Nesting Red-Eared Sliders (Trachemys scripta elegans) Exhibit Fidelity to Their Nesting Areas

Abstract

The tendency of sea turtles to return to previously used nesting areas is an important feature of their reproductive biology (e.g., Mortimer and Portier, 1989; Limpus et al., 1992; Boulon et al., 1996) but remains a little-studied aspect of freshwater turtle ecology. The painted turtle (Chrysemys picta) displays some degree of nesting-area fidelity (Christens and Bider, 1987), and snapping turtles (Chelydra serpentina) may or may not return to previously used nest areas (Congdon et al., 1987). In one detailed study, 74% of Ontario C. serpentina returned to the same nest area in the subsequent year (Loncke and Obbard, 1977). Spotted turtles (Clemmys guttata) and diamondback terrapins (Malaclemys terrapin centrata) may also exhibit nestingarea fidelity (Belmore, 1972; Lovich and Gibbons, 1990). Most species of freshwater turtles are known to make long overland movements, some of which are associated with nesting behavior (reviewed by Ernst et al., 1994). Slider turtles (Trachemys scripta) also make nesting migrations (Cagle, 1950, Moll and Legler, 1971, Morreale et al., 1984; Gibbons et al., 1990; Schubauer et al., 1990; Moll, 1994; Tucker, 1997; Burke et al., 1998), but little is known about their degree of nesting-area fidelity. Here, I examine the degree to which female redeared sliders (Trachemys scripta elegans) show nestingarea fidelity in riverine habitats in west-central Illinois. Turtles from these habitats have been studied extensively (e.g., Tucker and Moll, 1997; Tucker et al., 1998a,b). This study represents the first analysis of nesting-area fidelity in a freshwater turtle occupying riverine habitats. I caught turtles at seven nesting areas or on adjacent roads in west-central Illinois between 1994 and 2000 (Fig. 1; for details, see Tucker and Warner, 1999). I collected all females found at each nesting area including those preparing to nest and those that had nested and were returning to their aquatic habitats. The midline plastron length of each turtle was measured with calipers to the nearest 1 mm. Means are presented + 1 SD. All turtles were uniquely marked by drilling holes in one or more marginal scutes. After oviposition was induced with oxytocin, formerly gravid turtles were released at the original nesting areas 48 h after collection (Tucker et al., 1995). The frequency that turtles switched nesting areas among nesting areas was compared with two-way G tests (Sokal and Rohlf, 1981). Similar to the chi-square test, the G test evaluated the goodness of fit of the observed data relative to expected results. Values for P reported herein were obtained by substituting the G values into a computer chi-square function program (SAS Institute, Inc., Cary, NC, 1988). Wilcoxon rank sum test (Z) is used to compare means. Overall, 2913 captures of 2111 turtles were made at the seven nesting areas. Most turtles (374 turtles) were recaptured once, but 105 turtles were recaptured twice, 37 turtles were recaptured three times, and 23 turtles were recaptured four or more times. Some turtles were recaptured more than once per year because most females nest more than once per year (Tucker, 2001). Recaptures for all nesting areas totaled 802 captures. Of these recaptures, 40 (5.0%) were made at nesting areas different from the original one (Table 1). Only three turtles marked at Swan Lake (site 7) were later recaptured at Pohlman Slough (site 6), and one turtle marked at Pohlman Slough was recaptured at Swan Lake. Thus, 1.1% (four of 354) of recaptures made at Swan Lake and Pohlman Slough in Calhoun County were made at different nesting areas. In contrast, 8.0% (36 of 448 captures) of recaptures at nesting areas in Jersey County (sites 1-5) were made at nesting areas other than the original one (Table 1). The rate that turtles changed nesting areas was greater at the Jersey County sites than for those in Calhoun County (G = 23.45, 1 df, P < 0.0001). Turtles at Fowler Lake were more likely to switch nesting area than those from the other Jersey County nesting areas (G = 11.36, 2 df, P = 0.0034). Plastron length for 36 turtles that switched nesting areas from sites in Jersey County averaged 219 + 13.0 mm (range = 196-247 mm). Plastron length for the 412 other turtles recaptured that did not change nesting areas averaged 219 + 13.0 mm (range = 173-250 mm). These two means did not differ significantly (Z = 1.02, 1 df, P = 0.3054). The four turtles from Calhoun county that switched nesting areas also did not differ significantly from the 350 turtles recaptured at the original capture sites (Z = 1.21, 1 df, P = 0.2275. There was some indication that turtles tended to