Environmental Factors Affect Sexual Selection...

Fewer males in the environment changes a female butterfly's sexual selection. Presumably this is a chemical/hormonal pathway that responds to environmental changes to change the lust/promiscuity that the female butterfly feels.

Science & Technology at Scientific American.com: Female Butterflies Get Frisky When Males Become Scarce -- Researchers demonstrate how a parasite that ravages male butterflies drives females to seek multiple partners: "females do not resign themselves to forced virginity. Instead they become promiscuous scavengers, taking advantage of a single male's high capacity for mating"