Selection of environmental temperature and behavioral maintenance of body temperature are primary factors that may positively influence the survivorship, growth, reproduction, and physiological performance of aquatic and semiaquatic turtles (Ewert et al. 1994; O'Steen 1998; Rhen and Lang 1999; Sajwaj and Lang 2000). Maintenance of body temperature within an optimal range provides benefits to metabolic function, nutrient digestion, and immunological protection in turtles (Parmenter 1981; Huey 1982; Hammond et al. 1988; Knight et al. 1990; Rome et al. 1992; Lefevre and Brooks 1995); however, the need to remain cryptic or other factors affecting microhabitat selection may influence thermal preference in some species (Graham and Hutchison 1979; Spotila et al. 1984; Galbraith et al. 1987; Brown et al. 1990; Nebeker and Bury 2001). Many freshwater turtles, including North American softshell turtles (Family Trionychidae), thermoregulate by means of aerial and aquatic basking, and the ability to successfully balance foraging, social interaction, and predator avoidance with selecting preferred thermal niches may affect survivability in hatchling turtles. Janzen et al. (1992) and Lindeman (1993) reported that juvenile turtles, including hatchling softshell turtles, frequently bask in the wild.

Hatchling and yearling turtles placed in aquatic thermal gradients typically select warm (27°–33°C) temperatures within a narrow range. Selection of elevated temperatures in laboratory-based thermal gradients has been demonstrated in a few taxa from several families in North America: chelydrids (Chelydra serpentina; Schuett and Gatten 1980; Williamson et al. 1989; O'Steen 1998; Bury et al. 2000), emydids (Glyptemys insculpta, Tamplin 2006; Pseudemys nelsoni, Nebeker and Bury 2000; and Trachemys scripta, Crawford et al. 1983; Jarling et al. 1984, 1989; Bury et al. 2000), and trionychids (Apalone mutica; Nebeker and Bury 2001).

North American softshell turtles are highly aquatic, diurnal turtles that spend most of the time either foraging, basking in shallow water, or buried in the substrate (Williams and Christiansen 1981; Graham and Graham 1991, 1997; Ernst et al. 1994). Softshell turtles typically occupy lakes, rivers, and streams with soft bottoms composed of sand, gravel, or mixed sediments. Apalone spinifera is widely distributed across the central and southern United States with disjunct populations occurring in the Northeast, Northwest, and Southwest (Ernst et al. 1994). Across this range, A. spinifera is exposed to a wide variety of annual and diel temperatures. These turtles possess a high degree of thermoregulatory ability; in both air and water, adult individuals heat faster than they cool, and warming heart rates exceed cooling rates at the same temperature (Smith et al. 1981). Juveniles < 500 g may heat twice as fast as they cool, and A. spinifera can alter heat-exchange rates at levels that exceed other ectotherms of similar size (Smith et al. 1981). In Iowa, Williams and Christiansen (1981) reported that the body temperatures of an adult male at midday were frequently less than surface water temperature, suggesting that the turtle spent most of its time near the bottom despite the availability of warmer thermal environments. In laboratory studies, Nebeker and Bury (2001) determined that substrate differences affected temperature selection in hatchling A. mutica, the smooth softshell turtle. Most A. mutica selected 27°C in tests with a sand substrate but did not select a particular temperature when tests were run without a substrate.

Hutchison et al. (1966) reported a correlation of critical thermal maximum (CTM) and the temperature at which the loss of righting response occurred with geographic distribution and habitat in North American turtles; northern species and fully aquatic species demonstrated lower values; southern species and terrestrial species displayed higher values. Further, they determined that tolerance to high temperatures in 10 A. spinifera from St. John the Baptist Parish, Louisiana (mean CTM = 41.05°C); mean loss of righting response (= 37.2°C) was similar to other highly aquatic species and low compared to the intermediate values of semiaquatic turtles and the high values of terrestrial species. We tested the response of hatchling A. spinifera from Butler County, Iowa, in an aquatic thermal gradient. Our hypotheses were that hatchling A. spinifera will seek the warmest temperature available within their normal active range and that differences in substrate would influence thermal preference.

METHODS

We obtained 28 A. spinifera eggs from 2 separate nests on 7 and 13 June 2004 near the West Fork Cedar River in Butler County, Iowa. We removed eggs within 12 hours of deposition and numbered, weighed (± 0.01 g), measured (± 0.01 mm), and then incubated them in moistened sand at 27.5°C. Mean (± SD) egg mass (10.08 ± 0.82 g) and diameter (26.16 ± 0.85 mm) differed significantly (p < 0.001) between nests (nest 1: 10.74 ± 0.38 g, 26.77 ± 0.56 mm; nest 2: 9.41 ± 0.55g, 25.55 ± 0.62 mm). Hatching occurred between 69 and 72 days. Hatchlings were assigned a unique code and marked with wire and colored beads to identify individuals (Layfield et al. 1988). We maintained hatchlings in groups of 5–12 in 75–200-L aquaria at 20 ± 0.5°C and provided filtered water, a pea gravel substrate, basking platforms, and UV-A and UV-B lamps (12 h light:12 h dark cycle) that produced dry basking spots with temperatures as high as 28°C. We fed turtles chopped minnows (Pimephales promelas and Pimephales notatus) and pelleted turtle food 3–4 days per week; only individuals that appeared robust and healthy at 3 months of age were selected for testing, and data were subsequently discarded for any individual that did not survive for at least 1 year. Individuals were tested in groups of 6; groups were run in sequence so that each group and each individual was tested the same number of times.

We took morphological measurements (including mass, carapace length [CL], and plastron length [PL]) at hatching, at the start of all experimental tests (3 months) and at the completion of each set of trials (pea gravel substrate = 5 months; sand substrate = 7 months; sand/no sand substrate = 8 months). Hatchlings increased in mass and size from an initial mean (± SD) mass of 7.64 (± 0.56) g, mean (± SD) CL of 41.14 (± 1.25) mm, and mean PL of 27.81 (± 1.08) mm to a mean (± SD) mass of 34.81 (± 9.86) g, mean (± SD) CL of 64.99 (± 7.21) mm, and mean PL of 48.18 (± 5.23) mm at 8 months of age.

The gradient tank (Fig. 1) was 176 (l) × 84 (w)  × 15 (h) cm and composed of 6 equal-sized chambers, each 69 (l)  × 27 (w)  × 15 (h) cm, and a common area (15 [l]  × 176 [w]  × 15 [h] cm) through which turtles could easily move into and out of each chamber. The tank and chamber walls were constructed of molded polyethylene; 5–7-cm-thick insulating board was used to insulate the tank exterior and the interior of each chamber wall. Source water was thermally controlled, filtered, and aerated in 1 of 2 1500-: insulated tanks and then pumped through an insulated manifold to the gradient tank. We regulated chamber temperature by delivering varying amounts of warm (30°C) and cold (15°C) water to one end of each chamber; water was mixed via aeration and then flowed to an outlet valve at the opposite end of the chamber. Flow rate was controlled by needle valves and varied between 1.4 and 1.7 L per minute per chamber during experimental tests. We adjusted flow rates of each chamber periodically to maintain desired temperature. Each chamber held 2 aeration stones (a 15-cm-long bar at the chamber head and a 45-cm-long bar along the length of the chamber), and either a 2-cm substrate of pea gravel or sand or no substrate, depending on experimental treatment. Substrate treatments consisted of tests run with a pea gravel substrate in each chamber, a sand substrate in each chamber, or a sand substrate in the 4 coolest chambers (15°, 18°, 21°, and 24°C) and no substrate in the 2 warmest chambers (27° and 30°C).

View Figure

• Download as Powerpoint
• Download Figure

Water temperature (± 0.1°C) was measured continuously via remote-input sensors and checked against a manual probe inserted at several locations and intervals throughout the experiments. High flow rates and vigorous aeration prevented thermal stratification and maintained temperature to within 0.5°C of any one location in each chamber; locations in the common area near the outlet valves did show some thermal mixing, but these areas did not differ more than 1.4°C from the chamber interior. We maintained temperature increments between chambers at approximately 3°C intervals in gradient tests and 0°C in control tests. Within-chamber temperatures varied slightly during experimental tests; most runs produced temperature fluctuations of ≤ 1°C (e.g., 20°C = 19.6 – 20.4°C), and no runs had a temperature variation higher than 1.2°C within any one chamber. All runs were performed between 1200 and 1900 hours.

We numbered chambers (for control tests; n = 18, total observations = 1944) and assigned a temperature (for gradient tests; n = 18, total observations = 1944). In gradient tests, the high-temperature (30°C) chamber was opposite the low-temperature (15°C) chamber, and the gradient pattern was reversed on alternating test runs. Control and gradient tests were generally alternated and usually performed on successive days or in no more than 2 successive runs on the same day. In gradient tests, we placed turtles into the common area, at a location with the temperature nearest their acclimation temperature (20°C). In control tests, turtles were placed in the corresponding location, even though each chamber was held at 20°C. In control tests corresponding with the sand/no-sand substrate treatment, sand was removed from 2 adjoining chambers and alternated so that, in total, each chamber had an equal number of sand/no sand applications. Turtles were allowed 1 hour for acclimation, then observations of individual location were recorded every 10 minutes for 3 hours (18 total observations per individual per test run). Because the temperatures were similar, turtles in the common area were assigned to the closest chamber. During the acclimating period, cloacal temperatures of several turtles were checked with a digital thermometer and were ± 0.5°C of the water temperature.

We recorded locations of each turtle and noted turtles that switched chambers between 10-minute observations as relocated. We calculated mean number of chamber relocations per test, percentage of observations involving a relocation, and number of chambers visited per turtle for control and gradient tests. Because the first observation was not considered a potential relocation, maximum number of chamber relocations per individual per test was 17. We determined the mean number of observations for each chamber and compared these to observations from the chamber closest to the acclimation temperature (20°C). We used repeated-measures analysis of variance (ANOVA) (Abacus Concepts 1994) to determine if significant differences existed among observations from different chambers, different tests, and different substrate treatments. If significant differences existed, we used multiple comparisons tests to determine which temperatures in the gradient or which set of tests and substrate treatments produced significantly different observations.

RESULTS

Across all substrate treatments in the gradient tests, hatchling A. spinifera selected the warmest temperature available (30°C: 56.1% of observations) and avoided the 2 coldest temperatures available (15°C: 5.3% of observations; 18°C: 3.6% of observations) (Table 1). In control tests (20°C), turtles demonstrated a slight preference for the chambers at the ends of the tank, but most runs produced patternless distributions, and no chamber was selected significantly more often than the others (Fig. 2). Turtles relocated between chambers more often in control tests than gradient tests, and a significantly (p < 0.001) larger number of observations involved a chamber relocation in control tests. Mean (± SE) number of chamber relocations was 6.60 (± 0.45) for control tests and 2.60 (± 0.28) for gradient tests. Similarly, turtles visited significantly (p < 0.001) more chambers in control tests (4.07 ± 0.20) than gradient tests (2.37 ± 0.14). In control tests, 90.7% (± 3.4%) of hatchlings relocated at least once; in gradient runs, 60.2% (± 5.7%) of hatchlings relocated (changed temperatures) at least once. Multiple comparisons tests indicated that mean selection of chambers differed between control and gradient tests in all chambers except chambers 3 (21°C), 4 (24°C), and 5 (27°C).

Table 1. Number of observations of 12 Apalone spinifera hatchlings (age = 4–8 months), at 10-minute intervals for 3 hours, at each temperature in aquatic thermal gradient and control tests. Tests were performed either with a pea gravel or sand substrate in each chamber or with a sand substrate in the 4 chambers with the lowest temperatures (15°–24°C) and no substrate in the 2 chambers with the highest temperatures (27°C and 30°C). Control test observations were recorded in each chamber as though the gradient were in effect, although the temperature of all chambers was 20°C.

View Fullscreen

View Figure

• Download as Powerpoint
• Download Figure

In control tests, repeated-measures ANOVA indicated that the main effect of chamber number and the interactive effect of chamber and substrate were not significant. This suggests that no differences occurred in chamber selection within treatments, nor did substrate differences affect selection of certain chambers between treatments. In gradient tests, the main effect of chamber temperature significantly (p < 0.001) influenced chamber selection, and the interactive effect of temperature and substrate was significant (p < 0.01). Turtles chose 30°C significantly (p < 0.001) more often than other temperatures across all gradient tests and were observed more frequently in 30°C in tests conducted with a sand substrate than those conducted with a gravel substrate or without a substrate (Fig. 3). In tests conducted with sand in the 4 coolest chambers only, turtles chose the warmest temperature with a sand substrate (24°C) significantly (p < 0.001) more often than all other temperatures, except 30°C.

View Figure

• Download as Powerpoint
• Download Figure

In both control and gradient tests with a pea gravel substrate (Table 2), turtles often sat above the gravel on the bottom of the chamber or occasionally leaned against the chamber walls or floated on the surface with their limbs extended. A few turtles buried themselves under the pea gravel within a chamber; these individuals often remained buried for the majority of the test run, occasionally relocating to a warmer chamber if available. On a pea gravel substrate, frequency of burying behavior varied widely by individual. Interactions between individuals were typically aggressive; as turtles encountered each other, aggressive (biting and chasing) and subordinate behavior (withdrawing head, moving away) occurred frequently. Most interactions led to within-chamber movement or, occasionally, burying behavior rather than between-chamber movement. In gradient tests, individuals prompted to relocate from the warmest chamber often returned within several minutes.

Table 2. Number of observations of 12 Apalone spinifera hatchlings (age = 4–5 months), at 10-minute intervals for 3 hours, at each temperature in aquatic thermal gradient and control tests. Tests were performed with a pea gravel substrate in each chamber. Control test observations were recorded in each chamber as though the gradient were in effect, although the temperature of all chambers was 20°C.

View Fullscreen

In both control and gradient tests with a sand substrate (Table 3), turtles often buried themselves and moved under the sand within the chamber, occasionally emerging to either rebury or to swim into another chamber. Most interactions between individuals were neutral and passive, although aggressive (biting and chasing) and subordinate behavior (withdrawing head) was occasionally observed. In these instances, the subordinate individuals generally moved away quickly and buried in a nearby spot. In gradient runs, turtles would occasionally enter the common area, move to another chamber for several minutes, and then return to the warmest chamber. In control runs, turtles often moved about, changed chambers frequently, and appeared more active.

Table 3. Number of observations of 12 Apalone spinifera hatchlings (age = 5–7 months), at 10-minute intervals for 3 hours, at each temperature in aquatic thermal gradient and control tests. Tests were performed with a sand substrate in each chamber. Control test observations were recorded in each chamber as though the gradient were in effect, although the temperature of all chambers was 20°C.

View Fullscreen

In control tests with sand in 4 chambers and no substrate in 2 adjoining chambers (Table 4), turtles often chose chambers with sand and buried in the sand, often moving within the chamber without emerging. Turtles entering chambers without a substrate often explored the chamber and relocated within several 10-minute observations. In gradient tests with sand in the four coolest chambers (15°, 18°, 21°, and 24°C) and no substrate in the two warmest chambers (27° and 30°C), turtles in the warmest chambers often sat on the bottom or floated near the surface with their head and limbs extended; these individuals usually moved within the chamber every few observations but were generally inactive. Individuals that chose the warmest temperature available with a sand substrate (24°C) generally buried themselves and remained in the chamber for the majority of the test run.

Table 4. 3Number of observations of 12 Apalone spinifera hatchlings (age = 7–8 months), at 10-minute intervals for 3 hours, at each temperature in aquatic thermal gradient and control tests. Tests were performed with a sand substrate in the 4 chambers with the lowest temperatures (15°–24°C) and no substrate in the 2 chambers with the highest temperatures (27°C and 30°C). Control test observations were recorded in each chamber as though the gradient were in effect, although the temperature of all chambers was 20°C; bold indicates control test chambers without a sand substrate.

View Fullscreen

DISCUSSION

Juvenile turtles typically show increased vulnerability and have high levels of mortality compared to adults of the same species. Effective thermoregulation via selection of optimal thermal niches enhances growth and activity and may positively influence fitness in young turtles. The thermal preferences of only a few species of aquatic and semiaquatic turtles are known and fewer less of these studies addressed secondary factors, such as embryonic incubation temperature, nutritional state, or substrate type. In similar laboratory-based thermal gradients, hatchling and yearling emydids Glyptemys insculpta, Pseudemys nelsoni, and Trachemys scripta selected the warmest temperatures available in gradient runs of 12°–27°C and 15°–30°C (Bury et al. 2000; Nebeker and Bury 2000; Tamplin 2006; Tamplin, pers. obs.). In gradients of 18°–33°C, some P. nelsoni individuals selected 33°C, but the majority of observations were at 30°C (Nebeker and Bury 2000). Temperature selection in aquatic gradients by hatchling Chelydra serpentina were near 27°C in gradients of 12°–30°C (Bury et al. 2000), 24.5°–28°C in gradients of 20°–30°C (O'Steen 1998), 28°C in gradients of 12°–37°C (Williamson et al. 1989), and 29.8°C in gradients of 20°–32°C (Knight et al. 1990). Knight et al. (1990) determined that nutritional state did not influence temperature selection in 7-month-old C. serpentina, except that satiated individuals tended to move less frequently than starved individuals. Embryonic incubation temperature, in combination with nutritional state and time of day, differentially affected selected temperature of C. serpentina (Rhen and Lang 1999). Nebeker and Bury (2001) determined that differences in substrate affected temperature selection in hatchling A. mutica, a highly aquatic species. On a sand substrate, A. mutica selected intermediate temperatures in a gradient of 12°–36°C but did not select a particular temperature when the substrate was removed.

Hatchlings from at least several families of aquatic turtles are evidently able to select preferred temperatures within a narrow range. In the absence of other environmental factors, hatchlings typically select elevated temperatures, presumably optimizing metabolic processes and positively affecting growth and survivorship. Preferred temperature may differ between species or populations; emydid species that frequently bask and have southern distributions (P. nelsoni and T. scripta) may select warmer temperatures (30°C) than A. mutica and C. serpentina, 2 wide-ranging species with populations found at northern locations. Aerial basking and thermoregulation may occur more frequently in northern populations than southern populations of C. serpentina (Brown et al. 1990). In contrast, 6-month-old G. insculpta, an emydid species that is active at low temperatures and has a northern distribution, selected the warmest temperature available in a gradient of 12°–27°C (Tamplin 2006).

Our evidence indicates that hatchling North American A. spinifera can effectively detect temperature differences and select preferred temperatures within at least a 3°C range. In an aquatic thermal gradient with temperatures within their normal active range, hatchling A. spinifera select elevated temperatures and were able to thermoregulate. The majority of observations occurred in the warmest temperature available (30°C) across different substrates; however, effects of substrate were substantial, and turtles selected the warmest temperature and remained in place for longer periods when a sand substrate was available. When sand was removed from chambers with the 2 warmest temperatures, most observations occurred in the chamber with the warmest temperature with sand available (24°C). This suggests that substrate and the ability to remain cryptic substantially affected thermal preference. The ability to select spatial niches within their environment and effectively thermoregulate may be an important factor for aquatic and semiaquatic turtles. Further, the effect of secondary factors, such as substrate differences, may particularly affect those species that show frequent burying behavior, such as A. spinifera.