Here's why:
* They don't produce their own body heat: Unlike mammals and birds, desert tortoises don't have internal mechanisms to generate heat.
* They rely on the environment: They absorb heat from the sun and lose heat to the cooler environment.
* Basking behavior: Desert tortoises often bask in the sun to warm up, and seek shade or burrow underground to cool down.
Their cold-blooded nature is an adaptation to their desert environment. It allows them to conserve energy when it's hot by being less active and lowering their metabolic rate.