Living in large social groups can be advantageous for survival, but the neural mechanisms that regulate affiliation with large peer groups are poorly understood. Now, Fricker et al. have studied the behavioural tendency to associate with different group sizes in spiny mice (Acomys dimidiatus), which, in their natural habitat, live together in large mixed-sex groups. The authors found that an anterior cingulate cortex (ACC) to lateral septum (LS) circuit drives a preference for affiliation with larger over smaller groups.

The LS is a hub for social behaviours, including social recognition, and it has afferent and efferent connections with brain areas involved in social information processing, including the ACC. Using viral tracing techniques and expression of the immediate early gene (IEG) Fos as a marker of neuronal activation, the authors found that LS-projecting ACC neurons responded more strongly to exposure to a large group of unfamiliar same-sex conspecifics than to a small group.