Do Animals Grow Bigger On Islands?

Why do animals that are isolated on islands grow so big?!

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First documented in 1885 by biologist Edward Cope, biologists have been baffled by the fact that certain animals grow larger or smaller when isolated on an island for extended periods of time. Known as the "Island Rule", rodents tend to grow larger than average, while some carnivores become smaller than average. Biologists suspect that the growth patterns of these so-called "insular species" has had to do with availability of resources: smaller animals tend to grow larger while bigger animals tend to grow smaller.

A new study published by Duke University further cataloged the "island rule" in over 1,000 populations in 60 species around the world. In many ways, studying insular populations gives scientists insight into that animal's evolution. By comparing insular populations to their mainland cousins, scientists can hypothesize why they evolved apart. Additionally, biologists can learn about the potential speed of evolution. A 2006 compilation study in PLOS Biology found evolution happens faster on islands. Ultimately, why animals get smaller or larger seems to come down to resources and environmental factors: It helps explain mysteries like why the tortoises of the Galapagos grew to such a massive size.

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"Rodents are 17 times more likely to evolve to huge proportions on islands than anywhere else."

Mainland size variation informs predictive models of exceptional insular body size change in rodents (
"The tendency for island populations of mammalian taxa to diverge in body size from their mainland counterparts consistently in particular directions is both impressive for its regularity and, especially among rodents, troublesome for its exceptions."

Body Size of Mammals on Islands: The Island Rule Reexamined (
"Foster observed that there is a clear tendency for gigantism in insular rodents and perhaps marsupials, while dwarfism is characteristic of insular carnivores, lagomorphs, and artiodactyls."