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.