Obligate male death and sexual cannibalism in dark fishing spiders

Male dark fishing spiders (Dolomedes tenebrosus Araneae, Pisauridae) always die during their first and only copulation, making all males monogynous. Such obligate male death can be adaptive if it facilitates sexual cannibalism, and if sexual cannibalism results in male reproductive benefits, such as an advantage in sperm competition through reduced female remating. We first conducted an experiment to determine the extent to which D. tenebrosus (1) males are cannibalized by females and (2) females engage in remating, both of which are prerequisites for several adaptive hypotheses of male self-sacrifice. We then conducted an experiment to test the hypothesis that the cannibalism of the male by the female reduces the likelihood of female remating. We found that obligate male death appears to facilitate sexual cannibalism in D. tenebrosus; females always cannibalize males postcopulation. We also found that half of all females copulated with multiple males. Finally, we found no support for the hypothesis that cannibalizing a male reduces the likelihood of female remating. We were additionally able to document that even though males may appear to act as whole-body mating plugs immediately after death, by hanging from the female's genital opening, this rarely functions in preventing subsequent copulations (i.e. mate plugging). In summary, obligate male death and associated cannibalism in the dark fishing spider D. tenebrosus does not appear to function to reduce sperm competition through reduced female remating. •We examined pre- and postcopulatory cannibalistic behaviour of female Dolomedes tenebrosus.•We document the frequency of female multiple mating in this monogynous species.•Obligate male death appears to facilitate sexual cannibalism.•Postcopulatory sexual cannibalism appears to be obligatory.•Sexual cannibalism does not appear to reduce female remating.