of several small islands having "gently sloping sand and mud shores,
and interior areas of open sand and densely growing willows" in the
Mississippi River, near Fairport, Iowa. The same author wrote that the
egg-laying season is from late June to early July, and that the female
selected a place 10 to 60 feet inland "with an unobstructed view of
the open water." Farther north on the Mississippi River near Dubuque,
Iowa, Goldsmith (1945:447) found that _muticus_ preferred "clean,
somewhat level sandbars and sandy shores free from trash, weeds ...
and exposed to open view." The same species, however, may "make
unsatisfactory nests ... in any place they can find sand, even in the
weeds and bushes ... when the river is high, covering the sandy plots
..." Sometimes nests, which were "seldom nearer than six feet or more
than twenty-five feet from water ...," were submerged by a rise in
water level. In Missouri, Hurter (1911:251) found that individuals of
_muticus_ came "... out on the sandbars in the Mississippi River to
deposit their eggs ... At the end of May up to the middle of June ..."
Cahn (1937:182) wrote that the nesting season of _muticus_ is early
July near Meredosia, Illinois. Anderson (1958:212, Fig. 1) found nests
of _muticus_ along the Pearl River in Louisiana on an open sandbar
(not in gravel, fine sand or silt), whereas nests of _Graptemys_ were
confined to the landward margin of the sandbar.
The onset and length of the breeding season seems to be geared to the
climatic conditions under which the species occurs, and, as would be
expected, begins earlier and lasts longer in southern latitudes than
in northern latitudes. The period of deposition of eggs in _T. ferox_
(Florida) is from late March to mid-July, whereas that of northern
populations of _spinifer_ and _muticus_ (southern Great Lakes region)
is usually from mid-June to mid-July.
Seemingly there is little difference between species in preference of
nesting sites; a sandy substrate is probably preferred. Gravid females
of _ferox_ and _spinifer_ may wander overland some distance and select
places where the view of the water is obstructed by vegetation; both
species may wander little and nest in full view of the water.
Concerning _muticus_, it is of interest that of the many nests
discovered by Anderson (_loc. cit._) on an open sandbar, all were
those of _muticus_ and none was a nest of _spinifer_. The nests of
_muticus_ mentioned by Muller (_loc. cit._) and Goldsmith (_loc.
cit._) were on open sandbars. On June 4, 1953, six clutches of eggs
were found on an open sandbar of the Escambia River, Florida; all
hatchlings from those eggs that were successfully incubated were
_muticus_. On June 1, 1954, three nests were found on an open sandbar
of the same river (Pl. 50); the temperature within the nests at 6:30
a. m. was approximately 25° C. Two nests were dug in a sand substrate
on the level portion of the bar (Pl. 51, Fig. 1). The third clutch of
eggs was deposited in a sand-gravel substrate at the brim of the
incline from the shore (approximately 30 degrees and about five feet
above the water); the eggs of this clutch were arranged rather
symmetrically (Pl. 51, Fig. 2). Unfortunately, most of the eggs from
these three clutches failed to hatch. Although the data are far from
conclusive, I have the impression that _muticus_ limits its sites of
egg deposition to the open regions of sand bars and does not lay
inland where it must traverse vegetated areas unless preferred nesting
sites are submerged or otherwise unsatisfactory. Females of _spinifer_
may utilize open sandbars for deposition of eggs but not areas where
_muticus_ occurs. In areas where both _muticus_ and _spinifer_ occur,
the latter probably lays farther inland or on the landward margins of
sandbars.
Excavation of nests has been observed in _ferox_ (Hamilton, 1947:209),
_muticus_ (Muller, 1921:181-82; Goldsmith, 1945:448), and _spinifer_
(Newman, 1906:132-33; Cahn, 1937:191-92; Breckenridge, 1960:284).
Turtles leaving the water are cautious, usually stopping and extending
the neck to its greatest length, holding the head high, and sometimes
returning to the water for a short time. Depending on the condition of
the substrate and wariness of the female, nest construction may begin
immediately, or several holes may be dug and then abandoned. The
excavation on level ground or a slight incline is made by means of the
hind feet (Muller mentions digging with the forefeet; I agree with
Pope, 1949:321, and Conant, 1951:264, who consider Muller in error);
the forefeet are firmly planted and not moved during the excavation,
deposition of eggs or the filling of the nest cavity. The hind feet
are used alternately; cloacal water may be used to facilitate digging
or to provide a suitable substrate for the eggs. Cahn mentioned that
some sand may be flung four or five feet, and that during the digging
the head is held high. Breckenridge (_loc. cit._) reported that sand
was thrown a distance of ten feet. The nest may be completed in 16
minutes (Cahn, _loc. cit._) or less than 40 minutes (Newman, _loc.
cit._). Breckenridge recorded 17 eggs laid in six minutes, Cahn
recorded 12 eggs laid in eight minutes, and Hamilton recorded four
eggs laid in three minutes. The hind feet are used to arrange the eggs
and are used alternately to fill the nest cavity; sometimes a little
sand is scraped in before all the eggs are deposited. Muller recorded
the nest cavity as five inches in diameter and ten inches deep, the
finished nest appearing "as a small crater ... about a foot in
diameter, or where the surface is covered with pebbles, as a circular
area of clear sand." Goldsmith reported that the nest cavity was six
to nine inches in depth, and that after deposition and filling with
sand "By certain twisting movements with all four legs, she drags the
plastron around over the sand, making a perfect camouflage." Newman
found the nest flask-shaped having a depth of about six inches, and
diameters of about three inches at the bottom and one and one-half
inches in the neck. Hamilton described a flask-shaped nest, the
entrance of which would "barely permit the passage of an egg ... the
bottom, at a depth of five inches, being about the width of a quart
milk bottle." Cahn related that the "hole descended at an angle of
about 60°," and the eggs thus rolled down an inclined plane.
Possibly the nests of _ferox_ and _spinifer_ differ from those of
_muticus_ in being flask-shaped. A nest of _spinifer_ was reported by
Gehlbach and Collette (_loc. cit._) as having a neck three inches
across, a depth of six inches and a width of five inches at the
bottom. The nests of _muticus_ that I discovered on the Escambia River
were not flask-shaped; the eggs were five to seven inches below the
surface. Evermann and Clark (1920:594) reported eggs of _spinifer_
"generally at a depth of four to ten inches," and Breckenridge (_loc.
cit._) found the topmost eggs about five inches below the surface.
There may be behavioral differences between _ferox_ and _spinifer_ and
_muticus_. Hamilton (_loc. cit._) mentioned that _ferox_ proceeded
with its reproductive duties even when he stood only a few yards away.
Muller (_op. cit._:181) found that _muticus_ would run to the water if
disturbed, without completing deposition of eggs; the same behavior
was described by Cahn (_op. cit._:191) for _spinifer_. Newman
(1906:133) wrote that _spinifer_ will abandon nesting activities if
surprised before egg deposition begins, but will wait to cover the
eggs if interrupted while laying eggs. Goldsmith (1945:448) found that
an observer did not disturb females of _muticus_ when they were laying
eggs (females "could be approached and even touched"), but that, in
the presence of an observer, they would scurry toward the water
without covering the eggs and would not return to cover them. Turtles
frightened in the process of the construction of the nests would not
return to complete the original nest. Harper (1926:415) wrote that
_ferox_, after completing nesting activities, will crawl a few feet
from the nest and scuffle up the surface, presumably to decoy
predators that might otherwise destroy the eggs; this observation has
not been corroborated by other authors. Harper (_op. cit._:416)
recorded the observation of Allen Chesser, who says that females,
after egg deposition, often "... bury themselves, before they go ter
the water, an' stay there ten er twelve hours."
_Reproductive Potential_
Estimates of reproductive potentials are subject to variation of one
kind or another. Counts of oviducal eggs or those in nests may be
misleading, as in some individuals one or more eggs may have been
deposited previously. Mitsukuri (1905:263), Newman (1906:135), Muller
(1921:182), and Cahn (1937:183) have mentioned that the number of eggs
per clutch corresponds to the size of the female. Females of northern
populations may have larger clutches than females of the same size
from southern populations.
TABLE 8. Records in the Literature Pertaining to Number and Size
of Eggs of Three American Species of Trionyx.
===========+=====================+===================+=======================
| Number of eggs per | |
| clutch; oviducal | Size of eggs; | Authority
SPECIES | (o), nest (n); | ave. = average | and remarks
| ave. = average | |
-----------+---------------------+-------------------+-----------------------
_ferox_ | | 24 mm. | Agassiz (1857, pl. 7,
| | | fig. 20); nat. size.
| | |
| 22 (n) | ave. 31 mm. | Wright and Funkhouser
| | | (1915:120)
| | |
| some (o) | 32 mm. | "
| | |
| 20 (o) | ave. 25 mm., and | Goff and Goff
| | 12 gms. | (1935:156)
| | |
| 17 (o) | ave. 27 mm. | Hamilton (1947:209)
| | |
| 21 (o) | | "
| | |
| 7 (o) | | " (egg
| | | deposition probably
| | | interrupted)
-----------+---------------------+-------------------+-----------------------
_spinifer_ | | 29 mm., 26.5 mm. | Agassiz (1857, pl. 7,
| | | figs. 20 and 23,
| | | respectively); nat.
| | | size.
| | |
| 9, 12, 17, 18, 27 | | Eigenmann (1896:263);
| and 32 | | northern Indiana
| | |
| 9 to 24, ave. 18 | | Newman (1906:135);
| | | northern Indiana
| | |
| about 30 (n), 4 | 1.09 × 1 inch | Evermann and Clark
| (n), 3 (n) | | (1920:593-94);
| | | northern Indiana
| | |
| 21 (n and o) | (o) and some (n) | "
| | .93 × .93 inches; |
| | rest of (n) 1.07 |
| | × 1.07 inches |
| | |
| 32 (o) | ave. 1-1/4 inches | Force (1930:38);
| | | Oklahoma
| | |
| 9, 12, 13, 15, 17, | ave. 28.3 mm. | Cahn (1937:193);
| 19, 19, 21, 22, 23, | (217 eggs) | Illinois
| and 25; ave. 18 | |
| | |
| 12 (o), 26 (o), 24 | 22.0 to 28.5 mm. | Breckenridge
| (n), and 30 (n) | | (1944:187); Minnesota
| | |
| 21 (o) | 24 to 27.8 (ave. | Conant (1951:160);
| | 25.6 mm.) × | Michigan
| | 25.8 to 29 (ave. |
| | 27 mm.) |
| | |
| 22 (n), 22 or | | Hedrick and Holmes
| 23 (n) | | (1956:126); Minnesota
| | |
| 25 (n) | ave. 24 × 25.2 | Gehlbach and Collette
| | mm. | (1959:142); Nebraska
| | |
| 17 (n) | | Breckenridge
| | | (1960:284); Minnesota
-----------+---------------------+-------------------+-----------------------
_muticus_ | | about 22 mm. | Agassiz (1857, pl. 7,
| | | fig. 21); nat. size.
| | |
| 21 | about 20 mm. | Hurter (1911:249);
| | | Missouri
| | |
| 4, 12, 13, 16, 21, | ave. 2.3 cm. and | Muller (1921:182);
| 22, 26, and 33, all | 7 gms. | Iowa
| (n); ave. 22 | |
| | |
| 18 to 22, maximum | ave. 22.6 mm. | Cahn (1937:183);
| 31 | (116 eggs) | Illinois
| | |
| 93 from 5 nests, | variable--largest | Goldsmith (1945:449);
| ave. 18.6; 10, 10, | _ca._ 1-3/8 | Iowa
| 16, 17, 17, 19, 21, | inches, smallest |
| 21, 22, 22, 31, all | less than one |
| (n), ave. 18.7 | inch. |
-----------+---------------------+-------------------+-----------------------
Additional records of size of clutch are provided by data from dissected
females (Table 9). All females were collected from May through September
from localities south of latitude 36.5°. The number of eggs includes
those in both oviducts, and the number of ovarian follicles those in
both ovaries. The number and range in size of only the largest group of
follicles is listed; in some instances the size of follicles formed a
graded series, and the designation of a group was arbitrary.
TABLE 9. Length, Number of Oviducal Eggs, and Condition of Ovaries
in Adult Females of T. spinifer and T. muticus.
===========+========================+=========+======================
| | | Ovarian follicles
| | | (total)
SPECIES | Size of female | Eggs +---------+------------
| (plastral length, cm.) | (total) | |
| | | Number | Size (mm.)
-----------+------------------------+---------+---------+------------
_muticus_ | 14.4 | 6 | 14 | 15-18
| 16.3 | 9 | 4 | 15-17
| 16.5 | | 3 | 16
| 16.5 | 3 | 4 | 14-18
| 17.2 | | 13 | 14-21
| 27.0 | | 25 | 18-21
-----------+------------------------+---------+---------+------------
_spinifer_ | 16.2 | 7 | 4 | 16-20
| 16.2 | 7 | 5 | 18-20
| 16.2 | 7 | 1 | 18
| 16.3 | 6 | 5 | 16-18
| 16.3 | 4 | 5 | 15-19
| 16.8 | 6 | 1 | 18
| 17.3 | 3 | 2 | 17
| 18.3 | | 13 | 19-20
| 19.5 | | 2 | 17
| 19.8 | | 4 | 20
| 20.7 | | 11 | 15-18
| 21.5 | | 6 | 8-11
| 22.0 | | 13 | 11-14
| 23.5 | 8 | 12 | 20-24
| 25.5 | 11 | several | 18-22
| 25.8 | 13 | ? | 18-21
| 26.8 | 10 | 5 | 18-20
| 30.5 | 13 | 5 | 20-21
| | 16 | 16 | 16-21
| | 11 | 19 | 15-20
| | 17 | 23 | 18-22
| | 17 | 22 | 14-20
| | 8 | 15 | 18-22
-----------+------------------------+---------+---------+------------
Published data (Table 8) indicate that the average number of eggs per
clutch for the three American species is about 20, although the number
of eggs may exceed 30 in _spinifer_ and _muticus_. Except for those of
_ferox_, most of these records are based on observations in northern
latitudes (approximately 40°). My examination of females from southern
latitudes (below 36.5°) reveals no oviducal egg count greater than 17
and an average number of eggs per clutch of 9.6 per _spinifer_ (Table
9); that of _muticus_ is 7.3, as based on data given in Table 9 as
well as on egg-nest counts of 15, 6, 6, 6, 6, 5, 9, 8, and 8. Ovarian
follicles larger than 15 millimeters in diameter are arbitrarily
considered to comprise the next clutch that will be deposited in the
current season. Follicles of this size possibly are retained until the
following year or some may undergo regression; some of the included
follicles may not be representative of the succeeding egg complement.
The average number of follicles of the most enlarged groups is 9.0 for
_spinifer_ and 10.5 for _muticus_. Females in northern latitudes
probably have a greater reproductive potential than those in southern
latitudes if it is assumed that there is only one laying per season
for an individual; the maximum number of eggs laid at any one time
probably does not exceed 35. There is also an indication that larger
females deposit more eggs than smaller females (Table 9). Muller
(1921:184) mentioned two double eggs (each having two yolks) in the
complement of 33, indicating an abnormally large number and excessive
crowding of eggs in the oviducts. Simkins (1925:188) also mentioned
some eggs of a clutch (form and locality unknown) that were five or
six millimeters larger (about 31-32 mm.) than the rest, and which
"invariably bore twins." The largest number of eggs in a single nest
mentioned by Simkins is 22. If the presence of double-yolked eggs is
indicative of crowding of eggs in the oviducts, the egg complements of
22 and 33 indicate the approximate maximal number of eggs per clutch.
In the species _spinifer_, the average size of sexually mature females
is slightly smaller at some places in the south than in the north.
Therefore, smaller clutches are to be expected in the south.
Many of the females collected in June or July contained corpora lutea
four to eight millimeters in diameter in addition to enlarged ovarian
follicles. Presumably the corpora lutea indicate clutches deposited
earlier in the current season, and the enlarged follicles represent
clutches to be deposited in the current season. One female of
_muticus_ (OU 27593) obtained on July 10, contains oviducal eggs,
ovarian follicles 15-17 millimeters in diameter, and corpora lutea of
different sizes that exceed the number of oviducal eggs; possibly this
female was capable of laying three clutches each season. Corpora
lutea, representing ovulation points of eggs in the oviducts, are
approximately eight millimeters in diameter. In order to establish
definitely the reproductive potentials of any species of turtle, it is
desirable to know the approximate size of ovarian follicles that are
retained by sexually inactive females, and the rate of regression of
the corpora lutea. The data suggest that, in southern populations at
least, two and possibly three clutches of eggs are deposited in the
annual breeding season. Mitsukuri (_in_ Cagle, 1950:38) found that _T.
sinensis_ deposited four groups of eggs each season.
It is suggested that the seasonal reproductive potential of northern
populations (averaging about 20 eggs per clutch, and probably one
clutch per season) is less than that for southern populations
(averaging about 10 eggs per clutch, but three clutches per season).
But owing to variation, there may be no great discrepancy between the
actual potentials of northern and southern populations.
_Eggs_
The eggs of _Trionyx_ are white and spherical having a brittle shell.
Some eggs are occasionally abnormal in shape and size; overcrowding of
eggs in the oviducts may result in small, irregular-shaped eggs, or
large double-yolked eggs. Presumably enlargement of the eggs occurs in
the oviducts and within a short period after deposition prior to
complete hardening of the brittle shell; therefore some eggs in the
oviducts are smaller than those in nests.
The data concerning _ferox_ (Table 8) suggest that the maximum size of
eggs is 31 to 32 millimeters, whereas oviducal eggs are slightly
smaller, about 25 to 27 millimeters. Eggs of _spinifer_ from northern
latitudes (most from approximately 40°, Table 8) also vary in size,
oviducal eggs being as small as 22 millimeters in diameter and the
maximal size about 29 millimeters. Average extreme measurements (in
mm.) of oviducal eggs (number of eggs in parentheses) from females
taken in latitudes of 33 degrees or less are: 25 × 29 (11), 29 × 30
(11), 28 × 30 (13), 28 × 30 (10), 29 × 30 (5), 29 × 29 (8), 25 × 26
(17), 29 × 30 (5), and 28 × 29 (8). The average size of these eggs is
slightly larger than the oviducal eggs of which measurements are given
in Table 8, and suggest larger eggs from more southern latitudes. Eggs
of _muticus_ are smaller than those of _spinifer_ (Cahn, 1937:183) or
_ferox_; the average size of eggs from nests found in Iowa and
Illinois is 22 to 23 millimeters (Table 8). Nine oviducal eggs from a
female obtained in Lake Texoma, Oklahoma, averaged 22 × 23
millimeters. The largest eggs of _muticus_ are from the southernmost
locality; eight eggs from a nest found along the Escambia River,
Florida, averaged 26 × 27 millimeters.
In general, the data suggest that at each laying slightly smaller eggs
but larger numbers are laid by females in northern latitudes, whereas
larger but fewer eggs are laid by females from farther south.
_Incubation and Hatching_
Length of the incubation period seems to depend upon conditions of
heat and moisture, and, in general, to be geared to the prevailing
climatic conditions. Goff and Goff (1935:156) artificially incubated
some eggs of _ferox_ at temperatures varying from 82.3 to 89.2° F.,
and found that the incubation period was 64 days. Muller (1921:184)
wrote that the period of incubation of eggs of _muticus_ (natural
nests at temperatures about 90°., _op. cit._:182, and artificial
nests) in Iowa is from 70 to 75 days. Breckenridge (1944:187) stated
that _spinifer_ makes nests in Minnesota from June 14 to July 6, and
cited reports that indicate hatching in September. Hedrick and Holmes
(1956:126) discovered a nest of eggs in Minnesota on September 5; the
eggs were artificially incubated and some hatched on October 29.
Eigenmann (1896:263) found eggs as late as September in northern
Indiana that "contained young which would have been ready to hatch
about a month later." Cahn (1937:193) wrote that _spinifer_ in
Illinois lays in June or early July and that "young-of-the-year are
taken in late August and September." Some recently deposited eggs of
_muticus_ (as indicated by fresh turtle tracks, Pl. 50, Fig. 2) that I
obtained on June 1 were artificially incubated and hatched on August
4, indicating an approximate incubation period of 65 days. Dr. Paul K.
Anderson in the course of field work on the Pearl River, Louisiana
(1958:211), found that eggs collected on June 13 from a nest excavated
three to five days before, hatched on August 15, indicating an
incubation period of approximately 67 days. Eggs collected on May 17
to 25 (three clutches) hatched on August 4 to 6, indicating an
incubation period of approximately 77 days. In any latitude the
incubation period probably is at least 60 days. Eigenmann (_loc.
cit._), however, mentioned empty nests that were found in July; this
indicates early hatching or more probably the action of predators.
In northern latitudes eggs or young turtles may over-winter in the
nest if deposition is late in the season. In northern Indiana Evermann
and Clark (1920:595) found a nest on November 16 that contained
"well-formed young" and believed that the turtles would have wintered
in the nest. Conant (1951:160) was of the opinion that most eggs
probably hatch in early fall, but that some do not hatch until spring.
The hatching of eggs of _muticus_ has been described by Muller
(1921:183). According to him, the forelimbs first emerge through the
shell and enlarge the opening. There is an "egg tooth below the
flexible proboscis" but "it does not seem to be used in escape from
the eggs, and is dropped a week after hatching." Hatchlings burrow
almost straight upward through the sand leaving the egg shell below
the surface and a hole in the sand about an inch in diameter. Muller
found that young turtles emerge from the nests in the night or early
morning and always go downhill probably influenced in their movements
by the open sky and sloping beach. Anderson (1958:212-15) found that
hatchlings of _muticus_ leave nests within the first three hours after
sunset and travel a direct route to the water. He discovered that
hatchlings are active on the surface of the sand at night and
generally show a positive reaction to light (moonlight, flashlight),
whereas, in daytime, there is a negative reaction to bright sunlight
(causing the turtles to bury themselves in sand). Anderson believed
that the positive response to light at night is not correlated with
the water-approach behavior of hatchlings, but that movements to water
are possibly influenced by a negative reaction to dark masses of
environment (such as shadows formed by landward forests).
_Age and Growth_
Goff and Goff (1935:156) found that hatchlings of _ferox_ average 8.82
grams (extremes, 8.50 to 9.25); one of these, UMMZ 76755, is
illustrated in Plate 31. Muller (1921:184) recorded measurements of
five hatchlings of _muticus_; the average measurements (in cm.,
extremes in parentheses) were: length of carapace, 3.54 (3.43 to
3.67); width of carapace, 3.20 (3.10 to 3.25); length of plastron,
2.54 (2.47 to 2.60). I recorded measurements of 32 hatchlings (three
clutches combined) of _muticus_ on August 16; the turtles hatched on
August 4 to 6 from eggs collected along the Pearl River, Louisiana.
The average measurements (in mm., extremes in parentheses) of the 32
turtles were: length of carapace, 41.3 (34.0 to 45.0); width of
carapace, 38.6 (31.0 to 40.0); length of plastron, 30.1 (25.0 to
32.0). These turtles have circular umbilical scars averaging
approximately two millimeters in diameter. The smallest hatchling that
I have seen measures 21.0 millimeters in plastral length (_T. m.
muticus_, INHS 3458). There are no data to indicate a difference in
size of hatchlings among the American species of soft-shelled turtles.
The average plastral length of most hatchlings probably is 28.0 to
30.0 millimeters.
Owing to the lack of a horny epidermal covering of the carapace and
plastron, soft-shelled turtles are not so well suited to studies of
age and growth as are the "hard-shelled" species, which have visible
impressions of growth annuli on the epidermal scutes. Mattox
(1936:255) found annular rings in the long bones of specimens of
_Chrysemys_ and suggested that it is tenable to correlate the number
of rings with the age of the turtle.