Although these two animals are both bears, clearly they are entirely different. Becasue they live in pretty much opposite environments, they developed individual traits so they could survive, which is called adaption. While the polar bear's thick fur and fat serve as great protection against the cold, it wouldn't be very helpful in hot climates. And the sun bear, with it's short hair and lack of protection, would freeze to death in the polar bear's environment. Each animal, over time, has to develop new traits that allow it to adapt to it's particular environment.
These 2 different types of polar bears, may be the same type of animal, but as given in that text, they differ. These 2 different types of polar bears have different types of survival skills, in order to adapt to their environment. The Polar bear in the Northern Hemisphere has very thick fur, and other thick parts in it's body to keep them warm in the ice-covered areas that it lives in so that it wont die, due to the cold. While on the contrary, the bear in the tropical rainforest only has short hair, to keep them at a just-right temperature, so it wont die due to how hot it is. If both of these bears were to switch their environment, then they will die easily since they are not well adapted to that environment.
Answer: it's because they need to provide a lot of energy.
Explanation: fat cells have many mitochondria because they store a lot of energy. Muscle cells have many mitochondria, which allows them to respond quickly to the need for doing work.
