2008 NORTHEASTERN NATURALIST 15(3):417–430 Demography of an Island Population of Spotted Turtles (Clemmys guttata) at the Species’ Northern Range Limit Dan J. Reeves1 and Jacqueline D. Litzgus1,* Abstract - Demographic information from geographically isolated conspecific populations is important for understanding how a species is locally adapted, and can thus inform conservation decisions. Clemmys guttata (Spotted Turtle) is declining throughout its range in eastern North America due to habitat loss and fragmentation and collection of specimens for the pet trade. The objectives of our study were to describe the demography of a previously unstudied island population of Spotted Turtles and to make comparisons to conspecific mainland populations. We conducted mark-recapture surveys for turtles on a small (23.2-ha) island in eastern Georgian Bay, ON, Canada. Over seven sampling trips, 40 different turtles were captured 72 times: 23 females, 6 males, 10 juveniles, and 1 hatchling. Males had significantly larger straight-line carapace lengths and contour carapace lengths than females, whereas females had greater carapace heights than males. Adult females on the island were significantly smaller than females on the mainland. Density was estimated to be 1.7 turtles/ha for the entire island, and 21.4 turtles/ha in one wetland where turtles aggregated in spring. The adult sex ratio was significantly skewed in favor of females (1 male: 3.83 females). Our study provides information on the population ecology of Spotted Turtles in isolation, which is important for the creation of management plans for populations being fragmented by human activities. Introduction Natural history and life-history characteristics can differ among conspecific populations. Therefore, demographic information from geographically isolated populations is important for understanding how a species varies over its range, and can shed light on the environmental conditions that have led to locally adapted traits. Clemmys guttata Schneider (Spotted Turtle) is a relatively widely distributed species in eastern North America, with disjunct populations ranging from Maine southward along the Atlantic Coastal Plain to central Florida (Barnwell et al. 1997, Ernst et al. 1994), and westward along the southern shores of the Great Lakes from Ontario to Illinois (Wilson 1994). The Spotted Turtle is considered vulnerable, threatened, or endangered throughout its range; populations are thought to be declining largely due to habitat destruction and harvesting for the pet trade (Ernst et al. 1994, Litzgus 2004, Lovich 1989, Lovich and Jaworski 1988). The species was recently up-listed from Special Concern to Endangered in Canada by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC; Litzgus 2004). A few geographically separated populations have been relatively well-studied (e.g., Chippindale 1989; Cook et al. 1980; 1Department of Biology, Laurentian University, Sudbury, ON, P3E 2C6, Canada. *Corresponding author - jlitzgus@laurentian.ca. 418 Northeastern Naturalist Vol. 15, No. 3 Ernst 1970, 1976; Haxton and Berrill 1999; Litzgus and Brooks 1998a, 1998b, 2000; Litzgus and Mousseau 2004a, 2004b, 2006; Litzgus et al. 1999; Seburn 2003); however, there have not been any studies conducted on isolated island populations. Studying island populations can provide insight into how isolation affects population ecology, which will be important in understanding population persistence in the face of habitat fragmentation that leads to isolated populations. Thus, information on the demography of island populations is important to the conservation of endangered species. The objectives of our study were to describe the demography of a previously unstudied island population of Spotted Turtles and to make comparisons to other conspecific mainland populations. Field Site Description The study was conducted on a small (23.2-ha), privately-owned island along the east coast of Georgian Bay, ON, Canada. The island is isolated from the mainland by a minimum of 900 m of deep, open water and is approximately 50 m from the closest island, which is small and farther away from the mainland. The study island is at the same latitude (45oN) and is 10 km away from a mainland population of Spotted Turtles that has been the focus of an ongoing long-term (30-year) mark-recapture study (Litzgus 2006; Litzgus and Brooks 1998a, 1998b, 2000; Litzgus et al. 1999). The mainland site has been previously noted as an island, but due to low water levels in Georgian Bay, has been connected to the mainland by a short portage since at least 1991. In addition, even when the site was historically an island, it was separated from the mainland by about a 3-m wide channel of shallow water, and thus was not isolated from the mainland in terms of accessibility by Spotted Turtles. The island site was discovered while conducting reconnaissance surveys for previously unknown populations of Spotted Turtles in the summer of 2005. The habitats on the island are typified by elevated, exposed rock outcrops that are dotted with small pools of shallow water and small patches of upland forest, and a large shrub swamp that is located in the center of the island. Upland forest type on the island was classified using the Forest Ecosite Classification System (Chambers et al. 1997) as 14.1, which is characterized by Pinus strobus Linnaeus (White Pine)–Populus grandidentata Michx. (Bigtooth Aspen)–Quercus rubra Linnaeus (Red Oak)–dominated stands on dry to moderately fresh soil. Vegetation type was determined to be 28 using the Forest Ecosite Classification System (Chambers et al. 1997). The understory consists of moderate levels of hardwood and conifer regeneration, including Acer rubrum Linnaeus (Red Maple), Abies balsamea (L.) Mill. (Balsam Fir), White Pine, and Red Oak, and hardwood shrubs such as Corylus cornuta Marsh (Beaked Hazel), Vaccinium angustifolium Aiton (Lowbush Blueberry), Gaultheria procumbens Linnaeus (Wintergreen), and Lonicera canadensis Bartram ex Marsh (Fly Honeysuckle). Forest stands were commonly interrupted by areas with an open canopy, consisting of sections of bare rock with 2008 D.J. Reeves and J.D. Litzgus 419 patches of Juniperus communis Linnaeus (Juniper), Deschampsia flexuosa (L.) Trin. (Hairgrass), Cladina rangiferina Linnaeus (Reindeer Lichen), and assorted moss species. A 0.42-ha wetland where many turtles were captured consisted mainly of Sphagnum spp., Chamaedaphne calyculata (L.) Moench (Leatherleaf), Gaylussacia baccata (Wangenh) K. Koch (Huckleberry), Vaccinium macrocarpon Aiton (Large Cranberry), Aronia melanocarpa (Michx.) K.R. Robertson & Phipps (Black Chokecherry), Lowbush Blueberry, and a variety of floating algae. Rock pools of various sizes (from approximately 0.2 m2 to 5 m2) dotted the west side of the island. These rock pools were generally devoid of vegetation, but sometimes contained aquatic algae and sphagnum moss. Other reptile and amphibian species noted on the island included: Pantherophis [Elaphe] gloydi Conant (Eastern Fox Snake), Thamnophis sirtalis sirtalis Linnaeus (Eastern Garter Snake), Chrysemys picta Schneider (Painted Turtle), and Rana clamitans Latreille (Green Frog). The island is privately owned and has three cottages which are used seasonally, each with a dock onto Georgian Bay. Methods We used mark-recapture techniques to study the Spotted Turtles of the island site. We made a total of seven visits to the island between June 2005 and September 2007 (Table 1). Turtles were captured by hand while researchers walked the site; there were typically two researchers searching for turtles on each sampling visit. Upon initial capture each year (2005, 2006, and 2007), turtles were marked (Cagle 1939) if previously un-captured, weighed using a 300-g (± 2 g) Pesola spring scale, and midline (straight-line) carapace length and width, midline (straight-line) plastron length and width, and carapace heights were measured to 0.1 mm with calipers (± 0.05 stainless; Scherr-Tumico, China). Capture location was recorded with a hand-held GPS (Garmin Legend, Olathe, KS). Any injuries were sketched and noted. Sex was determined using secondary sexual characteristics (Ernst et al. 1994) and recorded; females were palpated to check for gravidity. Turtles Table 1. Date of visits to an island site in eastern Georgian Bay, ON and number of Spotted Turtles (Clemmys guttata) captured and marked. * indicates sampling dates used in population size estimate (see Table 4). MR = marked recaptures. Females Males Juveniles Total captures Total captures Total captures Date of visit (new captures) MR (new captures) MR (new captures) MR 10 June 2005 4 (4) 0 2 (2) 0 4 (4) 0 15 May 2006 3 (3) 0 0 (0) 0 2 (1) 1 20 June 2006 * 14 (8) 6 2 (2) 0 6 (3) 3 21 June 2006 * 7 (4) 3 1 (1) 0 3 (0) 3 6 July 2006 4 (2) 2 0 (0) 0 0 (0) 0 22 July 2006 * 9 (1) 8 2 (0) 2 5 (2) 3 10 September 2007 2 (1) 1 1 (1) 0 1 (1) 0 Totals 43 (23) 20 8 (6) 2 21 (11) 10 420 Northeastern Naturalist Vol. 15, No. 3 captured in 2006 were also measured for midline contour carapace and contour plastron length to account for the concavity of the male plastron (Ernst et al. 1994) with a flexible measuring tape (±1 mm). We tested for significant differences between males and females with respect to carapace length (CL), carapace width (CW), contour carapace length (CCL), plastron length (PL), plastron width (PW), contour plastron length (CPL), carapace height (CH), and body mass using independent sample t-tests. We tested whether the sex ratio differed from parity using a chi-square (χ2) analysis. Adult population size was estimated using three sampling dates (Table 1) and the CAPTURE2 program (Hines 1998). We used the Schnabel (1938) Mt time variation model, which is a direct extension of the Lincoln-Peterson model, to estimate population size (Braun 2005). The model assumes that: 1) the population is closed; 2) all animals are equally likely to be captured in each sample; and 3) marks are not lost, gained, or overlooked. Only adult turtles (carapace length of >102 mm for females and >105 mm for males; from Litzgus and Brooks 1998b) that showed secondary sexual characteristics (Ernst et al. 1994) were used in the population-size estimate. We tested for body-size variation between the island population and the mainland population of Spotted Turtles noted above. Only adult female turtles from each population were used due to the small number of males captured in the island population (Table 1). We tested for body-size differences between the island and mainland females using independent samples t-tests. Adult female size ratios were calculated by dividing mean mainland female measurement by mean island female measurement. Results Forty different Spotted Turtles were captured a total of 72 times over seven visits (Table 1) to the island site: 23 females, 6 males, and 11 juveniles (including 1 hatchling; Fig. 1). The adult sex ratio was 1 male: 3.83 females, which differed significantly from 1:1 (χ2 = 9.96, df = 1, P < 0.005). Five adults (17% of adult population) and one juvenile (9% of juvenile population) had missing limbs, and three adult females had stubbed tails (10% of adult population); no adult males or juveniles with stubbed tails were observed. Seventy-five percent of females (3 of 4) collected in 2005 were gravid, and 19% (4 of 21) of females collected in 2006 were gravid. None of the four females captured in both 2005 and 2006 was determined to be gravid in successive years. There was no significant size dimorphism between adult males and females with respect to CW, PL, PW, CPL, or body mass (P > 0.05 in all cases; Table 2). Males had significantly greater CL (t = -3.43, df = 27, P < 0.005) and CCL (t = -3.21, df = 18, P < 0.005) than females. However, females had greater CH than males (t = 3.58, df = 27, P < 0.005). Mainland females were significantly larger than island females in every trait measured (size ratios > 1 in all cases; Table 3). Estimated adult population size on the island was 31 turtles, and adult density was estimated to be 1.7 turtles/ 2008 D.J. Reeves and J.D. Litzgus 421 ha (Table 4). During one sampling period (22 July 2006), nine turtles were found in the 0.42-ha wetland noted above, giving a density of 21.4 turtles/ha in this wetland. An unusual aspect of the ecology of the island population of Spotted Turtles was the turtles’ use of rock pools during the active season. Of the 72 total captures, 44 (61%) occurred in rock pools; 24 female captures, 5 male captures, and 15 juvenile captures were from rock pools. A total of 20 (28%) of the captures occurred in the 0.42-ha wetland. Figure 1. Body-size (midline carapace length) frequency distribution of Spotted Turtles (Clemmys guttata) captured from an island site in eastern Georgian Bay, ON, Canada. Each turtle is represented once by its size at most recent capture. Table 2. Mean body size measurements ± standard errors for adult Spotted Turtles (Clemmys guttata) captured from an island population in eastern Georgian Bay, ON, Canada over seven visits between June 2005 and September 2007. Each individual is represented once by its size at most recent capture. * indicates a significant difference between the sexes (see text for statistical results). Body-size variable Females Males Carapace length (cm)* 10.89 ± 0.07 (N = 23) 11.70 ± 0.25 (N = 6) Carapace width (cm) 8.35 ± 0.06 (N = 23) 8.68 ± 0.18 (N = 6) Plastron length (cm) 9.68 ± 0.06 (N = 23) 9.70 ± 0.18 (N = 6) Plastron width (cm) 6.14 ± 0.04 (N = 23) 6.12 ± 0.16 (N = 6) Contour carapace length (cm)* 11.97 ± 0.10 (N = 18) 12.82 ± 0.34 (N = 4) Contour plastron length (cm) 9.74 ± 0.07 (N = 18) 9.60 ± 0.29 (N = 4) Carapace height (cm)* 4.07 ± 0.04 (N = 23) 3.71 ± 0.08 (N = 6) Body mass (g) 212.18 ± 4.08 (N = 22) 219.83 ± 7.73 (N = 6) 422 Northeastern Naturalist Vol. 15, No. 3 Table 4. Comparison of Spotted Turtle (Clemmys guttata) population sizes (method of estimation) and densities across the species’ range. Approximate latitudes of population locations are given in parentheses. Population size Number of Location (latitude) (model used) turtles captured Density (turtles/ha) Source Beidler Forest, SC (33oN) 36 (Lincoln index) 33 0.36 Litzgus and Mousseau 2004 Lancaster County, PA (40oN) 258 (Lincoln index) 180 79.60 Ernst 1976 Lockport Pr., IL (41.5oN) 258 (Schnabel) 98 2.20 Wilson 1994 Victoria County, ON (44oN) 55 (Lincoln index) 26 0.46 Haxton 1998 Mainland, Georgian Bay, ON (45oN) 187 (Peterson) 94 0.62 Litzgus 1996 Island, Georgian Bay, ON (45oN) 31 (Schnabel Mt) 27 1.34 Current study Table 3. Means (ranges) and size ratios for morphological traits of female Spotted Turtles (Clemmys guttata) from an island (current study) and a mainland site (J.D. Litzgus, unpubl. data). The results of independent t-tests for differences between populations are also provided. Units for all traits are cm, except g for mass. Size ratios were calculated as mean mainland trait divided by mean island trait. Mainland turtles were significantly larger than island turtles in all traits. Mainland Island Trait N Mean (range) N Mean (range) Size ratio t-value P-value Carapace length 66 11.57 (11.50–11.63) 21 10.88 (10.81–10.96) 1.062 5.82 less than 0.001 Carapace width 63 8.71 (8.66–8.76) 21 8.34 (8.28–8.40) 1.044 4.02 less than 0.001 Plastron length 66 10.16 (10.10–10.22) 21 9.68 (9.61–9.74) 1.049 4.32 less than 0.001 Plastron width 63 6.57 (6.53–6.61) 21 6.14 (6.09–6.18) 1.071 5.94 less than 0.001 Carapace height 38 4.37(4.34–4.41) 21 4.08 (4.04–4.11) 1.072 5.15 less than 0.001 Contour carapace length 38 13.25 (13.17–13.33) 18 11.97 (11.86–12.07) 1.107 8.48 less than 0.001 Contour plastron length 10 10.61 (10.52–10.70) 18 9.74 (9.68–9.81) 1.089 7.84 less than 0.001 Body mass 65 238.51(235.22–241.80) 20 212.38 (208.11–216.66) 1.123 4.16 less than 0.001 2008 D.J. Reeves and J.D. Litzgus 423 Discussion The majority of turtles captured at the island site were adults (28% of the 40 captures were juveniles). This finding is consistent with juvenile capture rates in other populations across the Spotted Turtle’s range, but is one of the highest values reported. Both at the mainland site in Georgian Bay, ON (Litzgus 1996) and in a South Carolina population (Litzgus and Mousseau 2004b), 14% of the captures were juveniles. In Massachusetts, 18% of captures were juveniles (Graham 1995), in Illinois up to 33% of annual captures were juveniles (Mauger 1990, 2004), and a 13% juvenile capture rate was observed in Pennsylvania (Ernst 1976). Juvenile Spotted Turtles tend to be more secretive and use different habitats than adults (Chippendale 1984, Ernst 1976), which can result in low capture rates for juveniles. On the other hand, low capture rates of juveniles may reflect low proportions of juveniles, which are expected for bet-hedging species (Roff 1992, Stearns 1976). However, we found relatively high juvenile numbers in the island population compared to most previous studies, which suggests that juveniles were readily captured, perhaps because suitable habitat for hiding is a limiting factor for juveniles at the island site. The adult sex ratio of the island population differed significantly from equality; it was heavily biased towards females. In contrast, most previous population ecology studies of Spotted Turtles found equal sex ratios (e.g., Graham 1995, Litzgus and Mousseau 2004b, Mauger 1990, McGee et al. 1989), including the closest mainland population (Litzgus 1996) to our island site. There are, however, other Spotted Turtle populations with skewed sex ratios favoring females (e.g., Haxton 1998 [1 male: 1.9 females]; Seburn 2003 [1 male: 3.5 females]), but none to the degree seen in our island population. One possible explanation is related to the fact that Spotted Turtles exhibit temperature sex determination, with lower egg incubation temperatures (22.5 to 27.0 oC) producing predominantly males, and higher temperatures (30 oC and above) producing 100% females (Ewert and Nelson 1991). Due to the relatively open canopy and shallow soils over bedrock present on the island, nesting may be limited to areas with high substrate temperature, thus skewing the sex ratio towards females. However, Spotted Turtles at the nearby mainland site also use open-canopy rock outcrops for nesting sites (Litzgus and Brooks 1998a, 2000) similar to those on the island, but the mainland population does not exhibit an unequal sex ratio (Litzgus 1996). Sampling bias can result in unequal sex ratios, and Spotted Turtle females and males use different habitats during some parts of the active season (Litzgus and Brooks 2000), so sampling efforts focusing on one type of habitat at one time of year could target one sex, influencing the sex ratio. For example, female Spotted Turtles may be more readily captured during the nesting season (mid-late June at the latitude of the island site; Litzgus and Brooks 1998a) when they tend to be more active than males (Litzgus and Mousseau 2004a, Morreale et al. 1984). However, we captured relatively few female turtles on some sampling dates during the nesting season (e.g., 10 June 2005, 424 Northeastern Naturalist Vol. 15, No. 3 21 June 2006), and many female turtles outside of the nesting season (e.g., 22 July 2006; Table 1). Furthermore, our sampling efforts encompassed several months (May, June, July, and September) during the active season, reducing the chances of a temporal bias, and we surveyed different habitat types at each visit. We conclude that it is unlikely that the skewed sex ratio in the island population is due to temporal or habitat sampling bias, but instead reflects a real deviation from equality. Future studies should confirm the sex ratio skew and examine possible explanations for the female-biased ratio. Spotted Turtles in the island population showed relatively high injury rates. Commonly reported injuries in turtles include tail loss and partial or complete amputation of limb(s). For example, Glyptemys insculpta (Le- Conte) (Wood Turtles) have been reported to have 24.5% tail injury rates and 9.6% limb amputation rates (Walde et al. 2003). Limb amputation is typically due to predation (e.g., Carroll and Ultsch 2006, Harding 1985, Harding and Bloomer 1979); however, turtles of the genera Clemmys and Glyptemys are known to cause conspecific tail damage during agonistic encounters (Ernst 1967, Kaufmann 1992). Spotted Turtles are known to exhibit forced insemination (Ernst 1976, Ernst and Barbour 1972), which may explain why all turtles with stubbed tails in our population were female. The proportion of adults in the island population with missing limbs (17%) was higher than that (5.8%) reported by Ernst (1976) for his Pennsylvania population. The prevalence of missing limbs in the island population may be due to higher catchability of injured animals, or may reflect a lack of suitable refugia from potential predators of turtles in the Northeast such as Procyon lotor Linnaeus (Raccoon) and Lutra canadensis Schreber (River Otter) (Brooks et al. 1991, Carroll and Ultsch 2006, Ernst et al. 1994). Spotted Turtles in the island population showed sexual size dimorphism in some traits but not others. Previous studies have reported differing results with respect to the direction and degree of sexual size dimorphism in Spotted Turtles. Ernst (1976) and Haxton (1998) reported no size differences between the sexes, others reported that males had larger carapace lengths (Litzgus et al. 2004), while others reported that females had larger plastron lengths with no difference in carapace lengths (Litzgus 1996, Litzgus and Mousseau 2004b). Sexual size dimorphism was evident in our population with respect to carapace length and contour carapace length, with males being the larger sex, and females having greater carapace heights. The greater carapace height in females is likely related to maximizing body volume to in turn maximize clutch size in a seasonal environment where brief and cool summers prevent high clutch frequencies (Iverson et al. 1993, 1997; Litzgus and Mousseau 2006). However, the small number of adult males captured (n = 6) inhibits drawing any concrete conclusions. Island females were smaller than mainland females in all traits measured. The differences in adult body size between the two populations are likely to be biologically relevant as they were readily apparent using the naked eye. These results are surprising due to the close proximity of the two popula2008 D.J. Reeves and J.D. Litzgus 425 tions; body size variation among conspecific populations is usually observed among widely separated populations and is attributed to climatic differences among regions (Bergmann 1847, Mayr 1956, McNab 1971, Scholander 1955). Our findings suggest that the island population is reproductively isolated from the mainland population. There is a positive relationship between female body size and clutch frequency, and between female body size and clutch size in the mainland population (Litzgus and Brooks 1998b) making it logical to assume that there would be a selective advantage for larger body size in females in the island population as well, since it is also at the northern extreme of the Spotted Turtle’s distribution. Such a trend has also been suggested in previous research on northern populations of small-bodied turtles (Galbraith et al. 1989, Iverson et al. 1993, Litzgus and Brooks 1998b, Murphy 1985). However, since the island females are smaller than mainland females, there may be some evolutionary advantage to having a smaller body size on the island. Other small-bodied turtle species, such as Emydura krefftii Gray (Fraser Island Short-necked River Turtle), also exhibit dwarfism on islands (Georges 1982). Island area may also influence body size (Maurer et al. 1992); a smaller island, such as our study site, may have fewer resources, driving animals on islands to be smaller, thus reducing the amount of total resources they need to survive. However, given the long generation time (≈25 years; Litzgus 2004) and great potential longevity of Spotted Turtles (110 years for females; Litzgus 2006), it is unlikely that the island turtles have had enough time to evolve smaller body sizes in response to limited resources or some other environmental factor, making it likely that the unusually small body sizes observed are due in large part to founder effects. Future work should focus on surveying other islands suitable for Spotted Turtles in the area. Primary productivity and turtle diets at the mainland and island sites should also be investigated. In addition, genetic studies would provide a glimpse into the evolutionary history of the island population and could suggest if, and for how long, this population has been reproductively isolated from mainland populations. The estimated size of the adult Spotted Turtle population at the island site was 31 individuals, and the actual number of adult turtles captured was 29. The number of new captures declined with each sampling visit, thus it is possible that our surveys resulted in capturing 94% of the adult turtles on the island, suggesting that the population size estimate was accurate and that the population size estimator used was appropriate. In addition, we met the assumptions of the Schnabel Mt model. The first assumption is that the population is closed; our data do not violate this assumption as it is unlikely that turtles could traverse the deep open water surrounding the island. The island is separated from the mainland by a minimum of 900 m of open water and a well-traveled boating waterway that is often choppy. There are also no known Spotted Turtle populations on any islands within 1 km of the study site, making it unlikely that a small-bodied turtle species would have access to other populations. The second assumption is equal catchability. Because 426 Northeastern Naturalist Vol. 15, No. 3 juveniles are known to be more secretive and potentially occupy different habitats than adults (Chippendale 1984, Ernst 1976), they were not used in the population size estimate. Although the adult sex ratio was 1 male: 3.83 females, we do not feel that we violated the equal catchability assumption as our sampling efforts suggest that the female-biased sex ratio is real (see above). The third assumption, that marks are not lost, gained, or overlooked was not violated because we used a widely accepted and tested method for marking turtles (Cagle 1939), with the same researcher (D.J. Reeves) being present at each of the sampling events. The density of adult Spotted Turtles was 1.7 turtles/ha across the entire island and 21.4 turtles/ha in a medium-sized wetland found on the island. Spotted Turtles are known to aggregate in small wetlands to breed in spring (Ernst 1967, 1970; Litzgus and Brooks 2000; Millam and Melvin 2001; Perillo 1997). The medium-sized wetland was the only open-canopy, permanent wetland on the island and is likely used as a breeding aggregation site for the population. Adult population density across the entire island was similar to adult population densities at the extremes of the species’ range, but was lower than in populations at the central portion of the range (Table 4). Reported population densities could be influenced by a variety of variables: variation in sampling intensity, latitudinal clines or habitat suitability, and reproductive success (Litzgus and Mousseau 2004b). It is unlikely that sampling efforts biased our population densities because the island was small and thus easy for researchers to survey for turtles. In addition, Litzgus and Mousseau (2004b) reported that while Spotted Turtle densities were highly variable among populations at different latitudes, there was no significant relationship between density and latitude. This result suggests that the habitat suitability of the site and the reproductive success of the island population are the primary causes of the relatively low population density. Spotted Turtles at the island site were often found in small rock pools. This behavior was unusual, and in fact, in the 16 years that J.D. Litzgus has been conducting field work on Spotted Turtles in other locations, she has never before observed turtles using rock pools. The pools were generally devoid of emergent vegetation and thus provided little cover for turtles; we therefore suspect that this unusual habitat use is related to thermoregulation and/or feeding. The rock pools are small and shallow and would therefore heat up quickly so that turtles within them could maintain relatively warm body temperatures while not being exposed, as would be the case when aerially basking. In addition, the pools are likely used by invertebrates for breeding, thus providing a food source for turtles. Future work should focus on measuring temperature and food availability in the rock pools. Our study, although preliminary, provides new natural history and demographic information on an island population of Spotted Turtles at the northern extreme of the species’ range. Given that this is a previously unstudied population, there are likely other currently unknown populations on other suitable small isolated islands in the area, increasing the importance of 2008 D.J. Reeves and J.D. Litzgus 427 field surveys along the coastal regions of eastern Georgian Bay, ON where there is an abundance of such islands. Our study also gives information that may be useful for the design of a management plan for a species that is declining across its range, with emphasis on isolated populations and populations at geographic extremes. Acknowledgments Financial support for the research came from Natural Sciences and Engineering Research Council (NSERC), the Endangered Species Recovery Fund of the World Wildlife Fund Canada and Environment Canada, the Ontario Ministry of Natural Resources Species at Risk Fund, the Canada-Ontario Agreement, and Laurentian University. The study was carried out under the guidelines of the Canadian Council on Animal Care and the Laurentian University Animal Care Committee (AUP# 2004-11-01). Field assistance was provided by R. Jones, S. Gray, and J. Enneson. J. Crowley provided comments on an earlier draft of the manuscript. Special thanks goes to Ron Black, Jake Rouse, and the Parry Sound Ontario Ministry of Natural Resources (OMNR). This study would not have been possible without the support of local island residents who allowed us access to their property and provided gracious support; they have requested to remain anonymous to protect the location of the population so that future generations may enjoy observing this federally endangered species. Literature Cited Barnwell, M.E., P.A. Meylan, and T. Walsh. 1997. The Spotted Turtle (Clemmys guttata) in central Florida. Chelonian Conservation and Biology 2:405–408. Bergmann, C. 1847. Über die Verhältnisse der Wärmeökonomie der tiere zu ihrer Grösse. Göttinger Studien. 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