Sexual selection is alive and well in the animal kingdom... there are brilliant colors, structures, dances and songs to attest to the power of female choice when it comes to selecting a mate. 'Beauty' in the animal kingdom naturally translates to a biologically fit and healthy individual, and the sexual structures we observe in the animal kingdom speak to a high level of fitness of the males that possess them. Honking horns, formidable phalanges, terrific teeth or fabulous feathers all attest to the quality of the genetic code carried within. However, sexual characteristics don't always need to be overly large or obvious in order to get the job done. Case in point: males of the Mexican guppy Poecilia sphenops seem to get by just fine with a little peach-fuzz on the upper lip.
Yes, you read that correctly.
Researchers hypothesized that epidermal outgrowths on the upper maxilla of these male fish (aka fish-staches) may be a sexually selected characteristic. Live-bearing fish of the family Poeciliidae are a well-studied system when it comes to sexual selection, and experimental research protocols for assessing female choice are therefore well established. Associative preference of females is strongly suggestive of mating success in guppies. In other words, if she chooses to hang out with you, you're pretty much guaranteed a shot at copulation. In order to address their 'Magnum P.I.' hypothesis, the researchers undertook a series of associative choice experiments using both live males (mustached versus cleanly shaven) and video playbacks of the same male either with his mustache or with it removed. Results from both experiments confirmed that the females preferred to associate with mustached males.
So what's in a mustache? Clearly there must be a biological function for this sexually selected trait. The authors suggest that the mustaches represent a multimodal sexual signal: they are appealing both visually (who can argue?) and tactilely. The rostral filaments have no sensory function for the male, but they are hypothesized to provide some 'stimulation' to the female. In a sexual behavior known as 'nipping,' males contact their snouts to the female genital area prior to copulation. If that snout happens to be home to several additional epidermal filaments we can perhaps assume that such contact is more tactilely pleasureable.
I'm not going to argue with that.