Fishing spider : evolutionary by - product

Fishing spider: evolutionary by-product Sexual cannibalism, per se, need not be adaptive for either sex. In a recent study of the fishing spider, Arnqvist and Henriksson4 argue that sexual cannibalism is a by-product of strong selection for female 'rapacity', or willingness to attack prey items. In this species, female fecundity is determined by the size that the female achieves in her final juvenile instar rather than as an adult. Rapacious females are likely to eat more, grow larger as juveniles, and subsequently be more fecund as adults than are less-rapacious females. Arnqvist and Henriksson hypothesize that a correlation between juvenile and adult rapacity accounts for sexual cannibalism in this species. Rapacious females of this species evidently gain little by being cannibalistic as adults; cannibalistic females do not become more fecund, and they suffer low fertilization rates. Male D. fimbriatus show no signs of complicity, and presumably do not benefit from sexual cannibalism. Thus the behavior is not likely to be adaptive for either sex in D. fimbriatus. Although the assumptions of Arnqvist and Henriksson's model have still to be tested, no other study has explored nonadaptive explanations of sexual cannibalism. The obvious effects of sexual cannibalism on male mating success and female fecundity have perhaps biased our view of why sexual cannibalism exists. As Arnqvist and Henriksson4 point out, sexual cannibalism may be an incidental by-product of selection on suites of correlated traits such as predation, territoriality or aggression. Researchers should bear this in mind when exploring the origin and maintenance of this phenomenon. Sexual cannibalism can have two potentially large effects on fitness that have made it particularly prone to adaptive explanations: it limits male future mating success to zero, and it increases female fecundity. Researchers have explored these aspects almost to the exclusion of all others. The recent work on Australian redback shiders and on fishing spiders opens the door to new lines of inquiry.