The hypothesis that masting by trees is a defensive strategy which satiates seed predators in mast years and starves them in the intervening periods is tested in 59 sets of data on the seed production and pre-dispersal seed-predation of 25 tree species. Twenty-lour of the 59 data-sets support the hypothesis and show a statistically significant positive relationship between the proportion of seeds surviving the pre-dispersal stage and the log10 of the crop size for the same year. Evidence that pre-dispersal seed survival increases with the length of the mast interval is poor.

A positive relationship between the strength of the masting habit and the maxintum observed pre-dispersal seed mortality in a sample of 15 tree species suggests that the masting habit is best developed in predator-prone species. A survey of seed crop frequencies in the woody plant flora of Nortli America shows masting species to be under-represented amongst shrubs and amongst trees which disperse their seeds in fleshy dispersal units. The selection pressures and evolutionary constraints which operate on the evolution of masting plants and their seed predators are discussed.