Minnesota; his data on rate of growth for the first five years do not
differ appreciably from those of Mitsukuri. As most females of
_spinifer_ are sexually mature when the carapace is about 11 inches
long, the age at sexual maturity is approximately 12 years according
to Breckenridge (_op. cit._:8, Fig. 4). The discrepancy in age of
females at the size of attainment of sexual maturity (Mitsukuri--six
years; Breckenridge--12 years) is, in part, rectified by the fact that
_Trionyx sinensis_ probably is a smaller species. Also, Breckenridge's
computation of the growth curve is based on continuously decreasing
increments of growth and seemingly eliminates consideration of the
probable marked decrease in rate of growth that occurs when sexual
maturity is attained--a phenomenon noted in other species of turtles.
I think that increments of growth of immature turtles are, on the
average, larger than those of sexually mature turtles. Judging from
these criteria, the age of a female of _spinifer_ at sexual maturity
is less than 12 years, and turtles having carapaces 17 to 18 inches in
length (maximal size for _spinifer_) would be older than 53 years
(_op. cit._:9). Occasional individuals, however, may greatly exceed
the usual growth rate in which event large adults may be younger than
50 years.
Females of _muticus_ are sexually mature when the plastron is 14.0 to
16.0 centimeters long, which corresponds to a carapace 19.6 to 22.4
centimeters (about 7-3/4 to 8-3/4 inches) long (average CL/PL
approximately 1.4, see Fig. 13). The smaller adult females probably
mature sexually in their sixth year, but most probably do so when
seven years old. Accordingly, some _T. spinifer emoryi_, which are
sexually mature at a plastral length of 16.0 centimeters, are also
sexually mature in their seventh year, whereas most _spinifer_
(sexually mature at a plastral length of 18.0 to 20.0 cm.,
corresponding to a length of carapace of 25.2 to 28.0 cm. or about 10
to 11 inches) probably become sexually mature in their ninth year, and
some when eight years old. Most males of _spinifer_ are sexually
mature when the plastron is 9.0 to 10.0 centimeters long (length of
carapace 12.6 to 14.0 cm. or 5 to 5-1/2 inches), whereas males of
_muticus_ and some _T. spinifer emoryi_ are sexually mature at a
plastral length of 8.0 to 9.0 centimeters (length of carapace 11.2 to
12.6 cm. or 4-1/2 to 5 inches). The smaller adult males are probably
sexually mature in their fourth growing season. Breckenridge (_op.
cit._:7, Tab. II) commented on the abundance of females between five
and 12 inches in length, and males that ranged in length from five to
seven inches. The abundance of turtles in these size ranges is
probably due, in part, to a slowing of the rate of growth indicating
the approach of sexual maturity; the abundance of the smallest males
is especially in accord with the size at sexual maturity of males
(about five inches).
The largest acceptable record of size of _spinifer_ is 18 inches in
length of carapace (Breckenridge, 1957:232). Stockwell (1878:402),
however, wrote that females of _spinifer_ attain "an extreme length of
from twenty-four to twenty-eight, and, in rare instances, thirty
inches, with an average length of carapace of fifteen to eighteen,"
and True (1893:152) mentioned lengths of two feet or even more.
Turtles 17 to 18 inches long are doubtless rare and probably about 60
years old. A specimen of _ferox_ lived the longest time in
captivity--25 years (Pope, 1949:304). Individuals of _ferox_ probably
exceed the maximum recorded length of carapace of 18-1/2 inches
(Agassiz, 1857:401). The head of a _ferox_ having a width of 3-1/2
inches (Wright and Funkhouser, 1915:120) corresponds to a length of
carapace of approximately 22-1/2 inches (PL/HW == 4.9; CL/PL == 1.3).
De Sola and Abrams (1933:12) wrote that _ferox_ in the Okefinokee
Swamp, Georgia, attains a length of two feet. The largest female of
_muticus_ of which I have record is 21.5 centimeters in plastral
length (KU 2308), a measurement corresponding to a carapace about 13
inches long.
Mortality
Man, in one sense or another, is a great enemy of soft-shelled
turtles. Those caught by fishermen are destroyed because of the
erroneous belief that they are harmful to fish populations. Some are
drowned in hoop-nets or gill nets used by commercial fishermen. Many
softshells are used by man for food. Herald (1949:118-19) reported the
results of spraying an area with DDT and mentioned a 10-inch
individual of _ferox_ that was eating a dead bluegill, and which
"probably died as a result of ingesting contaminated food."
Predation on eggs probably accounts for most mortality. Hamilton
(1947:209) reported tracks of spotted skunks, raccoons and foxes seen
about destroyed nests, and Cahn (1937:183) incriminated skunks and
raccoons. Goldsmith (1945:449) reported a raccoon that unearthed seven
nests in one night. Little and Keller (1937:221) wrote of egg shells
found in the sand (probably not as a result of hatching), and Muller
(1921:182) reported egg shells around dug-up nests, suggesting such
predators as "ground moles," raccoons and crows. Chesser (_in_ Harper,
1926:416) said that in the Okefinokee Swamp the jackdaw (fish crow),
raccoon, bear and domestic dogs will eat the eggs. Wright and
Funkhouser (1915:122) recorded a young _ferox_ in the stomach of a
water moccasin (_Agkistrodon piscivorus_), and suggested that young
soft-shells probably are food of larger snakes. Kellogg (1929:26)
wrote that stomachs of two alligators each contained one soft-shelled
turtle. Newman (1906:136) found that young captives were eaten by
individuals of _Chrysemys_ and _Sternothaerus_, and I found that they
were eaten by _Kinosternon_. Mitsukuri (1905:261-62) stated that
first- and second-year individuals of _T. sinensis_ are eaten by the
adults.
Breckenridge (1960) wrote that a clutch of eggs probably failed to
develop because of an "... unusually cool season." Evermann and Clark
(1920:595) stated that "many young appear to perish during the first
winter." They (_op. cit._:594) found two eggs submerged in two feet of
water and it is supposed that they never hatched. Dundee (1950:139)
reported remains of soft-shelled turtles left on the mud of a dried
swamp.
Parasites
Muller (1921:182) found maggots in a few eggs of a clutch, but thought
that only the infertile and decomposing eggs were infested. I removed
a hard, spherical cyst from the hind leg of a preserved softshell
(TU). A captive hatchling (TU 17304) died as the result of a
continuously enlarging and deepening hole on the top of its head; I
could not discern a visible parasite with the naked eye. I found 25
leeches (_Placobdella parasitica_, largest about 13 mm.; identified by
Dr. Kenneth B. Armitage, Department of Zoology, University of Kansas)
in association with 11 _T. m. muticus_ (number per turtle not known)
that were collected from the Kansas River at Lawrence, Douglas County,
Kansas. Evermann and Clark (1920:596) reported a few nematodes in the
stomachs of some _spinifer_, and three nematodes are listed by Harwood
(1932:46, 60, 62, 66) in the same species. Hughes, Higginbotham and
Clary (1941) have listed the known reptilian hosts of parasitic
trematodes, and Hughes, Baker and Dawson (1941) have done the same for
tapeworms. The species of parasites and their trionychid hosts are
listed below.
TREMATODA
_Trionyx ferox_: _Neopolystoma orbiculare_ _Vasotrema amydae_
_Neopolystoma rugosa_ _Vasotrema attenuatum_
_Polystomoides coronatus_ _Vasotrema robustum_
_Teloporia aspidonectes_
_Trionyx muticus_: _Crepidostomum cooperi_ _Opisthorchis ovalis_
_Trionyx spinifer_: _Hapalorhynchus evaginatus_ _Vasotrema amydae_
_Opisthorchis ovalis_ _Vasotrema attenuatum_
_Polystomoides coronatus_ _Vasotrema longitestis_
_Teloporia aspidonectes_ _Vasotrema robustum_
CESTODA
_Trionyx ferox_: _Proteocephalus trionychinus_
_Trionyx spinifer_: _Proteocephalus testudo_
NEMATODA
_Trionyx spinifer_: _Camallanus trispinosus_ _Spiroxys amydae_
_Falcaustra chelydrae_
Economic Importance
Several authors have mentioned softshells as a food item much sought
after by man. The commercial value of these turtles has been
summarized by Clark and Southall (1920:15-16). Softshells are consumed
in quantity only in small towns near the place of capture. They are
found only occasionally in the markets of large cities because the
turtles are little known and the palatability of their flesh is
unappreciated. Also, they do not stand shipment so well as other
turtles, and they are "not so meaty as the snapper; so there is more
waste" (Clark and Southall, _loc. cit._). Little and Keller (1937:221)
reported living individuals for sale at the market in Ciudad Juarez,
Chihuahua; however my inquiry at markets in Juarez in the summer of
1959 disclosed no evidence of recent sale of soft-shelled turtles. In
the southeastern United States the demand is perhaps greater than in
other regions. I have noted softshells in the market at New Orleans,
and Oliver (1955:19) has mentioned the sale of "some 146,600 pounds"
in one recent year in Florida. Over most of their range, however,
there probably is no general demand for softshells and no special
efforts are made to capture them. Softshells have been raised
successfully on "turtle farms" in Japan (Mitsukuri, 1905). True
(1893:152) wrote that "The eggs also are considered very excellent."
Softshells generally are condemned by fishermen because of the
mistaken belief that they are detrimental to fish populations. Food of
softshells is principally crawfish and insects. Fish comprise a small
proportion of the diet (frequency 1.9% in Michigan, Lagler, 1943: Tab.
9). Most of the fishes eaten seem to be small minnows. Probably fish
would comprise a larger percentage of the diet if they could be
caught. I doubt that a softshell can pursue and capture a healthy fish
in natural waters. Recently dead fish are eaten and perhaps fish eggs,
and senile and decrepit fishes. There is no evidence that soft-shelled
turtles are active predators on any kind of fish. Of course in
congested areas such as ponds of fish hatcheries, it is desirable to
eliminate the turtles. The known food habits of soft-shelled turtles
suggest that they compete with game fishes for food, but there is no
information on the intensity of competition (Lagler, _op. cit._:305).
The combined statements of many authors in their general accounts of
food habits (for instance, Babcock, 1919:425) have tended to create
the erroneous belief that soft-shelled turtles harm waterfowl. To my
knowledge the only basis for this belief is the statement of Wright
and Funkhouser (1915:123) that according to the natives of the
Okefinokee Swamp, the larger turtles "devour also such waterfowl as
are unfortunate enough to be taken unaware by these reptiles." Perhaps
an occasional waterfowl is eaten, but the present information on kinds
of food eaten certainly does not warrant the destruction of
soft-shelled turtles. There may be some mortality in congested areas
such as game refuges where young birds crowd the surface of the water.
The kind of bait successfully used in trapping softshell turtles
suggests that they are of some value as scavengers.
EVOLUTIONARY HISTORY
Before attempting to reconstruct the history of soft-shelled turtles
in North America, it will be helpful to summarize the salient facts
concerning the distribution and relationships of the living forms, and
to comment on fossils.
Distribution
The geographic range of the family Trionychidae in North America is
principally in the eastern two-thirds of the continent and contributes
to the well-known floral and faunal resemblance of eastern North
America to that of eastern Asia (Schmidt, 1946:149) because _Trionyx
ferox_ (see Fig. 18) resembles the species of the genus in Asia more
closely than it does any North American species. The Recent
distribution in America does not include the Neotropical region,
whereas the geographical range in the Old World extends south of the
equator (Fig. 1; Dunn, 1931:109, fig. 2; Gadow, 1909:333, fig. 72;
Hay, 1908:35, fig. 16).
American softshells occur in all river systems in the United States
and the two adjacent river systems on the east coast of México that
drain into the Gulf of México. Softshells inhabit streams of the Great
Plains and occur westward to the foothills of the Rocky Mountains in
the western tributaries of the Mississippi River. Only _T. s.
spinifer_ occurs in the southern part of the Great Lakes-St. Lawrence
drainage. Softshells are absent from the Atlantic Coast drainage
except the Hudson River and those rivers at least south of (and
including) the Pee Dee River in South Carolina.
_T. s. emoryi_ is not known to be indigenous west of the Río Grande
drainage, and has probably been introduced across the Continental
Divide via the Gila River in western New Mexico into the Colorado
River drainage of Arizona (Miller, 1946:46); the species undoubtedly
occurs in México on the Sonoran side of the Colorado River opposite
Baja California (Bogert and Oliver, 1945:417).
In the summer of 1959, I trapped turtles and with a specimen in hand
inquired about softshells occurring in the inland drainages of
northern México. From two collecting stations on the Río Nazas in
Durango, only specimens of _Pseudemys_ and _Kinosternon_ were
obtained; local inhabitants had neither seen nor heard of softshells.
Flooded conditions in August of 1959 permitted trapping in only one of
the inland drainages of northwestern Chihuahua, the Río Santa María;
only specimens of _Kinosternon_ were obtained. Local residents near
that river as well as those living near the Río Casa Grandes and Río
del Carmen had not seen or heard of softshells. A person that I judge
to be a competent observer reported seeing a softshell in June of 1958
in the Río Alamos (Arroyo Cuchujáqui) near Alamos, Sonora, in the Río
del Fuerte drainage on the west coast of México. I was a member of a
field party from the University of Kansas that visited that locality
in late January of 1959; only specimens of _Pseudemys_ and
_Kinosternon_ were collected. Possibly isolated populations occur in
streams of the Pacific Coast drainage of northern México. If so, they
may have entered Pacific Coast drainages by stream capture across the
Continental Divide. Several species of fish that are characteristic of
the Río Grande traversed the Sierra Madre Occidental at some former
time (presumably via the Río Conchos and Río Papigochic) and occur in
the Yáqui River drainage (Meek, 1904:xxxviii, xlvii; Miller,
1959:214-15, 217). Because of the probability that the Río Nazas at
some former time flowed north into the Río Grande (Meek, _op.
cit._:xxxiv), it is notable that softshells are absent in the Río
Nazas drainage; the Big Bend turtle, _Pseudemys scripta gaigeae_,
occurs in both drainages.
Relationships
Characters of _Trionyx ferox_ suggesting a closer resemblance to some
Old World members of the family than to the other three American species
are: large size; marked difference between juvenal and adult patterns on
the carapace; the marginal ridge; and the longitudinal ridgelike
prominences on the carapace, especially in juveniles. Other characters
of _ferox_ suggesting a corresponding, but less marked resemblance to
Old World species of _Trionyx_ are: the large size of the eighth pair of
pleurals; the absence of callosities on the epiplastron and preplastra;
frequent fusion of the hyoplastra and hypoplastra (more than in
_spinifer_ or _muticus_); and tolerance of marine waters (more than
_muticus_ or _spinifer_). Some fossils also suggest alliance with
_ferox_ and some Old World members of the genus in their large size,
large eighth pair of pleurals, and occurrence in marine deposits;
several Old World species have been reported at sea (_Pelochelys_, _T.
triunguis_, _T. sinensis_). _T. ferox_ is monotypic and has the most
southeasterly displaced, geographic range.
Because _ferox_ resembles softshells from the Old World more closely
than it does any American species, _ferox_ is assumed to be more closely
related to Old World softshells than to any American species, and,
because of resemblance to some fossils, _ferox_ is assumed to resemble
most closely the primitive, ancestral stock of softshells that occupied
North America. _T. spinifer_, _T. muticus_ and _T. ater_, which resemble
each other more closely than any of them resembles _T. ferox_ or any Old
World species, are considered autochthonous in North America. _T.
spinifer_ and _T. muticus_ are distinct, sympatric species. Burt
(1935:321) suggested that the two species "may be variants of the same
species." _T. ater_ is weakly differentiated from _T. spinifer emoryi_.
The species, _ferox_, _spinifer_ and _muticus_ are well-differentiated
and were considered by Agassiz (1857), Gray (1869) and Baur (1893) as
belonging to three different genera.
In the widely distributed _T. spinifer_, the subspecies _spinifer_,
_hartwegi_ and _asper_ closely resemble one another; _asper_ seems most
distinct, whereas _spinifer_ and _hartwegi_ are terminal populations of
an east-west cline in one character. The subspecies _pallidus_,
_guadalupensis_ and _emoryi_ resemble one another more closely than any
resembles any of the subspecies mentioned immediately above; _T. s.
pallidus_, however, is annectent. _T. s. pallidus_ and _guadalupensis_
represent terminal populations of clines in several characters, some of
which occur in _emoryi_, but that subspecies is more distinct from
_pallidus_ and _guadalupensis_ than those subspecies are from each
other. _T. s. emoryi_ is the most variable subspecies. _T. ater_, known
only from a restricted area in central Coahuila, is most closely related
to _T. s. emoryi_, and possesses some characters judged to represent the
attenuation of the geographic cline in _pallidus_, _guadalupensis_ and
_emoryi_ mentioned above. Some characters of _ater_ show alliance to the
species _muticus_. Of _T. muticus_, whose geographic range is removed
from that of _ater_, there are two subspecies. Four subspecies of
_spinifer_ (_spinifer_, _hartwegi_, _pallidus_ and _asper_) intergrade
in the Mississippi River drainage of Louisiana; few specimens, however,
are typical of _asper_. The subspecies of _muticus_ do not show definite
evidence of intergradation. To facilitate quick reference, the
occurrence of some characters that are shared by, or are approximated
in, two or more forms are listed in Table 10. In addition to external
characters, some ratios emphasize the clinal relationship between _T. s.
pallidus_, _guadalupensis_, and _emoryi_ mentioned above. Of especial
interest is the frequent resemblance of those subspecies and _T. ater_
to _T. ferox_ (dorsal pattern on limbs of adults, reduction in anterior
tuberculation, wide head, narrow carapace, and short snout), and the
less marked resemblance of _T. muticus_ to _T. ferox_; not shown in
Table 10 is the resemblance of _ferox_ to _T. muticus calvatus_ in
having thick, black-bordered postocular stripes. Some populations of _T.
s. emoryi_ resemble _T. muticus_ in the corresponding size at sexual
maturity and in having well-developed plastral callosities. It is
notable that the occurrence of _ater_, and to a lesser extent that of
_T. s. emoryi_, which resembles _ferox_ (and _muticus_), is in the
southwestern United States and northern México.
TABLE 10. Frequency of Selected Characters Among Species and
Subspecies of Trionyx in North America. Characters of muticus
Refer to the Typical Subspecies; Horizontal Dashes Connecting
X's Indicate that Computations for Those Subspecies Were
Combined; Vertical Dashes Indicate that the Subspecies Is
Intermediate Between the Adjacent Subspecies.
Column headings:
A: _ferox_
B: _spinifer_
C: _hartwegi_
D: _asper_
E: _pallidus_
F: _guadalupensis_
G: _emoryi_
H: _ater_
I: _muticus_
=====================================+===================================
| Species and subspecies
Characters +---+---+---+---+---+---+---+---+---
| A | B | C | D | E | F | G | H | I
-------------------------------------+---+---+---+---+---+---+---+---+---
Juvenal pattern: | | | | | | | | |
black spots, ocelli | | X | X | X | | | | |
| | | | | | | | |
white dots | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on snout: | | | | | | | | |
acute angle (reduced in _muticus_) | X | X | X | X | X | | | | X
| | | | | ¦ | | | |
triangular | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on side of head: | | | | | | | | |
contrasting marks | X | X | X | X | X | X | | |
| | | | | | ¦ | | |
non-contrasting marks (distinct | | | | | | ¦ | | |
stripe in _muticus_) | | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Pattern on limbs of adults: | | | | | | | | |
contrasting | | X | X | X | X | X | | |
| | | | | | ¦ | | |
reduced or absent | X | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Tuberculation (anterior edge of | | | | | | | | |
carapace): | | | | | | | | |
conical, equilateral | | X | X | X | X | | | |
| | | | | | | | |
reduced or absent | X | | | | | X | X | X | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Head (PL/HW, fig. 3): | | | | | | | | |
wide | X | | | X | | X | X | X |
| | | | | | | | |
narrow | | X | X | | X | | | | X
-------------------------------------+---+---+---+---+---+---+---+---+---
Carapace (CL/CW, fig. 4): | | | | | | | | |
wide | | X | X | X | X | X | | | X
| | | | | | ¦ | | |
narrow | X | | | | | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Level of Carapace Width (CL/PCW, | | | | | | | | |
fig. 5): | | | | | | | | |
middle of carapace | X | X | X | X | | | | | X
| | | | | | | | |
farther posteriorly | | | | | X | X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Snout (HW/SL, fig. 6): | | | | | | | | |
long | | X---X | X | X---X | | | X
| | | | | ¦ ¦ | | |
short | X | | | | X---X | X | X |
-------------------------------------+---+---+---+---+---+---+---+---+---
Fossils
The known occurrence of fossil trionychids throughout the world
indicates a former distribution more widespread than the family has
today; the principal difference in the former and present distributions
is the lack of living softshells in Europe.
I have not studied in detail the many fossil remains but such
examination as I have made of them suggests that many of the characters
used as a basis for distinguishing fossil forms in North America are
subject to individual variation or are of no diagnostic value in the
living species (Hummel, 1929:769). Knowledge of the variation in the
living species of the Old World would aid in adequately appraising the
North American fossils. Some osteological characters of the three living
American species (excluding _ater_) together with data on variation
within a given species are mentioned below. Some differences in skulls
of the three species already were mentioned in the section "Osteological
Characters." Because most fossil remains are those of the carapace and
plastron, attention is here given to those structures.