A non-sperm transferring genital trait under sexual selection: an experimental approach

Male genitalia are among the fastest evolving morphological characters, and at a general level sexual selection seems to be involved. But experimental determination of the functions of many remarkable genitalic elaborations is very rare. Here we present the first study to address experimentally the adaptive function of a male genital structure that is not involved in sperm transfer. Females of the orb-weaving spider Argiope bruennichi are sexually cannibalistic and polyandrous. The male increases his paternity by obstructing the female's insemination duct with a fragment of his complex genitalia (embolus tip). We manipulated males by detaching another species-specific structure, the median apophysis spur, and found that the spur promotes breakage of the embolus tip inside the female duct, but does not affect the probability and duration of copulation. These data are novel in that they suggest that a genitalic structure which does not transfer sperm nevertheless evolved in the context of sperm competition.