By Matt Walker
Editor, Earth News
A male mexican molly with moustache
Male molly fish in Mexico wear a moustache that makes them appear more sexy to females.
Scientists were unsure why male Mexican mollies wear an extravagant moustache-like structure on their top lip.
Now a study has revealed that female fish find the moustache sexually attractive, and it is likely to be a sexually selected trait.
As well as being visually-attractive, the moustache may be used to rub the female fish's genitals, exciting them.
Details are published in the journal Behavioral Ecology and Sociobiology.
As their name suggests, Mexican mollies (Poecilia sphenops) live in Mexico, being a fairly common species that resides in a range of habitats from small rivers to creeks and lakes.
The fish has a complicated mating behaviour, with males fertilising females internally, rather than spreading sperm over externally laid eggs.
Not all males are equal, however.
A close up of the sexy organ
Some possess a moustache-type structure growing from scales on their top lip.
Until now, its function has not been known as it has not been studied or described in any detail.
So zoologist Professor Ingo Schlupp of the University of Oklahoma in Norman, US and colleagues in the US and Germany decided to study it for the first time.
They caught from the wild a selection of male and female Mexican mollies, measuring the length of the moustache on those males found to be growing one.
The research team then conducted a series of experiments, placing male and female fish into tanks, and measuring how long females spent in the company of males sporting moustaches of various lengths, or none at all.
They also measured how female fish responded to videos of different males.
The results were clear: on experiments involving over 100 fish, females consistently preferred males with moustaches.
"The moustache-like structure seems to be a previously unrecognised sexually selected trait in poeciliid fishes," the researchers write in the journal.
Although the scientists only tested the visual attractiveness of the moustache, they strongly suspect it also has a tactile function.
"This is based on the general observation that males will touch the female's genital region with their mouth prior to mating," Prof Schlupp told the BBC.
This behaviour is known as 'nipping' and is being investigated further by the scientists.
But they think that the females can acquire information about the males this way.
In short, rubbing his moustache against a female's genitals may be a way for a male Mexican molly to advertise his attractiveness.
"Several other species, including catfish, have comparable structures, mostly with unknown functions," Prof Schlupp says.
Some zoologists have speculated that the long fleshy tentacles grown by male catfish might mimic larvae, allowing them to catch more prey.
That ability to catch more food might make them attractive mates to females.