Pattern of sperm transfer in redback spiders: implications for sperm competition and male sacrifice

Many sperm competition studies have identified copulation durationas an important predictor of paternity. This result is ofteninterpreted as a sperm transfer effect—it is assumed thatsperm transfer is limited by copulation duration. Here we testthe assumption of duration-dependent sperm transfer in the Australianredback spider, Latrodectus hasselti, in which a correlationbetween copulation duration and paternity has been implicatedin the evolution of a rare male self-sacrifice behavior. Maleredbacks facilitate sexual cannibalism by females during copulation.Sexual cannibalism is apparently adaptive for redback males,in part because it results in longer copulations (25 versus11 min.), and copulation duration is positively correlated withpaternity. We assessed sperm transfer in normal copulationsand in copulations that we terminated at 5, 10, or 20 min. Ourresults show that the paternity advantage of sexual cannibalismis not owing to time-dependent sperm transfer, as redback malestransfer the majority of their sperm within the first 5 minof copulation. This suggests that the link between copulationduration and paternity may instead be owing to cryptic femalechoice or the transfer of nongametic ejaculatory substances.Results further indicate that the act of cannibalism itselfmight play a role in mediating sperm transfer. This study highlightsthe importance of understanding mechanisms of sperm transferwhen attempting to interpret the outcome of sperm competitionstudies.