Mouse twins separated when young: A history of exploration doubles the heritability of boldness and differentially affects the heritability of measures of learning

Most quantifiable traits exhibit some degree of heritability. The heritability of physical traits is often high, but the heritability of some personality traits and intelligence can also be highly heritable. Importantly, estimates of heritability can change dramatically depending on such variables as the age or the environmental history of the sample from which the estimate is obtained. Interpretation of these changing estimates is complicated in studies of humans, where (based on correlational observations) environmental variables are hard to directly control or specify. Using laboratory mice, here we could control specific environmental variables. We assessed 58 groups of four full sibling male CD-1 genetically heterogeneous mice (n = 232). Using a standard full-sibling analysis, physical characteristics (body weight and brain weight) were highly heritable (h of body weight = 0.66 on a 0–1 scale), while behaviors indicative of a personality trait (exploration/boldness) and learning abilities (in a passive avoidance and egocentric maze task) were weakly-to-moderately heritable. Half of the siblings from each set of four were housed in an “enriched” environment, which provided extensive and varied opportunities for exploration. This enrichment treatment promoted improvements in learning and a shift toward a more bold personality type. Relative to animals in control (“impoverished” environments), the history of enrichment had significant impact on estimates of heritability. In particular, the heritability of behaviors related to the personality trait (exploration/boldness) more than doubled, and a similar increase was observed for learning (in the passive avoidance task). Physical traits (brain and body weight), however, were insensitive to environmental history (where in both environments, animals received the same diet). These results indicate that heritable traits can be responsive to variations in the environment, and moreover, that estimates of heritability of learning and personality traits are strongly influenced by environments that modulate those traits.