New neurons flatten social hierarchies.

As complex individualized behaviors in mice arise even when genes and environment are held constant, emerging social group structures should also depend on individual behavior. Adult hippocampal neurogenesis drives individual behavior and is involved in certain social behaviors. Would thus the emergence of social structure depend on adult-born neurons? To answer this question, we co-housed cohorts of female mice with intact and genetically diminished general adult neurogenesis in a large-scale automated tracking system for three months and analyzed the development of social behaviors over time. Not only did female mice develop stable hierarchies, we also found that, despite forming a functioning mouse society, their dominance behavior increasingly diverged over time. Specifically, neurogenesis-deficient mice showcased a greater increase in hierarchical behavior throughout the experiment. Within the social behaviors, mice showed no genotype-based preference, indicating intrinsic causes for behavioral differences. In mice with intact neurogenesis, social rank was positively correlated with adult hippocampal neurogenesis. Nevertheless, neurogenesis-deficient mice occupied higher ranks in the emerging hierarchy than their wild-type cohabitants, while displaying less individualization for this trait. The observed insistence on dominance might therefore reflect inflexible social strategies. Thus, social intelligence and flexibility, enabled by adult-born neurons, promote social adaptability, egality, and flatter hierarchies.