Major Urinary Proteins: Re-examining the functions in chemosensory communication

House mice (Mus musculus) communicate through chemical signals, and their scent marks convey a surprising amount of information about an individual (e.g., social status, health, genetic disease resistance, and inbreeding load), and also mediate kin recognition and inbreeding avoidance. Understanding how chemical signals provide such information has been a major challenge. Male house mice produce large quantities of protein in their urine (major urinary proteins or MUPs), which transport pheromones into urine and stabilize the release of these volatiles from scent marks over time. MUP production is testosterone-dependent and males produce three to five times more than females, which is likely a substantial metabolic cost. MUPs are encoded by 21 functional loci, and individuals appear to produce a variable number of MUP isoforms in their urine. Several studies have proposed that MUPs are highly polymorphic and provide a unique individual signature or "barcode" that mediates individual recognition, inbreeding avoidance, and other mating preferences. Our general aim is to determine whether MUPs provide signals of individual compatibility or quality to potential mates, and our first aim includes testing the following hypotheses: (1) MUPs show high levels of inter-individual variation and intra-individual consistency, and MUP diversity mediates individual recognition (as the barcode hypothesis assumes); (2) MUPs mediate sibling recognition and inbreeding avoidance; (3) Females prefer to mate with males that have higher levels of heterozygosity at MUP loci. If MUPs do not show high levels of inter-individual variation, as our preliminary findings indicate, we can confidently rule out the barcode hypothesis and we will focus on another hypothesis. It has been proposed that MUP production provides an honest advertisement of males` health and condition to rivals and potential mates (quality indicator hypothesis), and we will test the following hypotheses: (1) MUP production functions to enhance males` social status or mating success; (2) MUP production provides an honest indicator of males` health and condition; (3) MUP production signals high genetic quality, which provides genetic benefits in mate choice. We will study wild and wild-derived house mice under laboratory conditions and in semi-natural populations, and we will utilize a number of tools in molecular genetics and protein chemistry. These experiments, which require integrative work between behavioral biologists, geneticists, and chemists, will help to determine the function of MUPs, and how chemical signals convey information about genetic quality and compatibility.

House mice excrete large quantities of major urinary proteins (MUPs) that bind and stabilize the release of volatile pheromones from urinary scent marks, and our aim was to investigate their proposed signaling functions. Our first aim was to test whether MUPs provide an individual odor signature or barcode that mediates individual and kin recognition (barcode hypothesis). We conducted a large survey of MUP variation in wild populations of house mice and found that MUPs do not show unique or stable individual profiles, contrary to previous claims. Using state-of-the-art protein analyses, we discovered that mice express around nine MUPs per individual, and using DNA sequencing we found no individual variation in MUP genes whatsoever. Therefore, our results show that MUPs cannot mediate individual or kin recognition. Our second aim was to examine MUP regulation and test whether MUP excretion provides a reliable signal of male social status. We studied house mice living in large seminatural enclosures and found that males increased the excretion of MUPs after they became socially dominant. Using olfactory assays, we found that females were more attracted to the odor of dominant than subordinate males, but this result was not due to male MUP concentration per se. Further analyses showed that dominant males excrete higher levels of a particular MUP known to attract females (MUP20 or darcin), and a volatile pheromone known to influence female reproduction. Thus, our findings indicate that male mice regulate the excretion of overall MUP concentration and particular pheromones depending upon their social status, though it is the later that enhances their sexual attractiveness. Finally, we investigated the hypothesis that MUPs function to bind and transport toxic waste products (toxic waste hypothesis). We unexpectedly found the presence of an industrial chemical in the urine of our mice, which is bound by urinary proteins, as predicted by the toxic waste hypothesis. In summary, our findings challenge a long-standing explanation for the function of MUPs (individual and kin recognition). They provide support for the hypothesis that MUPs are regulated according to male social status, which help explain the olfactory females olfactory attraction to dominant males. Our results help to better understand chemical communication in mammals, and provide an unexpected relationship between chemical signals and toxic waste elimination.