Social Context Alters Chemical Signalling and Endocrine Profiles in Two Mouse Subspecies

Chemical signalling and endocrine state are known to covary with social environment, but the speed of their response to social transitions under natural conditions remains unclear. We examined how major urinary proteins (MUPs) and faecal steroid hormone metabolites respond to shifts between solitary housing and a complex social setting in two house mouse subspecies, Mus musculus musculus and M. m. domesticus. Using repeated sampling in semi-natural enclosures, we measured MUP, testosterone, and corticosterone metabolite concentrations across three contexts: before social exposure, during the social phase, and after return to isolation. Reproductive output was quantified within a 44-day window around each sampling event. Entry into the enclosures triggered a strong, coordinated response: urinary MUP concentrations increased in both sexes and subspecies, paralleled by rises in testosterone and corticosterone metabolites during early social exposure. In males, this pattern reversed quite quickly after removal, with MUPs and both hormone metabolites declining within 24 h of renewed isolation. Under stable social conditions, urinary MUP concentrations increased with current reproductive output, whereas testosterone and corticosterone metabolites showed no such association. MUP concentrations were also unrelated to either hormone, suggesting that socially responsive MUP investment is not directly coupled to steroid levels. Despite baseline differences between subspecies and the expected male bias in MUP production, the main context-dependent patterns were consistent across taxa. Our results demonstrate pronounced social plasticity in chemical signalling and endocrine state, and indicate that MUP concentrations more closely reflect reproductive outcomes than steroid metabolite concentrations.