Living things that are well adapted to their environment survive and reproduce. Those that are not well adapted don’t survive and reproduce. An adaptation is any characteristic that increases fitness, which is defined as the ability to survive and reproduce. What characteristic of the mice is an adaptation that increased their fitness?

An adaptation that increases a mouse's fitness could be physical, behavioral, or physiological.

1. Physical Adaptation: An example of a physical adaptation in mice might be fur color. Mice that live in a desert environment, for instance, may have lighter-colored fur that reflects sunlight and helps them blend in with their sandy surroundings to hide from predators. In contrast, mice living in a forested area might have darker fur to blend in with the dark soil and underbrush.

2. Behavioral Adaptation: A behavioral adaptation for mice could be nocturnal activity patterns. Being active at night reduces the chance of being seen by daytime predators. It also allows them to avoid the heat of the day in hot climates, which conserves water and energy.

3. Physiological Adaptation: A physiological adaptation might involve the mouse's metabolism. For example, mice living in colder environments might have a faster metabolism that generates more body heat, helping them to better withstand the cold temperatures.

The specific characteristic that is an adaptation depends on the environment the mouse lives in and the challenges it faces there.