In some species of dart-poison frogs (genus Dendrobates), males perform parental care. Some researchers have suggested that high relative parental investment by males in these species causes sex-role reversal. I argue here that, in general, three factors cause sexual selection: inequality of relative parental investment, variance in mate quality, and variability of parental investment. I review evidence from my previous research that contradicts the hypothesis that unequal relative parental investment can explain female mating strategies in two species of dart-poison frogs with male parental care. I then discuss the hypothesis that variability of male parental investment has an important effect on female mating strategies in these species.