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Abstract
Male animals typically attempt to mate with multiple females in order to increase their reproductive success. In some species, however, males instead invest in fertilizing the eggs of a single female. Monogyny (male monogamy) is found in a diverse assemblage of taxa, and recent theoretical work reveals that a male-biased sex ratio can favor the evolution of this relatively rare mating system. We integrate this theoretical framework with field observations and laboratory experiments involving the sexually size dimorphic fishing spider, Dolomedes tenebrosus. Results from mating trials revealed a novel form of self-sacrifice behavior where males spontaneously die when they copulate, thus all males are monogynous. Since all males experience obligate death when they mate, we set out to determine if male self-sacrifice in D. tenebrosus is adaptive. Self-sacrifice behavior (complicity in cannibalism or spontaneous death associated with copulation) can be adaptive if it facilitates sexual cannibalism, and if sexual cannibalism results in reproductive benefits for the self-sacrificing male. In a first series of experiments, we examine variation in female cannibalistic behavior and variation in female mating rate. We test the hypothesis that spontaneous male death facilitates postcopulatory sexual cannibalism and that sexual cannibalism reduces the likelihood of female remating. We found that spontaneous male death does indeed facilitate sexual cannibalism, as all females cannibalize males postcopulation. However, sexual cannibalism does not reduce the likelihood of female remating, thus D. tenebrosus males do not appear to receive a paternity advantage from postcopulatory sexual cannibalism. In a second series of experiments, we examine how sexual cannibalism affects female fecundity. We test the hypothesis that the consumption of a male by a female postcopulation results in an increase in the quantity and/or quality of offspring. We found that sexual cannibalism causes females to produce more and higher quality offspring. Specifically, when females were allowed to consume their mating partner, it resulted in a significant increase in the number, the size, and the survivorship of offspring. Our results document a novel case of adaptive self-sacrifice in the first non-araneoid spider, D. tenebrosus, providing evidence that males benefit from sexual cannibalism through paternal effort.
