Convective Heat Exchange in the Desert Tortoise: effects of wind speed, body size, and orientation

Abstract

The desert tortoise (Xerobates agassizii) has been listed as threatened since 1989. One focus of conservation efforts has been to try to understand the way in which the animals are affected by their environment, in order to better protect their habitats. This study focused on the thermal environment, measuring the effects of wind speed, body size, and orientation on convection coefficient and operative temperature of the desert tortoise. Trials were performed in a wind tunnel on aluminum casts of desert tortoises, at wind speeds of 0.5, 0.8, 1.0 1.3, 1.5, 1.8, and 2.0 m/s. Test runs were performed on two sizes of tortoise model, with carapace lengths 20 and 26 cm, to measure the effect of body size on convection coefficient and operative temperature. Finally, the small tortoise model was rotated 90° relative to the direction of airflow to measure the effect of orientation on these parameters. In general, convection coefficients increased with wind speed while operative temperatures decreased. Specifically, convection coefficients were found to increase proportionate to the 0.466 power of the wind speed, comparable to Mitchell's (1975) estimate for a cylinder at low Reynolds numbers. Size also had a significant effect on convection coefficient. Orientation, however, did not; placing the model perpendicular to airflow made only a 5% difference to measured convection coefficients. Convection could potentially contribute to the desert tortoise's thermoregulation by increasing the length of the tortoise's above-ground foraging period. These results indicate that this foraging period could be further enhanced at fast wind speeds or for larger tortoises. Additionally, the lack of significant effect of orientation on convection coefficient means that orientation is not important to the placement of operative temperature (T e) models by researchers in the field.