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, eutherians, birds, reptiles, amphibians, fish, crustaceans and plants - drawn from the literature (n = 1451). Although clutch mass (C in g) was largely a function of maternal size (Wf in g), offspring size (Wo in g) consistently had a small, positive effect, so that organisms with larger individual offspring tend to have somewhat larger clutch masses: log10 C = -0.31 + 0.78 log10 Wf + 0.14 log10 Wo (r2 = 0.93). The trade-off between offspring size and offspring number is so strong that fecundity (F, number of offspring/clutch) cannot be predicted without considering both maternal and offspring size, but when they are considered simultaneously, one equation fits all: log10 F = -0.32 + 0.79 log10 Wf -0.88 log10 Wo (r2 = 0.92). The effects of taxon in both regressions were significant but small relative to maternal and offspring size. Regressions on both maternal and offspring size 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 reasonable values and to test theoretical developments in discussions of life history.