What defines an endotherm?

An endotherm is defined as an animal that is capable of generating its own body heat through metabolic processes. This ability allows endotherms, such as birds and mammals, to maintain a constant internal body temperature regardless of the external environmental conditions. This thermoregulatory capability provides a significant advantage, as it enables these animals to remain active in a variety of habitats and weather conditions, even when temperatures drop.

The other options pertain to different categories of animals. For instance, the option that refers to an animal that cannot regulate its body temperature relates to ectotherms, which rely on environmental heat sources to maintain their body temperatures. The option suggesting reliance on external temperatures highlights how ectotherms function rather than describing endotherms. Hibernation, while a behavior that may be associated with endotherms to survive harsh conditions, is not a defining characteristic of all endothermic animals. Thus, the defining feature of endotherms is their capacity to produce heat internally, aligning with the provided answer.