INTERSPECIFIC VARIATION IN THE SIZE OF THE NUTRIENT INVESTMENT MADE BY MALE BUTTERFLIES DURING COPULATION.

In 1972 Trivers pointed out the importance of differences in parental investment between the sexes for understanding the behavioral differences between the sexes. He argued that because males usually invest little other than sperm in each of their offspring, selection should favor those males that maximize the number of eggs they fertilize, often by trying to copulate with as many females as possible. In contrast, females rarely gain by mating frequently (because they have a limited number of eggs) but instead should benefit by selectively mating with high quality males. This scenario applies well to most species of insects; however, there are clear exceptions in which males either offer a relatively large parental investment per offspring or provide nutritional rewards to females during copulation (Thornhill, 1976; Gwynne, 1983). In these species, as Trivers (1972) points out, some sex role reversal in the mating process is expected. Because a male's ability to invest in offspring or offer nutrients is presumably limited, "helpful" males are expected to show more choosiness than males in species that do not invest and females are expected to be more active in courtship to overcome male resistance to mating. Thus, for example, in giant water bugs (Belostomatidae), in which males provide considerable paternal care for the eggs, females are the initiators of courtship (Smith, 1979). Likewise, females of the Mormon cricket (Anabrus simplex) which receive a huge, nutritious spermatophylax from their mates, compete for males and attempt to induce males to copulate with them (in high density populations) but do not always succeed (Gwynne, 1981). In the Lepidoptera, recent evidence has shown that the secretions passed into the female's reproductive tract during mating may be viewed as an indirect paternal or nutritional investment in that the secretions are used, at least in part, for oogenesis by the female (Goss, 1977; Boggs and Gilbert, 1979; Boggs, 1981; Boggs and Watt, 1981). These secretions are contained within a cuticle-lined spermatophore formed in the corpus bursa of the female's reproductive tract by the male during copulation, and as loose secretion in the appendix bursa which if present is connected to the corpus bursa. Debate exists about whether these secretions represent a mating effort in that they insure fertilizations by causing the female to become unreceptive (Sugawara, 1979), or parental effort (see Low 1978; Alexander and Borgia, 1979). However, because these secretions cannot be produced in unlimited quantities by males and because they are assumed to have a positive effect of female fecundity they are viewed as having given rise to selection pressures that will shape mating behavior in the ways Trivers described for true parental investments (Gwynne, in press). Interspecific comparisons of lepidopteran mating behavior reveal that there is surprisingly little variation in the overall structure of mating behavior from species to species (Scott, 1972; Silberglied, 1977; Rutowski, 1982). In his review of lepidopteran mating behavior, Silberglied summarized by saying that "with few exceptions courtship follows a single basic pattern throughout the order" (p. 376). This apparent lack of strong interspecific differences in behavior suggests minor dif-