Final answer:
When faced with a sudden decrease in environmental temperature, endothermic animals conserve heat by using their fur, feathers, or fat for insulation, and through physiological responses like shivering and 'goose bumps'.
Step-by-step explanation:
When faced with a sudden drop in environmental temperature, an endothermic animal will typically conserve heat to maintain its body temperature. These animals possess adaptations such as fur, feathers, and fat that provide insulation and create an insulating layer of air between their skin and internal organs. For instance, mammals may increase body heat production through shivering, which is an involuntary increase in muscle activity. The contraction of arrector pili muscles causes hairs to stand on end ('goose bumps'), which can help to retain heat. Additionally, animals in cold climates like polar bears and arctic foxes have thick fur or use their fluffy tails as extra insulation to conserve heat.
Although hibernation, aestivation, or migration are methods to survive extreme temperatures, when specifically faced with an immediate drop in environment temperature, maintaining warmth through heat conservation strategies is the most direct and immediate response.