We perform a perturbation analysis (comparative statics) of how optimal reproductive effort and per offspring investment are jointly affected by diferent selective factors. The factors considered are: (1) mortality sources, classified according to affected stage ( juvenile or adult) and to its nature (avoidable or unavoidable), and (2) resource (energy) availability for the adult individual. The joint approach reveals both direct and indirect effects of each selective pressure. These interactive effects spring from the nonlinearity of reproductive expenditure, separated into a part devoted to endowing offspring (provisioning cost) and another part invested to make reproduction possible (requisite cost). The latter is envisioned as a reverse sigmoid function of fecundity (most models, so far, have considered only the first kind of cost). The indirect e!ects have the consequence of enlarging the class of selective pressures that can induce changes of offspring size and clutch size, as compared with current explanations. So, they illuminate new causes for some effects, and show new effects for some well-known selective causes. Several joint patterns in the two variables, shown by animals and plants in the field, can thus be given more appropriate interpretations than traditional, piecewise, ones.Instituto de Zoología y Ecología Tropical, Facultad de Ciencias, UCV.