CM – Island gigantism and dwarfism are the result of evolutionary island rule


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April 15, 2021

from Radboud University Nijmegen

It’s an old theory in evolutionary ecology: animal species on islands tend to become either giants or dwarfs compared to relatives on the mainland. However, since it was formulated in the 1960s, the « island rule » has been hotly debated by scientists. In a new paper in Nature Ecology and Evolution on April 15, the researchers resolved this debate by analyzing thousands of vertebrate species. They show that the effects of the island rule are widespread in mammals, birds, and reptiles, but less so in amphibians.

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Pygmy hippos and elephants on the Mediterranean islands are examples of large species that have shown dwarfism. On the flip side, after colonizing islands, small mainland species may have evolved into giants, resulting in curios such as the St. Kilda field mouse (twice the size of its mainland ancestor), the infamous Mauritius dodo (a giant pigeon) and the Komodo dragon.

In 1973, Leigh van Valen was the first to formulate the theory based on a study by mammologist J. Bristol Foster from 1964 that animal species follow an evolutionary pattern in terms of body size. Species on islands tend to become either giants or dwarfs compared to their mainland relatives. « Species are restricted to the environment on an island. The threat from predators is much less or nonexistent, » says Ana Benítez-Lopez, who did the research at Radboud University and is now doing research at Doñana Biological Station (EBD) – CSIC, Spain). « But there are also limited resources available. » However, so far, many studies have shown conflicting results, which has led to heated debates about this theory: Is it really a pattern or just an evolutionary accident?

The team of scientists at Radboud University, Doñana Biological Station, the National Museum of Natural Sciences and of Imperial College London has revised the island rule and tried to resolve this debate through a meta-analysis of over a thousand vertebrate species. They show that island rule effects are widespread in mammals, birds and reptiles, but less obvious in amphibians, which tend to be gigantic. The study also shows that the extent of island dwarfism and gigantism is more pronounced on smaller, more remote islands for mammals and reptiles.

They also found an influence of climate and seasonality on island rule. Small species of mammals and birds grew larger and large species stayed the same size to conserve heat in colder, harsher island environments. When seasons exist, the availability of resources for reptiles becomes less predictable, resulting in smaller reptile species becoming larger. Benítez-López summarizes: « With a wealth of data from museums and living specimens, we were able to rigorously demonstrate for the first time that island gigantism and dwarfism in vertebrates are a generalized pattern and not just an evolutionary accident. »

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Related titles :
Islands lead to evolutionary giants and dwarfs
Island gigantism and dwarfism the result of evolutionary island rule


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