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.
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).