Differences in brain weight, a morphological trait associated with the level of animal cognition, is usually evaluated via comparison of the respective scores in animals of different taxa. Selection for large and small brain size (in laboratory mice) is a technique that allows this role to be evaluated at the level of intraspecies variability. Selection for contrasting values of this trait were stopped at the level of F22, and lines with large and small brain weight were randomly bred inside each line. It was found that brain weight and behavioral differences (the prevalence of large brains in the solution of cognitive tasks) between these lines were maintained without selection. At the same time, selection for high scores of cognitive-task solution resulted in higher brain-weight scores in the selected strain. Therefore, in laboratory mice (widely used by neurobiologists as a model) the brain weight values could be the indicator of cognitive capacities development.

Mice selected for high score in the extrapolation test (EX line) and kept under conditions of “enriched environment” for 3 months demonstrated changes in locomotor and exploratory activity and enhanced reaction to novelty. The relative brain weight was higher and neurogenesis in the hippocampal fascia dentate was more intensive in this group. In non-selected mice, the changes were similar, but insignificant in many cases.

Aggression level of male mice EX, which belonged to line, selected for the high scores of cognitive test solution [ability to extrapolate the direction of food bait movement after it disappears from animal view] was compared to that of male mice from control unselected population CoEX. The standard opponent test was used. At the start of selection [F5] EX mice were more aggressive, while at F10-15 the pattern of differences changed and CoEX mice were significantly more aggressive. These data could be partly explained by differences between EX and CoEX in their anxiety and reaction to novelty. Highlights: Selection for cognitive trait induced changes in the intermale aggression Intermale aggression is presumably related to anxiety level Less aggressive mice are not afraid of novel food