Choosing a healthy mate: sexually attractive traits as reliable indicators of current disease status in house mice

Social interactions are critical for reproduction in many animals. Since several pathogens are transmitted by social contact, females searching for mating partners should select males that can signal being healthy. Not all signals, however, may be reliable, since males from a number of species can overcome behavioural symptoms of infection when mating opportunities are available. Here, we manipulated sickness status of male house mice, Mus musculus domesticus, by administering an immune challenge (lipopolysaccharide, LPS) and studied the consequences of this manipulation for two signals that function in mate attraction in this species: ultrasonic vocalizations and darcin (a urinary protein). Additionally, we quantified female visits to immune-challenged and control males, and the males' plasma testosterone levels. LPS-injected males had lower darcin and lower regular ultrasonic syllable production than control-injected males, while producing a larger number of high-frequency ultrasonic syllables. We conclude that immune-challenged male mice presented with a receptive female cannot maintain the production of sexually attractive signals. Females might use some of these cues when making mating decisions, since they spent significantly less time near LPS-injected males. Testosterone was reduced in LPS-injected males and could be a unifying mechanism downregulating both of the traits quantified. Darcin and ultrasonic vocalizations produced in the context of courtship may therefore function as reliable indicators of current health status.