Motherhood reshapes detection and response to male-derived chemosignals. From attraction to aversion?

Maternal aggression, the defence of the nest against conspecific adults (particularly males), is triggered by male-derived chemosignals that are otherwise attractive or rewarding to virgin females. This phenomenon suggests adaptations in the detection of male chemosignals by the vomeronasal organ (VNO) and/or in their central processing, during motherhood. This study aims to analyse if postpartum females undergo changes in VNO response to male-derived chemosignals that might explain the shift of behaviour towards males during motherhood. To investigate this, we exposed dyads of dams and virgin female mice to clean or male-soiled bedding to assess their chemoinvestigation (and sniffing/grooming to the cage mate) and processed their VNOs for pS6 immunohistochemistry as a proxy of neuronal activity. Mothers and virgin females showed similar chemoinvestigation of the bedding, with increased time exploring male-soiled bedding as compared to clean bedding. As for VNO activity, whereas in virgins male chemosignals increase pS6 immunostaining, dams show similar VNO activity in response to clean and male-soiled bedding. Furthermore, in dams but not virgins, VNO activity is negatively correlated with chemoinvestigation of the soiled bedding. In no case, the activity of the VNO is correlated with sniffing directed to the cage mate, thus suggesting a minor role of adult female pheromones in VNO activity in our experimental conditions. These findings show that motherhood modifies the VNO response to male-derived chemosignals, suggesting that during the postpartum period VNO functioning during chemoinvestigation of male-soiled bedding is modified maybe due to changes in the valence of male chemosignals.