from http://www.scienceblog.com/cms/node/7843 : A biologist at Washington University in St. Louis has been studying genital size in the male mosquitofish. His findings? Female fish prefer males with larger sexual organs, so having a large organ is an advantage when mating. On the other hand, a large organ causes more 'drag' in the water when fleeing predators, so well-hung male mosquitofish are more likely to be eaten -- an obvious disadvantage. So, be glad you're not a fish!