Rain Shadow Effect

The shaggy soft-haired mouse (Abrothrix hirta) changes in size based on which side of the Andes Mountains the individual lives on[1].

Scientists studying the skulls of 450 shaggy soft-haired mice (Abrothrix hirta), living on 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 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.

De la Sancha realized that the Rain Shadow Effect could explain why there was more food on the western side of the Andes, and thus, why the mice there were bigger.

The Rain Shadow Effect is a product of the way that water vapor travels over mountain ranges. "Essentially, one side of the mountain will be humid and rainy, and the other will have cold, dry air. On some mountains, the difference is extreme. One face can be a tropical rainforest, and the other side will be almost desert-like," says de la Sancha. "There is a Rain Shadow Effect in most mountains on the planet."

The 'Rain Shadow' indeed neatly matched up with the rodents' sizes—the first time, to de la Sancha's knowledge, that anyone has demonstrated the effects of the rain shadow on mammal size. And while so far it's only been shown for one species of mouse, de la Sancha suspects that he and his colleagues have hit on a larger truth—perhaps even the basis for a rule of its own someday.

This study was based on museum collections from Argentina, Chile, and the US, it's an amalgamation of years and years of collecting and big data sets.

Teta et al: Andean rain shadow effect drives phenotypic variation in a widely distributed Austral rodent in Journal of Biogeography - 2022