Assortative mating by size without a size-based preference: the female-sooner norm as a mate-guarding criterion

► The female-sooner norm is a new hypothesis to explain size-assortative mating. ► Males adopt a pairing decision rule based on female time to moult, not body size. ► It applies to several species of mate-guarding crustaceans. ► Mating preferences cannot be inferred directly from observed mating patterns.