One manifestation of individualization is a progressively differential response of individuals to the non-shared components of the same environment. Individualization has practical implications in the clinical setting, where subtle differences between patients are often decisive for the success of an intervention, yet there has been no suitable animal model to study its underlying biological mechanisms. Here we show that enriched environment (ENR) can serve as a model of brain individualization. We kept 40 isogenic female C57BL/6JRj mice for 3 months in ENR and compared these mice to an equally sized group of standard-housed control animals, looking at the effects on a wide range of phenotypes in terms of both means and variances. Although ENR influenced multiple parameters and restructured correlation patterns between them, it only increased differences among individuals in traits related to brain and behavior (adult hippocampal neurogenesis, motor cortex thickness, open field and object exploration), in agreement with the hypothesis of a specific activity-dependent development of brain individuality.

Individuals differ in their response to environmental stimuli, but the stability of individualized behaviors and their associated changes in brain plasticity are poorly understood. We developed a novel model of enriched environment to longitudinally monitor 40 inbred mice exploring 35 connected cages over periods of 3 to 6 months. We show that behavioral individuality that emerged during the first 3 months of environmental enrichment persisted when mice were withdrawn from the enriched environment for 3 additional months. Behavioral trajectories were associated with stable interindividual differences in adult hippocampal neurogenesis and persistent epigenetic effects on neuronal plasticity genes in the hippocampus. Using genome-wide DNA methylation sequencing, we show that one-third of the DNA methylation changes were maintained after withdrawal from the enriched environment. Our results suggest that, even under conditions that control genetic background and shared environment, early-life experiences result in lasting individualized changes in behavior, brain plasticity, and epigenetics.

We analyzed recruitment -environment relationships for 5 distinct Baltic Sea herring stocks inhabiting the areas of the Western Baltic (WBH), the Main Basin (MBH), the Gulf of Riga (GRH), the Bothnian Sea (BSH) and the Bothnian Bay (BBH). A number of hydro-climatic and biological predictors were tested for their effect on recruitment. Temperature was determined to be an important predictor for 3 of the stocks (MBH, GRH and BSH). However, spawning stock biomass was the major factor explaining recruitment for GRH and weight-at-age of the spawners was an important predictor of MBH recruitment. For 2 out of 4 stocks for which complete zooplankton data were available (BSH and MBH), food supply was also a significant predictor, suggesting that changes in climate and/or food web structure may indirectly affect herring recruitment via prey availability for the recruits or spawners. Our results emphasize both similarities and differences in the main regulators of recruitment dynamics for the 5 stocks that should be taken into consideration in the development of area-specific management strategies throughout the Baltic Sea basin.