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: 311/4 pounds (Goff and Goff, 1935:156); six pounds, lengths of carapace 101/2 and 13 inches (Hamilton, 1947:209); greatest width of head 31/2 inches (Wright and Funkhouser, 1915:120). A 101/2 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 91/2 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, 113/4, 121/2, and 13 inches; the smallest measurement corresponds to a plastral length of approximately 21.0 centimeters.