Why molly fishes?

Why mollies?  This is the question I most often hear regarding my master’s thesis research.  To be honest, I find myself wondering that same question when I’m submerged up to my shoulders in stinky fish water scrubbing the walls of their 150-gallon tanks.  Why do I do this?  Well, sailfin mollies (Poecilia latipinna) are an excellent example of a species subject to strong sexual selection.  Just like the male peacock with his outrageous tail feathers, male mollies have developed an enormous dorsal fin that serves to attract females.  How do such traits evolve?  One hypothesis – the handicap principle – suggests that because these enlarged traits are a burden to have, they must signal high mate quality to interested females.  So the debate is over – bigger is better, at least in the animal kingdom!  But wait – what’s a small male to do?  Well, smaller, less preferred male mollies have adopted an alternative mating strategy in which they try to sneak mating attempts on unsuspecting females.  While not so nice, these strategies are equally successful as the courtship strategy employed by large males (see courtship video below), so both strategies, sneaking and courting, are maintained in wild molly populations.  Pretty cool if you ask me.    

male courtship display

hey girl, do you like my tail feathers?

large male (50mm)

small male (25mm)