We all think we know what males and what females are, right? Males make sperm and chase females to sow those wild oats as far and wide as possible, while females prudently try to avoid being inseminated by all but the most desirable males. But in 1972, American sociobiologist Robert Trivers showed us that sex roles are really not about sperm and eggs; they’re about who invests the most. If the males invest very little (just a few cheap sperm cells) and the females a lot (large eggs, belly space, and breeding time), it’s more important for females to be choosy; and you get the traditional sex roles. But if both sexes invest similar amounts of time and effort (like in humans and in co-breeding birds such as grebes and jackdaws), both sexes are choosy towards one another. There are even some animals that explore the other extreme of the scale: in cave lice of the genus Neotrogla, the males produce huge, nutritious sperm packages that the starving females are eager to obtain. So much so, that evolution has blessed them with a penis-like organ (the “gynosome”) with which they penetrate males (who lack a penis) and snatch their spermatophore from their abdomens. This example, published today in Current Biology, is the latest addition to a growing list of animals with reversed sex roles. Other examples include phalaropes, seahorses, and scirtid beetles.