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Mate-copying is a form of social learning in which witnessing sexual interactions between conspecifics biases an observer individual's future mate-choice. Mate-copying exists in many vertebrates, as well as in Drosophila melanogaster. Here, we show that female fruit flies can copy the choice for mutant males (Curly-wing [Cy] mutants vs. wild types [WTs]) despite the fact that mating with Cy males induces a significant fitness cost for the observer female. When facing WT and Cy males, naive observer females of both phenotypes naturally prefer WT males. In a mate-copying experiment, naive observer Cy or WT females saw a demonstrator female copulating with either a Cy or a WT male aside a lonely male of the opposite phenotype. In the subsequent mate-choice test, the Cy and WT observer females did not change their already high natural preference for WT males after witnessing a WT male copulating …