North American Recent Soft-shelled Turtles (Family Trionychidae) Part 21

One of the immature softsh.e.l.ls (KU 51979, plastral length, 9.7 cm.) of the series from Lajitas is considered to be a female. It combines characteristics of both s.e.xes. 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 postl.a.b.i.al 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 s.e.xual difference in coloration of this series of softsh.e.l.ls 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).

_Deposition of Eggs_

Concerning _T. ferox_, Wright and Funkhouser (1915:122-23) wrote that deposition of eggs occurred in June and July in the Okefinokee Swamp on the sandy parts of the islands or in sandy fields in places exposed to the direct rays of the sun. The same authors recorded a gravid female taken on June 22 (_op. cit._:120), and a nest with eggs on June 26. Harper (1926:415) reported egg-laying in June in the Okefinokee Swamp. Goff and Goff (1935:156) found a female in search of a nesting site crawling toward a cleared area within a hammock at 11 a. m. on May 19, about 25 yards from the western sh.o.r.e of Lake Griffin, Florida. Carr (1940:107) stated that eggs in Florida "are laid from March to July 10. One individual laid her eggs on a block of ice which we had buried in the sand." Hamilton (1947:209) observed deposition of eggs near Fort Myers, Florida, in "a sandy roadbed slightly above the cypress swamp and ditch levels on either side of the road." ... either in ... "the ruts formed by cars or the slope of the roadbed"; dates of deposition of eggs recorded are March 30 at 11 a. m. in bright sun, and March 31 (from context, the date given as March 21 is considered an error) at 5 p. m. following a heavy rain. The daily temperatures at the time of Hamilton's observations "averaged 85 F., the first really warm spell of the season."

Eigenmann (1896:262) reported egg-laying of _spinifer_ in sand and gravel in June and July at Turkey Lake (= Lake Wawasee), Indiana. A turtle was seen digging a nest on June 26, and fresh nests of eggs were found on June 27 and July 9. Hedrick and Holmes (1956:126) wrote that a clutch of eggs of _spinifer_ in Minnesota was found about ten inches deep in sand about one foot from the river; a steep gravel bank was also cited as a favorite nesting site. Surface (1908:123) stated that eggs of _spinifer_ in Pennsylvania are laid in May, and the young hatch in August. Gehlbach and Collette (1959:142) found eggs of _spinifer_ on June 19 on a sand bank 15 feet from the edge of the Platte River in Nebraska. Breckenridge (1944:187) wrote that _spinifer_ in Minnesota nests on sandy beaches from June 14 to July 6.

Cahn (1937:193) stated that deposition of eggs in Illinois occurs in "June or early July: earlier in the southern part of the state, later in the northern portion." Force (1930:38) mentioned a gravid female from Oklahoma obtained on May 20. Evermann and Clark (1920:593) were of the opinion that _spinifer_ began laying about mid-June and continued until perhaps late July at Lake Maxinkuckee, Indiana; a female opened on June 14 had oviducal eggs, and the first nest was found on June 18. Nests were usually at the edge of an abrupt ascent in sand; one nest was found in black, mucky soil (_op. cit._:595).

Newman (1906:128) wrote that _spinifer_ in the same lake nests later than the other species of turtles, as a rule not earlier than the middle of June (but as early as June 10, _op. cit._:132), and rarely later than the middle of July; he observed deposition of eggs on June 22. Sites of deposition of eggs were mostly in soft sand not more than six feet from water; other sites found by Newman (_op. cit._:132-33) were a sandy, abandoned road bed separated from the sh.o.r.e by a strip of tall gra.s.s, a rock pile (the eggs being dropped into crevices and sand packed around them), among roots of a tree (the eggs being deposited between the roots and under them in a very irregular fas.h.i.+on), and in clay "so hard packed that one could scarcely break it with the fingers." Natural nest sites in hard clay and a rock pile seem incongruous with nesting habits of softsh.e.l.ls. I note that Newman's study was not begun until 1902 (_op. cit._:127), and it was that year that the water level of the lake was high, flooding the surrounding lowlands (Evermann and Clark, 1920:49-53). Perhaps some of the nests found by Newman were old and not natural because of conditions resulting from the receding water level.

Newman (_op. cit._:134-35) mentioned that in small sandy areas nests were frequently in contact and overlapped; he found one nest containing nine small eggs contiguous with 23 large eggs. Breckenridge (1944:187) reported a nest of 56 eggs of two slightly differing sizes, and probably from two females. Evermann and Clark (1920:594) discovered "probably 10 or 12 nests in a distance of a few yards" and mentioned one nest containing 25 eggs "that evidently belonged to two different sets ... In the bottom were 10 eggs that looked old ... and ... separated from them by sand, were 15 other eggs."

Nesting sites of _muticus_ were mentioned by Muller (1921:181) on one of several small islands having "gently sloping sand and mud sh.o.r.es, 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 un.o.bstructed 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 sh.o.r.es 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 lat.i.tudes than in northern lat.i.tudes. 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 sh.o.r.e (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 sc.r.a.ped 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 pa.s.sage 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. | Aga.s.siz (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. | Aga.s.siz (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. | Aga.s.siz (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 lat.i.tude 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 lat.i.tudes (approximately 40). My examination of females from southern lat.i.tudes (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 lat.i.tudes probably have a greater reproductive potential than those in southern lat.i.tudes if it is a.s.sumed 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 s.e.xually 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 s.e.xually 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 sh.e.l.l.

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 sh.e.l.l; 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 lat.i.tudes (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 lat.i.tudes 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 lat.i.tudes. 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 lat.i.tudes, 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 lat.i.tude 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 lat.i.tudes 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 sh.e.l.l 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 sh.e.l.l 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 ma.s.ses 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 ill.u.s.trated 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-sh.e.l.led turtles.

The average plastral length of most hatchlings probably is 28.0 to 30.0 millimeters.

Owing to the lack of a h.o.r.n.y epidermal covering of the carapace and plastron, soft-sh.e.l.led turtles are not so well suited to studies of age and growth as are the "hard-sh.e.l.led" 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 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 s.e.xually 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 s.e.xually mature when the carapace is about 11 inches long, the age at s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xually mature turtles. Judging from these criteria, the age of a female of _spinifer_ at s.e.xual 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.