Male insemination decisions and sperm quality influence paternity in the golden orb--weaving spider

In polyandrous species, paternity may be influenced by the timing and frequency of mating. Female spiders possess 2 genital openings that lead to separate sperm-storage structures. Thus, even when mating with a previously mated female, a male may reduce direct sperm competition by inseminating the opposite opening to her first mate. Such morphology may provide females with greater control over paternity. We examined simultaneously whether males avoided already inseminated female genital openings and whether this behavior varied with the time between successive matings. To explore these questions, we mated female golden orb weaver spiders, Nephila edulis, each to 2 males and manipulated the timing of their second mating. We documented male insemination patterns and explored the influence of male mating decisions on paternity success using the irradiated male technique. We found that 60% of males avoided sperm competition by discriminating against inseminated genital openings. Moreover, male mating behavior had a dramatic impact on the paternity success of irradiated males. When males inseminated the same genital opening, the competitive ability of the irradiated male's sperm was dramatically reduced resulting in lower paternity success. In contrast, when the 2 males inseminated opposite genital openings both males sired equal proportions of offspring regardless of their radiation status. There was no evidence that the timing of the second mating affected patterns of paternity. Our data suggest that differences in sperm quality may influence paternity success of N. edulis males under a sperm-competitive scenario. In contrast, females appear to have limited postmating control over paternity. Copyright 2008, Oxford University Press.