27.12.2022: Male house mice produce ultrasonic vocalizations (USVs), which are surprisingly complex and when made audible for human ears, they sound like the 'songs' of songbirds and whales. The functions of male courtship USVs are not understood, but it is often suggested that their songs enhance female sexual receptivity. This idea was recently tested for the first time by researchers at the Konrad Lorenz Institute of Ethology.
Male pheromones induce sexual receptivity
It has long been known that male house mice produce pheromones that induce changes in female reproductive physiology and behavior, including activating and accelerating estrous cycling. Pheromone-mediated estrous induction was discovered in domesticated laboratory mice over 60 years ago by Wesley Whitten (hence, it is often called the "Whitten effect"). Therefore, researchers aimed to confirm the Whitten effect in wild house mice for the first time, and test whether male courtship USVs similarly induce estrous and sexual receptivity.
Testing the effects of multiple male stimuli
The researchers conducted their study with wild house mice and began by recording male courtship USVs. They monitored females' estrous stage using vaginal cytology for two weeks, while keeping females isolated from males and male stimuli, and then they continued monitoring estrus for two more weeks while exposing females to different male stimuli, either (1) recordings of male USVs (which requires special loud speakers), (2) male scent, (3) both male scent and USV, or (4) control scent and sounds. This deign allowed them to test whether exposing females to a combination of both male scent and USVs has a more pronounced effect than either type of stimulus alone. Finally, the researchers paired the females with males to test whether any of these stimuli influenced females' reproduction.
Male songs not as sexy as pheromones
The researchers confirmed that male odor enhanced female estrous cycling, whereas exposure to USVs had no effect. Females exposed to both male USVs and odor went through more cycles than those exposed to male odor alone, suggesting that USVs might enhance the effect of male odor; however, the effect was not statistically significant. After pairing the mice, the researchers found that females showing male odor-induced cycling produced their first litter sooner than controls, whereas exposure to male USVs did not have such an effect.
Thus, courtship songs of male mice do not appear to influence female estrous cycling, but the researchers do not completely rule out this idea just yet. Their results raise the possibility that male USVs enhance the effect of male pheromones on female sexual receptivity. This was the first study to confirm the Whitten effect in wild house mice, and the first to show that male scent has a stronger effect on female sexual receptivity than male vocalizations. The problem now is explaining why male scent is more influentical than their songs on female sexual receptivity.
The article "Male scent but not courtship vocalizations induce estrus in wild female house mice" by Simon Wölfl, Sarah M. Zala and Dustin J. Penn was published in Physiology & Behaviour.