Males benefit from mating with virgin females for two reasons. First, they do not have to compete with rival males' sperm, ensuring paternity of all the eggs the female lays before she remates. Secondly, fecundity tends to decrease with female age in many insects, and hence virgin females are also the most fecund. The higher returns associated with mating with virgin females exert strong selection on males to become sexually mature before females, termed protandry (Wiklund and Fagerstrom, 1977). Protandry is also beneficial for females by minimizing the risk of dying unmated, since it reduces the time between emergence and mating (Fagerstrom and Wiklund, 1982). Female butterflies need to eclose at a time when suitable host plants for egg-laying are available, and males are often selected to eclose before females in order to maximize their mating success. This is most likely to occur in species with female monogamy or where there is a first-male mating advantage (e.g.
Wiklund and Fagerstrom, 1977; Bulmer, 1983; Iwasa et al., 1983; Wiklund et al., 1992; Wiklund and Kaitala, 1995). Early male emergence can also evolve in response to sperm competition in species where males, by mating with virgin females, fertilize a larger number of eggs than by mating with already mated females (e.g. Wedell, 1992). Protandry may also be the incidental by-product of selection for other life-history traits (e.g. body size), and not be selected per se (Wiklund and Solbreck, 1982; Thornhill and Alcock, 1983).
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