The function of claws can vary dramatically among and within turtle species. For example, the strong claws and forearms of several tortoise species (e.g., Gopherus polyphemus) are uniquely adapted for burrowing (Auffenberg 1966). Claws are often used to tear apart food when feeding and are sometimes used in courtship behaviors of many freshwater turtles (i.e., Graptemys species; Cagle 1948; Gibbons and Lovich 1990; Legler 1990). The number of claws can also vary geographically within a species (e.g., Terrepene carolina; Milstead 1969; Minx 1992).

Claws may also perform functions unique to each life history stage of the animal. For instance, hatchlings of the red-eared slider use their front claws to open the egg at hatching (Ewert 1979; Tucker 1995) and to dig out of nest cavities in the spring after overwintering (Tucker 1997, 1999a; Tucker and Packard 1998). As adults, female sliders use their hind feet and claws to dig nests, sometimes in extraordinarily hard-packed substrates such as gravel road shoulders (Tucker 1999b). Adult males often use their elongated front claws during courtship behaviors (Thomas 2002), and foreclaws are sometimes used by females during courtship as well (Lovich et al. 1990).

This variation in the function of claws prompted us to investigate several questions relating to adult sex differences and ontogenetic changes in claw morphology of the red-eared slider turtle. First, we asked whether the claws of newly hatched turtles in autumn differed from hatchlings caught in the spring after their migration from nests toward aquatic habitats. We expected that the front claws of newly hatched turtles would be longer than those of turtles whose claws had been worn during hatching and digging out of their nests. Secondly, we asked if the rear claws of adult females differed from those of adult males, considering that males do not construct nest cavities, and thus do not use their hind claws in this manner. We expected that hind claws of females would be longer than those of males, reflecting an adaptation for efficient nest construction by the female. Thirdly, we examined sex differences in front claw length. Based on results of previous work, we expected males to have longer front claws than females because of their frequent use during courtship (Thomas 2002).

Methods

We collected male and female red-eared sliders by hand in aquatic habitats in Calhoun County, Illinois. Collections were made in May through July 1997. We collected data on all turtles longer than 100-mm plastron length because males and females can be distinguished once they reach this size (Tucker et al. 1995).

Claw length for adult turtles was measured as the length of the claw on the third digit (method of Legler 1990). We also measured straight-line carapace length and midline plastron length for each turtle. Maximum carapace width and height were measured at the center of ventral scute three. All measurements were taken with calipers to the nearest 1 mm. All turtles were uniquely marked and then released alive immediately after measurements were made. No turtle was measured more than once.

Newly hatched turtles came from eggs incubated in the laboratory in 1998 and 1999. The use of drift fences and pitfall traps in 1999 allowed us to capture naturally incubated hatchlings soon after they started migrating towards water (Tucker 2000). Both laboratory- and naturally-incubated hatchlings used in this study were preserved for other studies (e.g., Tucker et al. 1998a).

Claw length of the third digit of hatchling turtles was measured from the preserved specimens with an ocular micrometer to 0.1 mm. Hatchling carapace and plastron length were measured prior to preservation with vernier calipers to 0.1 mm. We did not measure carapace width or height of hatchlings.

We used analysis of covariance (ANCOVA) to compare front and hind claw lengths of male versus female turtles. Carapace length was used as the covariate because it accounted for the most variance in front and hind claw length of males and females in stepwise multiple regressions including carapace length, width, height, and plastron length as independent variables (Tucker et al. 1998b). The data used in the analysis met the assumptions of normality and homogeneity of variance. However, slopes differed significantly between males and females for hind claw length versus carapace length, but slopes did not differ between sexes for front claw length versus carapace length. Thus, we used a heterogeneous slopes model for hind claw length and a homogeneous slopes model for front claw length (SAS 1996).

We used analysis of variance (ANOVA) to compare claw length between laboratory- and naturally-incubated hatchlings (after confirming that data met parametric assumptions). A covariate was not necessary because carapace and plastron length did not vary significantly between groups. Significance levels of p < 0.05 were used for all analyses.

Results

Males had significantly longer front claws than did females (F_1,173 = 1081.35, p < 0.0001). Front claw length for males was more than double the length of the front claws for females after adjusting for differences in carapace length (least squares means, 19.3 vs. 9.6 mm, respectively). Front claws of males and females did not differ in the rate at which the slope increased with carapace length (Fig. 1; carapace length by sex interaction, F_1,172 = 2.20, p = 0.1401).

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Females had relatively longer hind claws than males (F_1,172 = 29.38, p = 0.0376, 11.7 vs. 11.1 mm, respectively). Moreover, the slope of the increase in hind claw length relative to carapace length was greater in females (0.102) than in males (0.042) (Fig. 2; carapace length by sex interaction. F_1,172 = 44.41, p < 0.0001).

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Both front and hind claws of hatchlings newly emerged from the egg (i.e., laboratory incubated) were longer than those of hatchlings captured after they emerged from the nest (i.e., naturally incubated) and migrated to the drift fence where they were caught (front claws: F_1,164 = 42.16, p < 0.0001; hind claws: F_1,164 = 19.60, p < 0.0001; Table 1).

Table 1. Descriptive statistics for adult and hatchling red-eared sliders (Trachemys scripta elegans) from west-central Illinois. Mean ± SD (range).

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Discussion

Our results support previous findings that males and females differ in claw length for individuals longer than 100-mm carapace length (Cagle 1948; Gibbons and Lovich 1990; Legler 1990). Furthermore, the slopes of relationship between claw length versus carapace length for males and females (0.057 and 0.044, respectively) were similar to the slopes (0.04 for both sexes) reported by Gibbons and Greene (1990) for similar-sized males and females from South Carolina. The front claws of males go through a period of rapid growth while turtles are in the process of maturing (Gibbons and Greene 1990). However, we did not observe this period of rapid claw growth, because we only included sexually mature males in our analyses.

The elongated claws of male T. scripta (and of several other emydid turtles) are used during courtship, where males vibrate their claws across or near the female's face (i.e., titillation behavior). Titillation behaviors are commonly used during courtship by young males, but are rarely used by relatively old males, which primarily use biting or chasing behaviors during courtship (Thomas 2002). Adult females have also been observed using titillation behavior during courtship (Lovich et al. 1990).

Elongated front claws are 1 of 5 sexually dimorphic traits reviewed by Gibbons and Lovich (1990), whereas Legler (1990) found 28 traits that varied sexually among all species of the genus Trachemys. We now report hind claw length as a previously unrecognized sexually dimorphic trait in T. scripta. Females in this study had relatively long hind claws compared to males despite the fact that their claws could have been worn down during pervious nesting seasons. In addition, the slope of the increase in hind claw length relative to carapace length was greater in females (0.102) than in males (0.042). Thus, female hind claws increased in length at a rapid rate in individuals longer than 100-mm carapace length and maintained this rate of increase even in large females (Fig. 2). This result is of interest because relatively large, presumably older, females have had more chances to wear down their hind claws due to their greater number of nesting events than smaller, younger individuals.

We suggest that increased hind claw length is important to nesting females in that females with relatively long hind claws are more efficient at nest construction. Intense predation on nesting turtles has been observed (Tucker et al. 1999), and females capable of rapidly constructing nests might be exposed to terrestrial predators for shorter periods of time than those that take longer in nest construction. Our hypothesis could be tested by comparing rates of nest construction in females with their hind claws clipped short to those with unaltered hind claws.

The difference in claw length in recently emerged hatchlings from eggs versus those that had migrated from nests towards water has not been previously reported. The observed difference in hatchling claw length may reflect claw wear that occurs once hatchling turtles leave their eggs. The turtles caught in our drift fence had dug their way out of nests and had moved some distance to the drift fence. Since turtles grow very little from hatching to migration (Tucker et al. 1998a), they must hatch with the claw length needed to complete the process of escaping the nest and reaching aquatic habitats, and longer claws would presumably make this process more efficient. Although statistically significant, the differences in claw length between laboratory- and naturally-hatched individuals were relatively small. The biological significance of these differences in claw size can be evaluated by comparing nest emergence and movement speeds of newly hatched turtles with claws experimentally shortened to hatchlings with unaltered claws.