We perform a perturbation analysis (comparative statics) of how optimal reproductive e!ort and per o!spring investment are jointly a!ected by di!erent selective factors. The factors considered are: (1) mortality sources, classi"ed according to a!ected 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 e!ects of each selective pressure. These interactive e!ects spring from the nonlinearity of reproductive expenditure, separated into a part devoted to endowing o!spring (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 "rst kind of cost).
The indirect e!ects have the consequence of enlarging the class of selective pressures that can induce changes of o!spring size and clutch size, as compared with current explanations. So, they illuminate new causes for some e!ects, and show new e!ects for some well-known selective causes. Several joint patterns in the two variables, shown by animals and plants in the "eld, can thus be given more appropriate interpretations than traditional, piecewise, ones.