JOINT EFFECTS OF MATERNAL AND OFFSPRING SIZES ON CLUTCH MASS AND FECUNDITY IN PLANTS AND ANIMALS

The allometries of clutch mass and number of offspring per clutch are strikingly similar in data from eight different taxa - metatherians, e utherians, birds, reptiles, amphibians. fish, crustaceans and plants - drawn from the literature (n = 1451). Although clutch mass (C in g) w as largely a function of maternal size (W-f in g), offspring size (W-o in g) consistently had a small, positive effect, so that organisms wi th larger individual offspring tend to have somewhat larger clutch mas ses: log(10) C = -0.31 + 0.78 log(10) W-f + 0.14 log(10) W-o (r(2) = 0 .93). The trade-off between offspring size and offspring number is so strong that fecundity (F, number of offspring/clutch) cannot be predic ted without considering both maternal and offspring size, but when the y are considered simultaneously, one equation fits all: log(10) F = -0 .32 + 0.79 log(10) W-f-0.88 log(10) W-o (r(2) = 0.92). The effects of taxon in both regressions were significant but small relative to mater nal and offspring size. Regressions on both maternal and offspring siz e appear to be powerful predictors of total clutch mass and fecundity across much of the living world. These relations can therefore be used to establish norms for comparisons, to constrain speculation to reaso nable values and to test theoretical developments in discussions of li fe history.