Criteria for demonstrating postcopulatory female choice.

The recent discussion of Birkhead (1998) may cause confusion regarding the nature of cryptic female choice and how to test for its occurrence. Here I attempt to (1) clarify the distinction between cryptic female choice and ‘‘sperm choice,’’ the primary focus of Birkhead’s discussion; (2) present more appropriate criteria for demonstrating cryptic female choice; (3) describe errors resulting from an either-or approach to male and female effects on sperm usage; and (4) discuss the usefulness of the kinds of indirect evidence that are often crucial in testing evolutionary hypotheses. I will treat each of these points in turn.