Rodents Evolve to Nightmarish Proportions on Islands

When you think of rodents, you probably think of tiny mammals. You don't normally think of 100-pound behemoths. Now, researchers have analyzed size data for rodents worldwide to distinguish the truly massive mice and giant gerbils from regular-sized rodents and have found that rodents are 17 times more likely to evolve to nightmarish proportions on islands than elsewhere.

In this latest study, the researchers analyzed data from more than 1,000 rodent populations representing more than 60 species across the globe. This included mice, rats, squirrels, hamsters and porcupines that ranged in size from the tiny .2-ounce harvest mouse to the nearly 50-pound North American beaver.

The researchers actually looked at various rodents to see which ones became large for their kind over time. In other words, they were looking at which parts of the world are home to the biggest hamsters or the smallest squirrels.

So what did they find? It turns out that islands are the answer. Rodent populations on the 182 islands they looked at weighted in among the top or bottom 2.5 percent for size for their species.

Coues' rice rat, as an example, have grown to almost twice the size of their mainland counterparts on the island of Cozumel.

"Deer mice, too, are nearly twice as big on the Gulf Islands off the coast of Vancouver than on the North American mainland," said Paul Durst, one of the researchers, in a news release.

Surprisingly, though, dwarfs were also found on islands. More specifically, hot, dry islands caused rodents to shrink; for example, an Asian tree squirrel has shrunk by half since arriving on the Thai island of Ko Lan.

"They don't have the resources they need to get big," said Durst.

The findings reveal that islands are places of extremes. On lush islands, rodents can grow to monstrous proportions while on hot, dry islands, they can shrink. This, in turn, reveals how the environment can greatly impact evolution.

The findings are published in the journal Proceedings of the Royal Society B: Biological Sciences.

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