In our society, we think of anxious behavior as being a disadvantage. But that's because, for the most part, we live in a nutrient-rich, low-danger environment. In the rat equivalent to our world, the relaxed rat lives a comfortable life. It is likely to reach a high social standing, and it doesn't have to worry about where its next meal is coming from. An anxious rat, on the other hand, doesn't do so well. It is more likely to have a low social standing and suffer from diabetes and heart disease.
In another environment, however, the tables turn. The anxious, guarded behavior of the low-nurtured rat is an advantage in an environment where food is scarce and danger is high. The low nurtured rat is more likely to keep a low profile and respond quickly to stress. In the same environment, a relaxed rat might be a little too relaxed. It may be more likely to let down its guard and be eaten by a predator.