Thermal Requirements And Specific Dynamic Action In Two Species Of Hatchling Freshwater Turtles

The hatchling Chinese striped-necked turtles (Ocadia sinensis Gray) and red-eared slider turtles {Trachemys scripta elegans Wied) were used as model animals to study their thermal requirements and specific dynamic action. The thermal tolerance and thermal dependence of locomotor performance of hatchling turtles were studied in the thermal requirement experiment, and two thermal environments, one with and the other without thermal gradients, were designed to study diel variation in body temperature of hatchling turtles. In the experiment of specific dynamic action, the oxygen consumption was measured in hatchling turtles after they ingested food, either as a single meal or as double meals, to test for the influence of food type and feeding frequency on specific dynamic action. The main results and conclusions are summarized as the following: I . The thermal requirement in two species of hatchling turtlesIn the hatchling O. sinensis, the upper (CTMax) and lower (CTMin) limits of thermal tolerance were 41.9 ℃ and 1.8 ℃, respectively. The corresponding values in the hatchling T. s. elegans were 41.9 ℃ and 1.6 ℃, respectively. CTMax and CTMin did not significantly differ between the two turtles.In the environment with thermal gradients, there was significant diel variation in body, water and air temperatures, and daily means of body and water temperatures were nearly the same, both being greater than daily mean of air temperature. Significant variation in body, water and air temperatures was also found in the environment without thermal gradients, but there were no significant differences in daily means of body, water and air temperatures. The existence of thermal gradients was a necessary condition needed by turtles to behaviorally regulate body temperatures within their voluntary range. Selected body temperature (Tsel) of O. sinensis varied from 25.4 ℃ to 29.2 ℃, with turtles at the time of 02:00 to 06:00 h exhibiting lower Tsel than did those at other time, and Tsel of T. s. elegans varied from 26.6 ℃ to 30.4 ℃, with turtles at the time of 00:00 to 10:00 h exhibiting lowerTsel than did those at other time. Both of the maximum and the minimum of Tsel did not significantly differ between the two species. Body temperatures were positively correlated with both water and air temperatures in the two species. Body temperatures of turtles in the environment with thermal gradients were higher than those of them in the environment without thermal gradients when the influence of variation in air temperature was removed, and the difference was the consequence of turtles in the thermal gradients regulating body temperatures.Locomotor performances of turtles were highly dependent on their body temperatures. Sprint speed increased with body temperature increasing within the range from 18 to 39 , and decreased at the body temperature of 41 °C. In general, turtles at relatively higher body temperatures had better locomotor performance than did those at lower or extremely high body temperatures. A partial correlation analysis showed that sprint speed was positively correlated with both maximum length of continuous locomotion and stop numbers of turtle in the racetrack, and the maximum length of continuous locomotion was negatively correlated with stop numbers. II. Specific dynamic action in two species of hatchling turtlesIn the single meal experiment feeding two different types of food, food ingested by O. sinensis of the two experimental groups differed significantly in wet mass, dry mass, lipid content and energy content, but not in lean dry mass. T. s. elegans feeding mealworms consumed less food than did turtles feeding shell-free shrimps in terms of dry mass, lean dry mass and lipid content, whereas turtles of both experimental groups did not differ in food intake in terms of wet mass and energy. Food passage time of the Chinese striped-necked turtles ingesting mealworms and shell-free shrimps was 42.9 h and 39.8 h, respectively. To the red-eared slider turtles, food passage time was 46.4 h and 40.3 h, respectively. Food passage time was significantly longer in the both turtles ingesting mealworms than in the both turtles ingesting shrimps. Temporal variation in oxygen consumption over 72 h after feeding was evident in the ingesting turtles but not in the unfed controls, suggesting that feeding exert an effect on metabolic rate (and thus, SDA) in O. sinensis and T. s. elegans. Oxygen consumption was significantly higher in the experimental O. sinensis than in the control ones within the time interval from 4.5-45 h after feeding. T. s. elegans of both experimental groups consumed significantly more oxygen than did control turtles during the period between 4.5 and 33 h following feeding. The duration of the SDAwas significantly longer in the hatchling O. sinensis than in the hatchling T. s. elegans. The peak metabolic rate and the magnitude of the SDA effect both did not differ between T. s. elegans feeding the two different types of food. The peak metabolic rate and the magnitude of the SDA effect was greater in O. sinensis ingesting mealworms than in the turtles ingesting shrimps, but these difference was absent when the influence of variation in ingested energy was removed. However, the SDA coefficient was a little higher in the two species of turtles ingesting shrimps than in the turtles ingesting mealworms, respectively.In the double meal experiment feeding mealworm, temporal variation in oxygen consumption over 96 h after the initial feeding was evident in the experimental O. sinensis but not in the control ones. The first peak metabolic rate occurred at 9.3 h after the initial feeding, whereas the second peak metabolic rate at 35.4 h. Oxygen consumption was significantly higher in the experimental O. sinensis than in the control ones within the time interval from 6-72 h after the initial feeding. Turtles in the double meal experiment ingested more energy and hence had a prolonged duration of the SDA effect than did those in the single meal experiment, but the magnitudes of the SDA effect did not significantly differ between both experimental treatments when the influence of variation in ingested energy was removed.These results show that food type and feeding frequency have different influence on the SDA of animals, but the magnitude of the SDA effect remained nearly constant when the influence of variation in ingested energy was removed. Therefore the energetic cost associated with ingestion is mainly determined by energy content of food ingested by animals. When comparing these data with the corresponding data collected in other species, we find that the time to reach a peak of SDA, duration of SDA and the peak of SDA differ considerably among species.