Individual variation in behavior and physiology is a widespread and ecologically functional
phenomenon in nature in virtually all vertebrate species. Due to domestication of laboratory
animals, studies may suffer from a strong selection bias. This paper summarizes behavioral,
neuroendocrine and neurobiological studies using the natural individual variation in rats
and mice. Individual behavioral characteristics appear to be consistent over time and across
situations. The individual variation has at least two dimensions in which the quality of the�…