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.
Mitsukuri (1905:265) reported that in hatchlings of Trionyx sinensis the [575] 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 73/4 to 83/4 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 51/2 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 41/2 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 [576] 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 181/2 inches (Agassiz, 1857:401). The head of a ferox having a width of 31/2 inches (Wright and Funkhouser, 1915:120) corresponds to a length of carapace of approximately 221/2 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.