Final answer:
The statement is true; ectotherms depend on external heat sources, like the sun, for body heat regulation, contrasting with endotherms that produce body heat metabolically.
Step-by-step explanation:
The statement 'Ectotherms receive most, or all, of their body heat from an external source, usually directly from the sun' is true. Ectotherms, such as reptiles and amphibians, rely heavily on ambient environmental temperatures to regulate their body heat. They engage in behaviors like basking in the sun or seeking shade to manage their body temperature because they have a slow metabolic rate that does not generate sufficient heat for thermoregulation. In contrast, endotherms generate metabolic heat internally, which allows them to maintain a more constant body temperature regardless of external conditions.
For example, reptiles may bask in the sunlight to absorb heat and raise their body temperature, or find shade or burrows to cool down when necessary. This behavioral thermoregulation is essential for ectotherms since their level of activity is greatly influenced by external temperatures, which can affect the activity of the enzymes within their bodies.