Female hunger can explain variation in cannibalistic behavior despite male sacrifice in redback spiders

Male Australian redback spiders (Latrodectus hasselti Thorell: Theridiidae) place their abdomens directly over their mate's mouthparts during copulation, increasing the likelihood of sexual cannibalism. Male sacrifice may be adaptive because cannibalized males increase their paternity relative to those that are not eaten. Despite male sacrifice behavior, however, up to 50% of laboratory matings may end without sexual cannibalism. Here, I report a similar pattern in the field, where males were not cannibalized in 35% of observed matings (6/17). I examined variation in female cannibalistic behavior by evaluating the following three hypotheses for the occurrence of cannibalism from the female perspective: (1) the mistaken identity hypothesis proposes that females sometimes cannibalize males because they mistake them for prey, (2) the mate rejection hypothesis predicts that females cannibalize males who are unacceptable as mates, and (3) the feeding opportunism hypothesis predicts that hungry females are more likely to be cannibalistic. Field observations refuted the first two hypotheses: females recognized males as potential mates (i.e., nonprey), and cannibalized and noncannibalized males were not phenotypically different. The feeding opportunism hypothesis was supported. In staged field matings, cannibalistic females were hungrier than their noncannibalistic counter-parts. Moreover, a logistic regression analysis indicated that hunger was a significant predictor of cannibalism. Because redback males are below the typical prey size that females accept, well-fed females are less likely to consume their mates, despite the vulnerable mating posture. These results indicate that, although males facilitate sexual cannibalism, their fate may depend on the female's physical condition.