Foster's rule

[1][2] It is part of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts.

The rule was first formulated by van Valen in 1973[3][4] based on the study by mammalogist J. Bristol Foster in 1964.

Foster proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed because of the absence of some of the predators of the mainland, and larger creatures become smaller when food resources are limited because of land area constraints.

[7] The idea was expanded upon in The Theory of Island Biogeography, by Robert MacArthur and Edward O. Wilson.

[9] There are some cases that do not neatly fit the rule; for example, artiodactyls have on several islands evolved into both dwarf and giant forms.