In areas where Azurill, not Marill, come out of Marill eggs, females make up three of every four births to Marill and Azumarill mothers. Yet after the Azurill evolve, males and females are equal. This is because an Azurill's large, bouncy tail is one of the most remarkable ways to store and alter genetic material that nature has ever devised; one based not on the circumstances of birth, but on ability and merit.
Unlike most rodents (and indeed mammalian pokemon), the reproductive organs of the Marill line are stored in the tail. In Azurill, and to a lesser extent in its evolutions, the tail is also used for everything from battle to locomotion, and it is how it is used in this phase which determines a "female" Azurill's adult gender. Marill and Azumarill females lay many eggs which take a long time to hatch, and must be strong enough fighters to protect their clutch from predators. Consequently, a tail used more often as a bludgeoning weapon creates a female in adulthood. The male of the species does not participate in raising children, but must woo females with grand splashing displays and maximizes reproductive success by preserving its own survival. This is why Azurill tails used for splashing and bouncing vast distances develop into male organs.
It was once thought that Azurill warriors became women and the weak became men, but this is untrue; the quick and the flashy become men and the weak get eaten by predators. Yet far more choose to stay and fight, so one in four Azurill must be made male at birth to maintain balance.
This method of gender selection, although unusual, is astonishingly successful; Marill are far more common in places near Sea Incense than in places where they are born into that form.