Social communication in the house mouse relies heavily on pheromone-carrying major urinary proteins (MUPs), which delay release of pheromones thus extending longevity of the scent signal. Moreover, MUPs appear to play an important role in individual recognition. In the last few years, new research has led to some important advances. It has been shown that MUPs without their volatile ligands are able to activate neurons in vomeronasal organ and elicit behavioural and physiological responses in the signal receiver. Furthermore, increasing evidence has been found showing that, contrary to the traditional view, MUP expression is condition and state dependent, and that this variation may provide additional information about an individual. Progress has also been made in the description of MUP-like proteins in other rodents; as yet, however, the protein variability typical of the house mouse has not been observed in any other species. Despite these new results, the concept of MUPs has remained more or less unchanged from the date they were first recognized as an identity signal. The aim of this review is to summarise recent knowledge about MUPs and to discuss previous findings in the light of novel facts. Special attention is paid to the consequences the new results may have on our understanding of the individual recognition role of MUPs.
The presence of large amounts of proteins in mammal urine is usually associated with a pathological condition and indicates serious renal lesions. However, there are few species with obligate proteinuria indicating that they must derive some benefit from this condition. Urinary proteins have been most extensively studied in the house mouse and the rat, and findings to date indicate that their function in intraspecific communication is complex and not yet fully understood. Other proteins of the same protein family as MUPs have been also found in urine of some other rodent species, and still less is known about these. In this study we demonstrate the existence of urinary lipocalins in Mastomys coucha for the first time. Our results support the hypothesis that urinary proteins may play an important role in chemical communication, and level of polymorphism of these proteins in different rodents may help us to understand their specific function.