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

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Abstract
Many sperm competition studies have identified copulation duration as an important predictor of paternity. This result is often interpreted as a sperm transfer effect—it is assumed that sperm transfer is limited by copulation duration. Here we test the assumption of duration-dependent sperm transfer in the Australian redback spider, Latrodectus hasselti, in which a correlation between copulation duration and paternity has been implicated in the evolution of a rare male self-sacrifice behavior. Male redbacks facilitate sexual cannibalism by females during copulation. Sexual cannibalism is apparently adaptive for redback males, in part because it results in longer copulations (25 versus 11 min.), and copulation duration is positively correlated with paternity. We assessed sperm transfer in normal copulations and in copulations that we terminated at 5, 10, or 20 min. Our results show that the paternity advantage of sexual cannibalism is not owing to time-dependent sperm transfer, as redback males transfer the majority of their sperm within the first 5 min of copulation. This suggests that the link between copulation duration and paternity may instead be owing to cryptic female choice or the transfer of nongametic ejaculatory substances. Results further indicate that the act of cannibalism itself might play a role in mediating sperm transfer. This study highlights the importance of understanding mechanisms of sperm transfer when attempting to interpret the outcome of sperm competition studies.