Male insects of many species donate nutrients to their females at mating, and the females can use these nutrients for egg production and somatic maintenance. These male-derived nutrients represent male investment in reproduction. The relative investment by each sex in reproduction has been postulated to be a determinant of the mating system and possible operation of sexual selection. However, the effect of male nutrient donations on female fitness is not well understood. I examine the effect on female fitness of male nutrients donated at mating in terms of the role of male nutrients in the female's nutrient budget for egg production. Nonnutritional constraints on total egg mass, the timing of egg production, the quality and quantity of adult female feeding, and alternative functions of male nutrient donations are incorporated into a model describing the role of male-derived nutrients. The model predicts the conditions under which changes in the availability of male-derived nutrients to individual females will be accompanied by changes in female fecundity, in adult female feeding, and/or in the allocation of the total nutrient pool to egg production. The model also can be used to predict the relative importance of male nutrient donations to female reproduction and hence the likelihood of sexual selection mediated by nutrient donations. Furthermore, the model can be used to predict associations of reproductive and foraging traits within a species or population. Data from the literature corroborate the assumption that male-derived nutrients may differ in composition from nutrients available to the female from adult feeding. Literature data also support the predictions that changes in male nutrient donation result in changes in female fecundity only when adult female feeding is restricted and that, given the opportunity, females on restricted adult diets ingest more nutrients from males. Data are not currently available to test other predictions of the model adequately. Finally, new data for two species of Lepidoptera with rich adult diets are presented that support the model's prediction that adult female feeding and the acquisition of male nutrients via mating should be inversely correlated when fecundity is constrained by nonnutritional factors. Thus, male-derived nutrients may affect female fitness through effects on foraging as well as on fecundity.
