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New research involving the Museum is now looking into how and why the tiny mainland dormouse grew to such incredible sizes when isolated on islands
· Dormice on the island of Formentera are about 50% bigger than the mainland ones. Those on Majorca used to be slightly bigger still, and those on Sicily about the size of a cat
· The study, led by the University of York, will be published in Proceedings of the Royal Society B on Wednesday 4 November
Many of the islands that are scattered throughout the Mediterranean Sea were once home to an extraordinary array of giant and miniature animals. These included giant dormice, which were once found on Sicily and across the Balearic Islands
Despite their name, dormice are not actually mice but belong to a separate family of rodents which contains around 30 different species. Between around 6 and 5.3 million years ago, one of these species managed to get across to different islands of the Mediterranean before being cut off and isolated. Over the next few million years, on each island the rodents grew to extraordinary sizes.
Dr Victoria Herridge is a researcher at the Museum and an author on the paper. She comments, ‘One of the most common evolutionary responses in mammals to becoming isolated on islands is to change size, with many large mammals evolving to become smaller, and many small mammals evolving to become bigger. This is known as the Island Rule or Island Syndrome, and in some case this can be really extreme, with tiny 1m-tall dwarf elephants living alongside cat-sized giant dormice on Sicily 300,000 years ago.’
By looking at each fossil species and comparing them to living dormice, the team, led by University of York’s Jesse Hennekam, were able to show that even though the giant rodents all likely started off at a similar size, the subtle environmental differences on each island, such as the climate, geography and predation risk, meant that they went down slightly different evolutionary routes.
This resulted in some of the dormice becoming gigantic herbivores and others becoming giant carnivores.
Dr Herridge explains, ‘Islands are critical to our understanding of evolution — pioneers of evolutionary biology like Charles Darwin and Alfred Russel Wallace developed their theory of evolution by natural selection in part because of what they saw and collected on islands in the Galapagos and Indonesia.’
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