Dr. Grey performed an experiment to test how temperature affects activity levels in mice. She put one group of mice in a cold environment and another group of mice in a hot environment. Everything else about the two environments was the same, including the type and amount of food and water. She allowed the experiment to last for a few generations of mice and tested the activity levels of the different groups. There was no change in the activity levels of the mice, but she noticed that the mice in the cold environment had much darker, thicker fur than the mice in the hot environment. What scientific question could she ask based on her results?