Same-sex mounting in single-housed C57BL/6J female mice is not coupled to estrous state

Female-female mounting is widespread in mammals. Although the factors that regulate the display of female-female mounting likely vary according to species and to behavioral context, a large body of work has investigated the relationship of female-female mounting to female hormonal status. While this relationship has been well explored in ovariectomized and hormonally manipulated female rodents, less is known about the potential relationship between female-female mounting and estrous state in naturally cycling female rodents. Recent studies found that short-term social isolation robustly promotes same-sex mounting in C57BL/6J female mice. In the current study, we tested whether displays of female-female mounting by naturally cycling single-housed C57BL/6J females during interactions with naturally cycling, group-housed stimulus females depend on the estrous state of either female (n = 34 independent pairs). We found that the likelihood of mounting by single-housed females did not differ according to their estrous state. Moreover, single-housed females displayed mounting in trials that included receptive stimulus females, as well as in trials that included non-receptive stimulus females, and same-sex mounting was observed across all resident-stimulus female estrous state combinations. These findings indicate that same-sex mounting by single-housed C57BL/6J females is not coupled to the estrous state of either female in the pair. Future work remains to determine the factors that regulate the occurrence of female-female mounting in this species and behavioral context.