Male damselflies detect female mating status: importance for postcopulatory sexual selection

The existence of postcopulatory sexual selection is now clearly established in many animal species. In Odonates, males remove sperm during copulation from the bursa copulatrix and (when physically accessible) from the spermatheca. We used these model organisms to test the relative importance of sperm competition and cryptic female choice for copulation duration in laboratory experiments. If long copulations evolved only because of sperm competition, males should prolong copulation with previously mated females, and use this extra time to remove/displace the stored sperm. In species without a spermatheca (or when it is physically inaccessible), copulation duration should be similar in mated and virgin females. The cryptic female choice hypothesis predicts that copulations should be prolonged (acting as copulatory courtship) when males cannot physically remove sperm from the spermatheca but not if females do not have a spermatheca. We found that male damselflies can detect the mating status of females probably using chemical sensilla in their genitalia. Copulation duration with mated females was almost twice as long as with virgins in species with a spermatheca, but this behaviour was probably not the result of sperm competition, because in our model species, males could not remove sperm from this organ. The duration of copulation did not increase in species without a spermatheca. We conclude that even in odonates, where sperm removal is widespread, females have retained control over sperm reserves in their spermatheca(e), and males prolong copulation with mated females to elicit rival sperm ejection and/or to induce females to use their sperm in fertilization.