In most organisms it is the female who contributes the most to the reproduction of offspring as she must invest an egg and then carry the zygote. Males, on the other hand, need only provide a sperm that is of low cost. In “Pieris napi,” however, mating is unusually costly to males as the ejaculate matter produced contains not only sperm but accessory substances as well. These substances average 15% of male body mass and are incorporated into female soma and reproductive tissues during the mating process.[7] Therefore, the nuptial gift given by “Pieris napi” males qualifies both as paternal investment and mating effort. This system is unlike other types of butterflies such as the “Pararge aegeria” where female reproductive effort is independent of male ejaculate.[8]
The amount of ejaculate of virgin males during mating is larger than that of non-virgin males. Females therefore must mate more frequently with non-virgin males in order to obtain the necessary amount of male-derived nutrition.