What is the island rule in simple terms?

Among animals that end up living on islands for a long time, big ones such as elephants tend to become smaller and small ones such as mice tend to become larger.

Why does this happen?

A common explanation is that limited food on islands disadvantages large animals, while fewer predators and competitors make larger size advantageous for small animals.

Island Rule

🔬 Natural Sciences By CampusDown Wiki Editorial Team Last updated October 26, 2026

The island rule is the generalization in evolutionary biology that, among animals isolated on islands, species that are large on the mainland tend to become smaller over generations (insular dwarfism) while small species tend to become larger (insular gigantism). It is also called Foster's rule, after J. Bristol Foster, who described the pattern in mammals in 1964.

Contents

1. Origins and observations
2. Ecological explanations
3. Meta-analyses and debate
4. FAQ
5. References

Origins and observations

Comparing mammals on islands off the coast of British Columbia with their mainland relatives, the Canadian biologist Bristol Foster (1964) reported that rodents tended to be larger on islands while artiodactyls and carnivores tended to be smaller. Leigh Van Valen (1973) called this the island rule. The fossil record contains many dramatic examples of dwarfism, such as the pygmy elephants and hippopotamuses of Mediterranean islands, including the small elephants of Malta and Sicily. Lister (1989) showed that red deer on Jersey shrank greatly within only a few thousand years during the last interglacial, and the small-bodied hominin Homo floresiensis, reported from the Indonesian island of Flores by Brown et al. (2004), is also discussed in relation to insular dwarfism.

Ecological explanations

The main factors invoked to explain the island rule are resources, predation and competition. Because islands are small, large animals may struggle to find enough food, making smaller bodies advantageous. Small animals, by contrast, face fewer predators and competitors on islands, so larger bodies may help them monopolize resources or reproduce. Case (1978) offered a general explanation combining food availability and release from competition, and Heaney (1978) showed that body size in Southeast Asian squirrels varies with island area. Lomolino (1985, 2005) described a continuous relationship in which smaller species grow larger and larger species shrink, a tendency to converge on an optimal size. The rule is often compared with other geographic patterns in body size such as Bergmann's rule.

Meta-analyses and debate

How general the island rule is remains debated. Meiri, Cooper and Purvis (2008) argued that once phylogeny is controlled for, the pattern across mammals is weak and different groups show different trends. Raia and Meiri (2006) reported from fossil data that dwarfing in large mammals depends on food resources and the presence of predators. By contrast, Lomolino et al. (2013) analyzed fossil and living species together and argued that the pattern is ancient and widespread, and Benítez-López et al. (2021), analyzing data on more than a thousand species of terrestrial vertebrates, found consistent trends matching the island rule in mammals, birds and reptiles. Whittaker and Fernández-Palacios (2007) treat the topic as part of the evolutionary dimension of island biogeography.

FAQ

What is the island rule in simple terms?: Among animals that end up living on islands for a long time, big ones such as elephants tend to become smaller and small ones such as mice tend to become larger.
Why does this happen?: A common explanation is that limited food on islands disadvantages large animals, while fewer predators and competitors make larger size advantageous for small animals.

Sources reviewed: 2026-10-26

References

Foster, J. B. (1964). Evolution of mammals on islands. Nature, 202(4929), 234–235.
Van Valen, L. (1973). Pattern and the balance of nature. Evolutionary Theory, 1, 31–49.
Heaney, L. R. (1978). Island area and body size of insular mammals: Evidence from the tri-colored squirrel (Callosciurus prevosti) of Southeast Asia. Evolution, 32(1), 29–44.
Case, T. J. (1978). A general explanation for insular body size trends in terrestrial vertebrates. Ecology, 59(1), 1–18.
Lomolino, M. V. (1985). Body size of mammals on islands: The island rule reexamined. American Naturalist, 125(2), 310–316.
Lomolino, M. V. (2005). Body size evolution in insular vertebrates: Generality of the island rule. Journal of Biogeography, 32(10), 1683–1699.
Meiri, S., Cooper, N., & Purvis, A. (2008). The island rule: Made to be broken? Proceedings of the Royal Society B, 275(1631), 141–148.
Brown, P., et al. (2004). A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature, 431(7012), 1055–1061.
Lister, A. M. (1989). Rapid dwarfing of red deer on Jersey in the Last Interglacial. Nature, 342(6249), 539–542.
Raia, P., & Meiri, S. (2006). The island rule in large mammals: Paleontology meets ecology. Evolution, 60(8), 1731–1742.
Lomolino, M. V., et al. (2013). Of mice and mammoths: Generality and antiquity of the island rule. Journal of Biogeography, 40(8), 1427–1439.
Benítez-López, A., et al. (2021). The island rule explains consistent patterns of body size evolution in terrestrial vertebrates. Nature Ecology & Evolution, 5(6), 768–786.
Whittaker, R. J., & Fernández-Palacios, J. M. (2007). Island Biogeography: Ecology, Evolution, and Conservation (2nd ed.). Oxford: Oxford University Press.

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This article was last updated on October 26, 2026. It is based on widely recognized original works and textbooks; when citing it in a paper, please check the original sources listed in the references.