Appearancesdeceive:female“resistance”behaviour inasepsidflyisnotatestofmaleabilitytoholdon

Female rejections of males are crucial events in sexual selection by female choice and sexually antagonistic coevolution, but there are few detailed studies of the process of rejection. Female struggles when mounted by males are often assumed to function to dislodge the male. But this study, in which female receptivity was manipulated by using females of different ages, showed that this “dislodgement” hypothesis is incorrect in a group (sepsid flies) in which energetic female shaking behaviour was previously interpreted as female attempts to dislodge males. Mounts in Archisepsis diversiformis often failed, but males were nevertheless seldom thrown off; instead, they almost always dismounted while the female was quiet. Males also showed signs of being in control of dismounts, as they dismounted more quickly if the female had recently been mounted by another male. Predictions from two other hypotheses for the function of female resistance behaviour also either failed or were not consistently supported: (1) females resist in order to filter males with respect to their ability to hold on to the female or outlast her resistance, or to court while mounted (“male endurance/ female exhaustion” hypothesis); (2) females resist in order to sense the male’s grip on her wings and thus filter males with respect to their species-specific clamps or to elicit other male courtship (“male screening” hypothesis). Several predictions of a further possibility, that (3) females resist in order to communicate their lack of receptivity to the male, and to induce him to leave (“communication” hypothesis), were confirmed. Although one type of data did not fit easily with the communication hypothesis, overall it was the most likely explanation for female shaking behaviour. Our results call into question conclusions from previous studies regarding male-female conflict in this and other groups, and suggest testable alternative hypotheses. A survey of behaviour in other flies (which are presumably indicative of other animals in this respect) indicates that female “resistance” behaviour probably has a variety of functions. In sum, facile interpretations of a forceful resistance function should not be accepted without careful analyses.