Which characteristic is typical of ectothermic animals?

Ectothermic animals, also known as cold-blooded animals, rely on external environmental temperatures to regulate their body heat. This means they do not generate their own body heat like endothermic or warm-blooded animals do. Instead, their metabolic processes and overall activity levels are heavily influenced by the temperature of their surroundings. This characteristic allows ectothermic animals to thrive in a variety of habitats, but it also makes them dependent on ambient temperatures for thermoregulation and can limit their activity to warmer periods of the day or season.

In contrast, the other options do not accurately reflect the defining traits of ectothermic animals. The statement about generating body heat is applicable to endothermic animals, as they maintain a constant body temperature through metabolic processes. The assertion regarding size does not hold true universally; ectothermic animals encompass species of various sizes, which can be larger than their endothermic counterparts. Lastly, while some ectothermic species can have specific habitat requirements, they are not universally dependent on constant water temperature to survive. Many can tolerate a range of temperatures, adapting their behavior accordingly. Thus, the reliance on external temperatures is the key characteristic that distinguishes ectothermic animals.