North American Recent Soft-shelled Turtles (Family Trionychidae) Part 2

The Extremes Precede the Mean (in parentheses).

=============+============================ SPECIES | Plastral length (cm.) -------------+---------+------------------ _ferox_ | males | 17.0-26.0 (20.0) | females | 23.3-34.0 (27.9) | | _spinifer_ | males | 13.8-16.0 (14.4) | females | 26.0-31.0 (28.0) | | _muticus_ | males | 11.8-14.0 (12.3) | females | 17.7-21.5 (18.9) -------------+---------+------------------

_Pattern_

Secondary s.e.xual differences in pattern are probably more p.r.o.nounced in soft-sh.e.l.led turtles than in other species of turtles, except perhaps for the well-known melanism and concomitant obliteration of pattern acquired by some adult males of the _scripta_ section of the genus _Pseudemys_.

The difference in pattern between the s.e.xes of American species varies with size of the individual and with the species and subspecies. The juvenal pattern of some individuals of _T. spinifer asper_ differs according to s.e.x. In the other species and subspecies, there are no secondary s.e.xual differences in the juvenal pattern. That pattern in females of all species and subspecies is partly or entirely obscured by a mottled and blotched pattern as growth proceeds. This mottled and blotched pattern is present on females not yet s.e.xually mature, and is of contrasting lichenlike figures, and in other individuals is less contrasting and a more uniform coloration. The largest males of _T.

spinifer_ retain a conspicuous juvenal pattern; in those of _muticus_ the pattern may be well-defined or partly modified and obscured, whereas in large males of _ferox_ the juvenal pattern is ill-defined or absent. No male normally acquires a contrasting mottled and blotched pattern on the carapace. The pattern on the carapace of many large individuals of _ferox_ is not distinctive as to s.e.x.

On the dorsal surface of the soft parts of the body there is a contrasting pattern in adult males and hatchlings of some forms, but in most large females the pattern is usually reduced to a near-uniform coloration; the pattern on adult males of _ferox_ and _muticus_ is not contrasting and resembles that on large females.

_Coloration_

Because most specimens examined were preserved, the detection of secondary s.e.xual differences in coloration was difficult. There is one difference in coloration between the s.e.xes in the subspecies _T. s.

emoryi_. Males from the Rio Grande drainage, at least those from the Big Bend region of Texas, and southwestward in the Rio Conchos into Chihuahua, Mexico, are bright orange on the side of head (postl.a.b.i.al and postocular pale areas); an orange tinge also occurs in pale stripes on the snout, and pale orange blotches sometimes occur on the dorsal surfaces of limbs, especially the hind limbs. The coloration of these areas on females is pale yellow, lacking orange.

_Tuberculation_

In all subspecies of _spinifer_ the carapace of adult males is "sandpapery" owing to abundant, small, spiny tubercles distributed over its surface; all females lack spiny tubercles on the surface of the carapace.

_Length of Tail_

Elongation of the prea.n.a.l region of the tail resulting in the extension of the cloacal opening beyond the posterior edge of the carapace occurs in males of several kinds of turtles, including _Trionyx_, at least in those from Louisiana, Texas, and Lake Texoma, Oklahoma (Webb, 1956:121). Probably this elongation is characteristic of males of all American softsh.e.l.ls. Some females of _spinifer_ and _muticus_ that exceed the maximum size attained by males have the tip of the tail and cloacal opening extending a short distance beyond the posterior edge of the carapace. Some large females of _ferox_ have more elongate tails than those of _spinifer_ and _muticus_.

_Width of Alveolar Surfaces of Jaws_

Stejneger (1944:34-36, pl. 6) commented on a series of large skulls of _ferox_ mostly from Kissimmee, Florida, some of which had conspicuously expanded alveolar surfaces. He suggested that the condition was confined to large males. A scattergram (Fig. 2) based on measurements obtained from 45 skulls of _ferox_ shows widened alveolar surfaces of the upper jaws on some of the larger skulls. Because the maximal size of adult males is unknown and the difference in size between the s.e.xes of _ferox_ is slight, such large skulls might represent either s.e.x. The s.e.x had been recorded for only three of the 45 skulls; none of the three exceeded 82 millimeters in basicranial length or had widened alveolar surfaces. Some of the larger skulls of approximately the same size differ markedly in width of the alveolar surfaces; this difference suggests that both s.e.xes are included and that the s.e.xes may be of approximately the same maximal size. On the other hand, the variation observed in skulls is possibly confined to one s.e.x. To judge from what is known of the maximal sizes of the s.e.xes of _spinifer_ and _muticus_ (see Table 2), skulls of _ferox_ of more than 85 millimeters in basicranial length probably are of females. The largest alcoholic male (dissected) of _ferox_ that I examined had a width of head of approximately 46.5 millimeters; that measurement corresponds to a basicranial length of 70 to 75 millimeters. The specimen of which measurements are depicted by the uppermost symbol in the scattergram (represented by KU 16528) was recorded as a female.

Large females of _T. s. asper_ from rivers emptying into the Atlantic Ocean have broadened alveolar surfaces.

[Ill.u.s.tration: FIG. 2. Basicranial length and greatest width of alveolar surface of upper jaw on 45 skulls of _T. ferox_. Some skulls (s.e.x unknown) in which the basicranial length exceeds 85 mm. develop widened alveolar surfaces of the jaws.]

_Length of Claw_

Secondary s.e.xual differences in length of claw on the forelimb are p.r.o.nounced in some kinds of turtles. Cahn (1937:178) stated that the female of _Trionyx muticus_ usually has long claws on the hind feet, while the male has long claws on the forefeet, but I am unable to substantiate his statement. Measurements of length of the third claw on the hind limb taken in 41 males and 45 females of _spinifer_ from Louisiana showed no secondary s.e.xual difference.

Ontogenetic Variation

_Pattern_

In all species and subspecies the juvenal pattern is replaced in females as growth proceeds by a mottled and blotched pattern that is contrasting or of nearly uniform coloration. The blotched pattern (of lichenlike figures) is evident on the carapaces of most females that have plastra so long as 8.0 centimeters. The contrasting juvenal pattern on the dorsal surfaces of the soft parts of the body is correspondingly modified in females, but at a size larger than 8.0 centimeters. Size of ocelli (OD/PL) in _T. s. spinifer_ and _hartwegi_ seems to vary ontogenetically (see section on Geographic Variation).

Some hatchlings have blotched patterns (_T. spinifer asper_, TU 16689.2, plastral length, 3.5 cm.); the largest females examined that did not show any evidence of mottling were two _asper_ having plastrons 7.6 and 8.0 centimeters in length. Variation in color and pattern probably is modified greatly by the environment (Heude _in_ Stejneger, 1907:518, footnote d) and the physiological condition of the individual. Smith, Nixon and Minton (1949:92) reported that a female of _T. s. hartwegi_ developed a striking melanistic pattern in captivity and they concluded that patterns of soft-sh.e.l.led turtles may be produced not only by conventional chromatoph.o.r.es, but also by other depositions, both intra- and extracellular. TU 16170, taken from brackish water at Delacroix Island, St. Bernard Parish, Louisiana, is the only adult male I have seen that had a blotched pattern (orange-brown in life) on the carapace in addition to the juvenal pattern. One female of _muticus_, KU 48229, having a plastral length 14.5 centimeters, retained a well-defined juvenal pattern, and lacked a mottled and blotched pattern (see Pl. 46).

_Tuberculation_

Males of the subspecies of _spinifer_ develop small, sharp tubercles on the dorsal surface of the carapace when s.e.xually mature. As growth proceeds, the minute prominences along the anterior edge of the carapace on hatchlings of both s.e.xes of _spinifer_ change in shape to conical projections or low, flattened, scarcely-elevated prominences, depending on the subspecies (Fig. 8).

Large females of _spinifer_ and _ferox_ acquire enlarged, flattened k.n.o.bs in the nuchal region and posteriorly in the center of the carapace.

_Length of Tail_

The prea.n.a.l region of the tail rapidly elongates in males of all soft-sh.e.l.ls when they are s.e.xually mature.

_Width of Alveolar Surfaces of Jaws_

The alveolar surfaces of the jaws are conspicuously broadened in large adults of _ferox_, and females of that population of _T. s. asper_ in the Atlantic Coast drainage.

_Ratios_

Width of head increases at a rate slightly slower than does the length of the plastron (PL/HW, Fig. 3). The change in proportions is most p.r.o.nounced at a plastral length of 7.5 to 8.0 centimeters. In general, the head is narrowest in _muticus_ and widest in _ferox_. _T.

s. asper_ and _emoryi_ seemingly have the widest heads among the subspecies of _spinifer_. Geographically width of head increases from _spinifer_ and _hartwegi_ through _pallidus_ and _guadalupensis_ to _emoryi_. _T. ater_ terminates the cline; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _asper_ in having wide heads (average PL/HW of 4.93).

[Ill.u.s.tration: FIG. 3. Ratio of length of plastron to width of head (PL/HW) in some American species and subspecies of the genus _Trionyx_. The size of each sample is given in parentheses following an indication of the range (< =="" less="" than,=""> = greater than) in length of plastron (in cm.) of each sample. The horizontal line indicates the observed variation; the vertical line, the mean; the white rectangle, four standard deviations; and the black rectangle, four standard errors of the mean. There is some ontogenetic variation in PL/HW. The head is narrowest in _muticus_ and widest in _ferox_.]

The carapace increases in width more slowly than it increases in length (CL/CW, Fig. 4). The change in proportions is most p.r.o.nounced when the carapace is 8.0 to 8.5 centimeters in length. Ontogenetically _muticus_ varies least and _ferox_ most; large specimens of _ferox_ have narrower carapaces than _muticus_ of corresponding size. There is also an indication of a geographical gradient that parallels the cline mentioned above for PL/HW. There is a gradual decrease in width of carapace from _pallidus_ through _guadalupensis_ to _emoryi_. Of the subspecies of _spinifer_, _emoryi_ has the narrowest carapace and resembles _ferox_. In _T. ater_ this cline is accentuated and terminates; 12 specimens, ranging in plastral length from 9.6 to 18.4 centimeters, resemble _ferox_ and _emoryi_ in having narrow carapaces (average CL/CW of 1.32).

_Osteological Characters_

Closure of the anterior, paravertebral fontanelles on the bony carapace, and size and number of plastral callosities are subject to ontogenetic variation (see sections ent.i.tled "Carapace" and "Plastron").

[Ill.u.s.tration: FIG. 4. Ratio of length of carapace to width of carapace (CL/CW) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3. There is some ontogenetic variation in CL/CW (least in _muticus_). The carapace is narrowest in _ferox_ and _emoryi_, and widest in _muticus_, _pallidus_ and _asper_.]

[Ill.u.s.tration: FIG. 5. Pattern on dorsal surface of snout of some American species and subspecies of the genus _Trionyx_. Note the gradual transition in pattern from that of _hartwegi_ (b) and _asper_ (c) to that of _emoryi_ (h).

a. _T. ferox_ (UMMZ 102276, 1/3) b. _T. spinifer hartwegi_ (KU 46742, 3/4) c. _T. spinifer asper_ (KU 50842, 1) d. _T. spinifer pallidus_ (KU 2958, 1/2) e. _T. spinifer pallidus_ (KU 2934, 1/2) f. _T. spinifer pallidus_ (KU 2947, 1/2) g. _T. spinifer guadalupensis_ (TU 10165, 2/3) h. _T. spinifer emoryi_ (KU 48218, 2/3) i. _T. muticus muticus_ (KU 48236, 2/3) ]

Geographic Variation

Geographic variation occurs in _Trionyx spinifer_ and _T. muticus_.

The variant populations of _spinifer_ are segregated into six subspecies, those of _muticus_ into two. In the subspecies of _spinifer_ there is both group variation and clinal variation.

Group Variation