Ectotherms are animals that rely on external sources of heat to regulate their body temperature. Unlike endotherms, which generate heat internally, ectotherms absorb warmth from their environment, such as sunlight or warm surfaces. This group includes many species, such as reptiles, amphibians, and fish. Because ectotherms depend on their surroundings, their activity levels can change with temperature fluctuations. For example, they may become more active in warm weather and sluggish in cooler conditions. This adaptation helps them conserve energy, as they do not need to expend resources to maintain a constant body temperature.