These Mice Grow Bigger on the Rainier Sides of Mountains: It Might be a New Rule of Nature

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Scientists studying mice from the Andes Mountains in Patagonia noticed something they couldn't explain: the mice from the western side of the mountains were bigger than the ones from the east, but DNA said that they were all from the same species. The researchers examined the skulls of 450 mice from the southern tip of South America, and found that existing biological laws didn't explain the size differences. Instead, in a new paper in the Journal of Biogeography, the scientists put forth a new hypothesis: the mice on the western slopes were bigger because that side of the mountain range gets more rain, which means there's more plentiful food for the mice to eat.

[...] De la Sancha and his colleagues realized this might be related to what biologists call the "resource rule." "This rule suggests that where there are more resources, individuals from the same species tend to be larger than where there are fewer resources," says de la Sancha. "For instance, some deer mice that are found in deserts and other habitats tend to be smaller in drier portions of their habitats. Another hypothesis suggests that some animals tend to be smaller in mountains versus adjacent plains in North America. Our study found a mixed result of these rules."

The sizes of mice seemed to be following the resource rule, but the question still remained: why were there more resources on the western slopes of the southern Andes than on the eastern slopes? De la Sancha had a "Eureka!" moment while teaching a class of undergraduates at Chicago State University.

"Believe it or not, when I was teaching ecology, one of the things that I was teaching about was the rain shadow effect," says de la Sancha.

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