Magnificent Moustaches Make Male Mexican Mollies Miles More Macho

As the name suggests, Mexican Mollies (Poecilia sphenops) originate from small rivers, creeks and lakes in Mexico and their domesticated relatives have been familiar aquarium fish for years in the UK. These fun little fish have complicated mating patterns, with the males rather unusually fertilising the females internally instead of laying sperm over the eggs externally like most species. But the males are even more unusual in that some of them sport "moustache-like" epidermal outgrowths from the edge of the scales on their top lips.

These growths don't appear to provide any type of sensory function and their size isn't linked to the size of the fish, so scientists were initially at a loss to explain these extravagant facial features. However a new study discovered that female fish find the moustaches sexually attractive and they are therefore likely to be a sexually selected trait.

The results of the study were conclusive, showing that in experiments involving over 100 fish caught from the wild, females consistently preferred those males with moustaches - and the longer the moustache, the better. But rather than just being visually attractive, the moustaches also served a tactile function.

Since the mating behaviour of P. sphenops involves rubbing of the male’s snout against the female's genital region before mating (a behaviour known as 'nipping'), it seems that the moustache may also convey important tactile signals to the female, allowing the male Mexican molly to advertise his attractiveness. Those males that were lucky enough to boast these moustaches therefore had a distinct advantage when it came to mating, and if the study data is correct, size does indeed matter.