What characteristic defines ectotherms?

Ectotherms are defined by their reliance on external environmental conditions to regulate their body temperature, which is why they are often referred to as "cold-blooded" animals. They do not have the physiological mechanisms to maintain a stable internal temperature regardless of the external environment. Instead, their body temperature fluctuates with the surrounding conditions, which can affect their metabolic processes, activity levels, and behaviors.

As a result of their dependence on external heat sources, ectothermic organisms can exhibit varying physiological and behavioral adaptations to help manage their body temperature, such as basking in the sun to warm up or seeking shade or burrowing to cool down. This characteristic is fundamentally different from endotherms, which can generate and maintain a consistent internal temperature through metabolic processes.

The other options do not accurately describe ectotherms; for example, while some ectotherms may be terrestrial, this is not a defining characteristic of the group as a whole, since many ectothermic species are also aquatic. Similarly, ectotherms do not produce heat through metabolic processes to the degree that endotherms do, so this characteristic also cannot apply. Overall, the characteristic that specifically defines ectotherms is their inability to maintain a consistent internal temperature independent of the