Dietary Effects on Temperature Selection in Juvenile Desert Tortoises (Gopherus agassizii)
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Body temperature of reptiles and other ectothermic animals conforms to ambient temperatures within a heterothermal environment. Optimal temperatures for digestion may depend on the quality and quantity of the diet, particularly in herbivorous ectotherms like desert tortoises. Two nutritional groups of tortoises were fed different diets, one received a high fiber chow and the other received a low fiber chow (Experiment 1). Then all tortoises were switched to the same diet containing an intermediate amount of fiber (Experiment 2). In a laboratory thermal gradient, individuals had the opportunity to select body temperatures from a range of 10 to 4S °C. In these experiments, quantity of food eaten was more important to temperature selection than quality of the diet. Consumption increased as a function of body mass and was positively correlated with selected body temperature both during the day and at night. Temperatures selected during daytime were significantly higher than at night for Experiment 1, but this difference disappeared in Experiment 2. Desert tortoises showed a behavioral selectivity for the quality of food consumed, because, through differential intake, individuals consumed approximately equal amounts of fiber in Experiment 1. As a result of this selectivity, diet quality was an insignificant factor in temperature selection. However, significant increases in the rate of mass gain based on an increase in consumption provided evidence of energy assimilation used for growth. These results provide insight into behavior of the tortoises in their natural environment. Foraging and food intake are main factors in behavioral temperature selection and, hence, the tortoises' ecological niche.