We‘ve found the presence of attractive or unattractive male mice among four littermate male mice by behavior-based measurement with video camera tracking system. This trend of preference disappeared by hiding male mice with four-layered air-permeable filter. Furthermore, genetically blind female mice showed completely different trend of preference against the same male mice set, indicating that appearance may be one of major factors of male attractiveness.
To increase or decrease male attractiveness, we‘ve tried several approaches.
1) One male mouse and one female mouse are bred as a mating pair for three months to and see whether the female develops a preference for the male.
2) When a female mouse is meeting with an unattractive male mouse, isoproterenol is injected into the female mouse just before the meeting to see if she will mistake it for a crush on the male mouse.
3) Optogenetic stimulation of dopaminergic nerves that project to the nucleus accumbens only when approaching an unattractive male mouse during the male preference test.
4) Unattractive male mice were given daily subcutaneous injections of testosterone for a week to see if their attractiveness would increase.
In these approaches, only testosterone injections were able to increase the attractiveness of the unattractive male mice. It is possible that testosterone directly affected the brain, influencing attitude and pheromone release, since the changes were seen within a week. We hope to elucidate the mechanism in the future.