Major urinary protein levels are associated with social status and context in mouse social hierarchies

Abstract

We have previously shown that male mice living in groups of 12 males establish and maintain stable linear social hierarchies with each individual having a defined social rank. However, it is not clear which social cues mice use to signal and recognize their relative social status within their hierarchy. In this study, we investigate how individual social status both in pairs and in groups affects the levels of major urinary proteins (MUPs) and specifically MUP20 in urine. We housed groups of adult outbred CD1 male mice in a complex social environment for three weeks and collected urine samples from all individuals repeatedly. We found that dominant males produce more MUPs than subordinates when housed in pairs and that the production of MUPs and MUP20 is significantly higher in alpha males compared with all other individuals in a social hierarchy. Furthermore, we found that hepatic mRNA expression of Mup3 and Mup20 is significantly higher in alpha males than in subordinate males. We also show that alpha males have lower urinary creatinine levels consistent with these males urinating more than others living in hierarchies. These differences emerged within one week of animals being housed together in social hierarchies. This study demonstrates that as males transition to become alpha males, they undergo physiological changes that contribute to communication of their social status that may have implications for the energetic demands of maintaining dominance.

Keywords: MUP20; creatinine; major urinary protein; social dominance; social hierarchy.

Figure 1.

Win–loss sociomatrices showing total wins and losses by each individual in group housing. Cells are colour-indexed from white (no wins) to darker colours (highest number of wins). Winners of each contest are listed in rows and losers are listed in columns according to I & SI rank order. All six cohorts formed a significantly linear social dominance hierarchy. Empty cells indicate that the frequency of aggressive interactions was 0. (Online version in colour.)

Figure 2.

β coefficients for pairwise comparison for rank comparisons from GLMM for differences in total MUP levels. Positive values indicate ranks in rows having higher MUP levels than individuals in columns. β coefficients in coloured squares are statistically significant and darker colour indicates lower p-value (*p < 0.05; *p < 0.01; ***p < 0.001). Standard errors of all β coefficients were ranged between 0.09 and 0.10. (Online version in colour.)

Figure 3.

Average daily levels of (a) total MUPs and (b) creatinine by social rank over three weeks of group housing. Data are boxplots showing median (horizontal bars), IQR (boxes) and 95% CI (whiskers).

Figure 4.

Daily levels of MUP20 by social status over three weeks of group housing. Data are boxplots showing median (horizontal bars), IQR (boxes) and 95% CI (whiskers). (Online version in colour.)