Which characteristic is true for ectotherms?

Ectotherms, commonly referred to as "cold-blooded" animals, regulate their body temperature based on the temperature of their surroundings. This means that ectotherms rely on external heat sources to warm their bodies and are unable to maintain a constant internal body temperature like endotherms (warm-blooded animals). Their physiological processes, such as metabolism and behavior, are influenced by external temperature conditions, which can impact their activity levels, digestion, and overall energy expenditure.

In contrast, the other characteristics mentioned do not accurately define ectotherms. For instance, maintaining a constant internal body temperature is characteristic of endotherms. The assertion that ectotherms only inhabit aquatic environments is incorrect; many ectotherms can be found in terrestrial habitats as well. Lastly, the presence of lungs for respiration does not exclusively identify ectotherms, as some ectothermic organisms, like certain amphibians, may also utilize skin for respiration, while others, like reptiles, possess lungs. Thus, the defining characteristic of ectotherms is their dependence on environmental temperatures for body heat regulation.