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A comparison of diel nest temperature and nest site selection for two sympatric species of freshwater turtles.

INTRODUCTION

It is well established that, in general, shallow soil depths have higher diurnal and lower nocturnal temperatures than deeper sites (Chudnovskii, 1948; Shul'gin, 1957; Hassett and Banwart, 1992). Thus, oviparous reptiles that construct subterranean nests subject their eggs to temperature regimes that are influenced by the depth of oviposition.

Temperature effects on egg development include duration of embryogenesis (Yntema, 1978; Packard et al., 1987), embryo survival (Moll and Legler, 1971; Packard and Packard, 1988), sex determination (Bull, 1980; Ewert and Nelson, 1991), morphology (Yntema, 1960; Bustard, 1969), and body size of hatchlings (Van Damme et al., 1992; Packard et al., 1993). Thus, nesting females could alter nest depths to affect temperature regimes experienced by the eggs. Alternatively, temperature patterns could be modified through selection of nest sites with physical attributes that affect nest temperature.

There is a positive correlation between female body size and nest depth among species of turtles (Carr, 1952). Thus, sympatric species of different-sized turtles with overlapping nesting seasons would be expected either to (1) construct nests that have predictable and dissimilar diel temperature patterns that reflect interspecific differences in nest depth or (2) exhibit interspecific differences in the selection of nest sites which compensate for the effect of depth on nest temperature.

Adult eastern mud turtles, Kinosternon subrubrum (maximum carapace length 12.5 cm) are substantially smaller than Florida cooters, Pseudemys floridana (maximum carapace length 40.3 cm) (Goff and Goff, 1932; Burke et al., 1993; Ernst et al., 1994). At our study site, these two species are sympatric and have overlapping nesting seasons (see Gibbons and Greene, 1990). Therefore, we predicted that K. subrubrum nests should either (1) have lower nocturnal and higher diurnal temperatures than P. floridana or (2) exhibit micro-habitat characteristics that differ from P. floridana nests and differentially modify the effects of nest depth on nest temperature. To address these predictions we (1) determined the mean nest depth for each species, (2) recorded diel temperature patterns from natural nests, and (3) quantified interspecific differences in microhabitat characteristics that exhibited the potential to alter nest temperature.

MATERIALS AND METHODS

The study was conducted in terrestrial habitats adjacent to Ellenton Bay, a 10-ha Carolina bay wetland on the Savannah River Site near Aiken, South Carolina (see Sharitz and Gibbons, 1982, for a site description). During the 1993 (1 May through 20 July) and 1994 (26 April through 2 July) nesting seasons, biologists intercepted Kinosternon subrubrum and Pseudemys floridana exiting Ellenton Bay at a terrestrial aluminum drift fence that surrounds the bay. The drift fence was continually monitored during daylight. Upon capture, female turtles were transported to a laboratory and X-radiographed to assess reproductive condition. Gravid turtles were fitted with waterproof 5-g radio transmitters (transmitter weight [less than or equal to] 5% of body mass) and marked individually on the carapace with either paint or vinyl numbers. Each turtle was released into the water at Ellenton Bay, generally within 24 h. When the turtles re-emerged, biologists gently lifted the nesting turtles over the drift fence. Turtles were left undisturbed for 40 min and subsequently monitored covertly via radio telemetry until nesting ensued.

A biologist carefully excavated each probable nest site by hand to verify the presence of eggs. One thermocouple temperature probe was placed within each nest and was buried horizontally to ground surface for several centimeters to assure accurate measurement of temperature at a single depth. Diel temperature patterns of natural nests were determined by recording nest temperatures hourly over 24-h periods. Temperatures were recorded with a Cole-Parmer Digi-Sense digital thermometer calibrated by an ice bath (see Taylor and Jackson, 1986).

No attempt was made to protect nests from predation as they were concurrently monitored as part of a nest survival study. To negate any possible effects due to metabolic heating and standardize nest measurements of both species, thermocouples were placed adjacent to the uppermost egg of each nest and nest depth was recorded as the distance from ground surface to the top of the uppermost egg.

Four abiotic factors known to affect turtle nest temperatures were examined to determine their potential effect on nest temperature: soil composition, slope of the terrain, soil moisture, and amount of sunlight reaching the nest (Paukstis et al., 1984; Vogt and Bull, 1984; Packard et al., 1993). Terrestrial habitats adjacent to Ellenton Bay exhibit only minor differences in soil composition, and the terrain is flat [maximum slope [less than] 6% (Rogers, 1990)]. For these reasons, soil composition and slope were not considered to differ substantially among nest sites and will not be discussed further.

To quantify soil moisture, soil cores (surface to 15 cm deep) were collected 1 m from nest sites. Cores were collected on a single day, weighed, oven-dried concurrently for 48 h at 60 C (to a constant mass), and reweighed. Percent soil moisture was calculated by dividing the difference between dry soil mass and original soil mass by original soil mass. Nests that were depredated between the time that temperature measurements were recorded and the day that soil cores were taken were not included in the soil moisture analysis due to concern that depredation disturbance would affect moisture levels.

The amount of sunlight (measured in lux) reaching the ground surface above each nest was recorded hourly over 24-h periods. Sunlight measurements were recorded by hand with an INS DX-100 digital lux meter (accuracy [+ or -] 2%). Nest temperature and sunlight measurements were collected simultaneously on four nonconsecutive cloudless days (24-h periods) with minimal winds and similar air temperatures. Because Kinosternon subrubrum are more abundant than Pseudemys floridana at our study site, a ratio of one P. floridana nest to three K. subrubrum nests was maintained on each day that measurements were taken. Interspecific differences in nest depth and soil moisture were compared using t-tests (SAS Institute, Inc., 1985).

RESULTS

Data were collected for a total of 21 Kinosternon subrubrum and seven Pseudemys floridana nests, although some nets provided partial data due to nest depredation. Of these, 17 K. subrubrum and six P. floridana nests escaped predation long enough to permit collection of soil moisture levels.

Nest depth for Kinosternon subrubrum ([Mathematical Expression Omitted], SE [+ or -] 0.41 cm, n = 21) was significantly shallower than for Pseudemys floridana ([Mathematical Expression Omitted], SE [+ or -] 0.64 cm, n = 7; t = 2.93, P [less than] 0.01). Mean nest temperatures for K. subrubrum were generally cooler at night than those exhibited by P. floridana nests, but nest temperatures for the two species approximately paralleled each other from 1000 until 1400 h [ILLUSTRATION FOR FIGURE 1 OMITTED]. Temperatures for both species peaked at approximately 1500 h, with P. floridana nests achieving slightly higher maximum temperatures than K. subrubrum nests. The P floridana nests maintained higher temperatures than K. subrubrum nests for the remainder of the 24-h cycle.

Mean percent soil moisture did not differ significantly between Kinosternon subrubrum ([Mathematical Expression Omitted], SE [+ or -] 0.27%, n = 17) and Pseudemys floridana nests ([Mathematical Expression Omitted], SE [+ or -] 1.31%, n = 6; t = 1.39, P = 0.18). Comparison of sunlight measurements indicated that at 1300 h, ground surfaces above K. subrubrum nests were exposed to approximately half as much sunlight as P. floridana nests [ILLUSTRATION FOR FIGURE 2 OMITTED].

DISCUSSION

The diel temperature patterns of the shallow Kinosternon subrubrum and deeper Pseudemys floridana nests only partially conformed to the expected patterns based on nest depth. As predicted, the K. subrubrum nests remained generally cooler than P. floridana nests at night. Contrary to our prediction, K. subrurum nest temperatures did not surpass P. floridana nest temperatures during the day. The most likely explanation for this discrepancy is the difference in the amount of sunlight experienced by the nests of each species. We attribute differences in the amount of solar exposure to differences in the amount of vegetative cover at the nest sites selected by each species. Our qualitative observations indicate that P. floridana nested in sparsely vegetated open fields, where exposure to direct sunlight was high. In contrast, K. subrubrum nested in thick vegetation or near the edge of tree lines, where exposure to sunlight was reduced.

Sustained developmental temperatures of 40 C are probably lethal to developing turtle embryos (Cagle, 1950; Yntema, 1978; Packard et al., 1987). Burger (1976) reported significantly higher mortality in shallow nests of the northern diamondback terrapin (Malaclemys terrapin terrapin) due to exposure to high temperatures. At our study site, we measured temperatures of 40 C in unshaded shallow ([approximately equal to] 5 cm) soils for several hours daily during summer months. Thus, the shallow-nesting Kinosternon subrubrum may require substantial amounts of shade to avoid temperature-related mortality.

Because solar exposure is the abiotic factor that has the greatest influence on temperature at the depths at which these turtles nest (Shul'gin, 1957), it is not surprising that turtles would react to this factor to modify nest temperature regimes. Schwarzkopf and Brooks (1987) also reported the use of solar exposure to modify nest temperature by a northern population of painted turtles (Chrysemys picta). In a Canadian nesting ground that experiences cool ambient temperatures during embryonic development, C. picta chose nest sites that were exposed to significant amounts of sunlight by selecting S-facing slopes with sparse vegetation. Schwarzkopf and Brooks suggest that local climatic conditions influence nest site selection. The results of our study suggest that nest depth is also an important determinant that influences nest site selection in turtles.

Acknowledgments. - We thank T. Tuberville, C. Kean and K. Wilson for field assistance. Constructive comments on earlier drafts of the manuscript were provided by J. W. Gibbons, J. Congdon, R. Kennett, J. Krenz and S. Doody. Research and manuscript preparation were supported by contract DE-AC09-76SR00-819 between the U.S. Department of Energy and the University of Georgia's Savannah River Ecology Laboratory.

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Author:Bodie, J. Russell; Smith, Kelley R.; Burke, Vincent J.
Publication:The American Midland Naturalist
Date:Jul 1, 1996
Words:2292
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