Scent-marking displays provide honest signals of health and infection

Males of many species produce scent marks and other olfactory signals that function to intimidate rivals and attract females. It has been suggested that scent marks provide an honest, cheat-proof display of an individual’s health and condition. Here we report several findings that address this hypothesis in wild-derived house mice (Mus musculus domesticus). (1) We exposed males to female odor, which induces an increase in testosterone, and found that sexual stimulation significantly increased the males’ scent-marking and the attractiveness of their scent marks to females. (2) We challenged sexually stimulated males with a nonreplicating strain of bacteria (Salmonella enterica C5TS) to activate immunity and found that this significantly decreased the males’ scent-marking and the attractiveness of their scent marks to females. (3) We collected scent marks from infected and sham-infected males when they were sexually stimulated or not, and we found that females could significantly discriminate the scent marks of infected versus control males, but only when the males were sexually stimulated. Taken together, our results indicate that male mice modulate their scent-marking display depending on their health and perceived mating opportunities. Increased scent marking enhances males’ attractiveness to females, scent marks provide an honest indicator of bacterial infection (and perhaps immune activation), and females are able to assess the health of males more easily when males mark at a high rate.