Sexual cannibalism in the garden spider Araneus diadematus

In natural populations, courting males of Araneus diadematus are often consumed by females before they have successfully copulated. Despite the possible nutritional benefits of sexual cannibalism for females, the male can derive no benefit by being consumed before copulation. In this study, females that consumed a single male significantly increased their body mass, regardless of the quality of their diet. The implication is that, for A. diadematus, sexual cannibalism increases female fecundity. In experimentally controlled courtship sequences, larger males were less likely to be cannibalized than smaller males, but female size had no effect on male mating success. The mating success of males was not influenced by the age of the male, indicating that cannibalism is not the result of male senility. Arachnids are notorious for the predatory nature of their mating and courtship behaviour, and a variety of male adaptations have evolved apparently in response to the rapaciousness of the female. For example, males of the crab spider, Xysticus, throw several lines of silk over the female before attempting to copulate with her, although this does not appreciably inhibit her movements (Bristowe 1958). The jaws of female Tetragnatha orb-weavers are held open with specialized curved spurs on the jaws of the males (Bristowe 1958). Males of several araneid orb-weavers attempt to mate with ecdysing females when cannibalism is physically impossible, while in other species courtship is not initiated by the male until the female is feeding on a prey item (see Robinson 1982 for a review). The nuptial gifts offered by males to the females of the nursery web spider, Pisaura mirabilis, may also be a mechanism for reducing cannibalism (Bristowe 1958) although Austad & Thornhill (1986) found little supporting evidence for this. Despite the appeal of sexual cannibalism in the popular literature, no study has recorded more than anecdotal observations of the relative frequency of cannibalism in spiders; for example, cannibalism has been noted in dictynids (Jackson 1979), saiticids (Jackson 1980) and araneids (e.g. Robinson & Robinson 1980). Jackson (1980) reported cannibalism in 1% of 1834 interactions between courting males and females, although it is not clear how many different individuals this represented. Furthermore, these studies have not * Present Address: School of Biological Science, University of New South Wales, P.O. Box 1, Kensington, Sydney, N,S.W. 2033, Australia. always indicated whether cannibalism occurs before, during or after copulation. The evolutionary significance of sexual cannibalism after, or during copulation has been examined theoretically by Buskirk et al. (1984). Their model shows that a male may increase his fitness by being eaten by his mate if the chances of subsequent matings are low, and the nutritional value of his body makes a significant difference to the successful development of his offspring. Thus, under some circumstances, males may be expected to sacrifice their lives after or even during copulation. However, sexual cannibalism in orb-weaving spiders commonly takes place before copulation (e.g. Austin & Anderson 1978; Robinson & Robinson 1980) although there are exceptions (see Grasshoff 1964; Blanke 1983). The model proposed by Buskirk et al. (1984) does not apply in this case; while the female may still benefit from a nutritious meal, there is no benefit of this behaviour to the courting male. Gould (1984) criticized Buskirk et al.'s (1984) model and claimed that while sexual cannibalism with active male complicity may be favoured, it has evolved rarely, if ever. Instead, he argued that the indiscriminate predatory behaviour of females may have evolved for some other reason, and that some males are eaten despite any adaptations to escape (see also Jamieson 1986). Bristowe (1958) and Jones (1983) have suggested that older males are less agile, and that sexual cannibalism is more likely for older males. Other attempts to evaluate the evolutionary significance of sexual cannibalism have focused on its influence on the courtship behaviour of spiders (e.g. Robinson 1982).