Clutch Frequency in the Red-Eared Slider (Trachemys scripta elegans)

Clutch frequency, or how often eggs are laid within and among years, is a critical life-history trait for any species of turtle (Gibbons and Greene, 1990; Iverson, 1992). Clutch frequency has been studied in greatest detail for sea turtles where clutches may be laid every 10 to 20 days (Hirth, 1980; Frazer and Richardson, 1986; Bjorndal and Carr, 1989; Rostal et al., 1997). However, female sea turtles do not generally nest in successive years (Mortimer and Carr, 1987). Freshwater turtles are also known to produce more than one clutch per season with internesting intervals ranging from 16 days for Chrysemys picta (Iverson and Smith, 1993) to over 30 days for Kinosternon subrubrum (Gibbons, 1983). Some species of Sternotherus may have even longer internesting intervals (Cox and Marion, 1978; Etchberger and Ehrhart, 1987). In contrast, internesting intervals range around 30 days for terrestrial turtles and tortoises (Turner et al., 1986; Jackson, 1991; Diaz-Paniagua et al., 1996; Wallis et al., 1999). Estimating clutch frequency is difficult, and few of the studies cited above determined clutch frequency. Often only internesting interval is known because the percentage of turtles nesting in successive years was not determined (for an exception, see Wallis et al., 1999). Clutch frequency can only be determined by following marked females in long-term studies that then estimate the number of clutches per year and nesting patterns in successive years for a particular species (i.e., Frazer et al., 1990). Species such as Trachemys scripta, the slider, are known to nest more than once per year (Cagle, 1950; Iverson, 1977; Thomhill, 1982; Gibbons, 1982; Jackson, 1988; Gibbons and Greene, 1990; Tucker et al., 1998a,b; Tucker, 1999). Although the ability to account for the whereabouts of each female in each nesting season is the ideal, there are other methods that allow estimates of clutch frequency within and among years. These techniques include examination from ovarian anatomy (e.g., Thomhill, 1982), plotting the frequency that nesting females are captured through the nesting season 19 8a. Variation in carapace morphology and (e.g., Iverson and Smith, 1993), or using recapture records from year to year (e.g., Iverson, 1991). Estimates based on different techniques result in large differences in fecundity estimates for Trachemys scripta. Working under the assumption that the whereabouts of all females during the nesting season were known, Frazer et al. (1990) estimated clutch frequency at 1.10 clutches per year (in South Carolina Gibbons et al., 1982). However, total fecundity was reduced because only 27.17-47.11% of females apparently nested in any given year. The estimate for total annual fecundity for South Carolina Trachemys scripta scripta was, thus, 2.56 eggs per year. I examined reproduction frequency in the red-eared slider (T s. elegans) in west-central Illinois. I estimate clutch frequency and year-to-year nesting frequency for two populations using three separate techniques, including examination of ovarian anatomy, distribution of captures through the nesting season, and recaptures made from turtles collected from 1994 to 1999. I ask whether my data based on large numbers of females collected over a longer-term study agree with conclusions of Cagle (1950) and Thornhill (1982). Ovarian Anatomy.-Nineteen females were caught by trapping on 1 June 1994 and sacrificed by decapitation. Reproductive tracts were dissected from the turtles and scored following methods of Thornhill (1982). I counted oviductal eggs, corpora lutea, and enlarged follicles. Enlarged follicles (diameter > 17 mm) were counted as a potential second clutch, whereas midsized follicles (diameter between 16 mm and 14 mm) were scored as a possible third clutch. Living Turtles.-Except for the turtles killed in 1994, all females were uniquely marked by drilling holes in one or more marginal scutes and then released alive after capture. Living females were collected at or near two nesting areas. Nesting areas examined include those near Stump Lake in Jersey County and those near Swan Lake in Calhoun County, Illinois (Tucker, 1997; Tucker et al., 1998a,b). To equalize collecting effort, each site was visited once per day with the site visited first alternated daily. Reproductive data were collected for each gravid female (Tucker et al., 1998a,b). Oviposition was induced for all turtles found gravid by intramuscular injections of oxytocin (Ewert and Legler, 1978). Formerly gravid females were released 48 h after oviposition to reduce adverse impacts of oxytocin use (Iverson and Smith, 1993; Tucker et al., 1995). Females found at nesting areas but without oviductal eggs were assumed to have laid eggs prior to discovery and were marked and recorded as well. I compared means using the Kruskal-Wallis test (= H) or the Wilcoxon rank sums test (= Z), both nonparametric methods. Tests were performed with SAS version 6.12 (SAS Institute, Inc., Cary, NC, 1996). Analysis of covariance was not used because variables tested were independent of turtle size. Ovarian Anatomy.-All females examined contained one set of corpora lutea and a corresponding set of oviductal eggs. All females also contained one set of enlarged follicles suggesting that they all would have laid two clutches. Moreover, 13 of 19 females (68%) also contained sets of midsized follicles that may have represented potential third clutch. Clutch frequency was estimated to be 2.68 clutches per year (= 100% a d S ith, 1 93), or using recapture reear to year (e.g., Iverson, 1 91). 664