From “Huge Hamsters and Pint-Sized Porcupines Thrive On Islands: Researchers Test ‘Island Rule’ of Rodent Evolution” (ScienceDaily, Mar. 23, 2012), we learn,
From miniature elephants to monster mice, and even Hobbit-sized humans, size changes in island animals are well-known to science. Biologists have long believed that large animals evolving on islands tend to get smaller, while small animals tend to get bigger, a generalization they call “the island rule.”
But some animals still prove a puzzle. “Large animals like elephants and deer have a pretty consistent pattern. They all tend to get small. But it’s more complicated for other animals such as rodents,” Durst said.
Over the years, a number of scientists have questioned the generality of the island rule, arguing that size trends on islands aren’t as consistent as originally thought.
The key factor in determining which rodents got bigger and which got smaller, they found, is original size on the mainland. Rodents above a cutoff size of 253 grams — a group including beavers, groundhogs, squirrels and hamsters — generally got smaller over time, whereas those that started out below 253 grams generally got bigger.
Is this a design of some kind?
Anyway, it’s nice to see someone doing some useful work on evolution, instead of just spouting Darwin.
The cutoff size suggested makes sense. Larger animals are at risk of outgrowing their food supply on an island from which they cannot just migrate. Small rodents’ numbers are mainly constrained by their predators, few of whom may have ever made it out to the island. Their food supply is not a huge issue if they can live on a very little. So the one group benefits from getting smaller whereas the other may not find an obvious barrier to getting larger (and perhaps lazier?). Thoughts?