New law of nature: Rats grow bigger on rainy mountains

"There are a lot of ecogeographic rules that scientists use to explain trends we see repeatedly in nature," said Noé de la Sancha, a research associate at the Field Museum in Chicago, an assistant professor of environmental science and research at DePaul University, and the paper's corresponding author.

The mice that De la Sancha and his colleagues studied in this study were furry soft-haired mice called Abrothrix hirta

De la Sancha's colleague Pablo Teta, who works at the "Bernardino Rivadavia" Argentine Museum of Natural Sciences in Buenos Aires, Argentina, began studying the shaggy soft-haired mouse as part of his doctoral dissertation

There are many natural "rules" that explain the patterns we see in our lives

To find the regularities that explain the differences in size, the researchers used statistical analysis to compare measurements from 450 mouse skulls

De la Sancha and his colleagues realized that this might have something to do with what biologists call "resource rules

The size of the mice seems to follow the resource rules, but the question remains: Why do the western slopes of the South Andes have more resources than the eastern slopes?

"Believe it or not, when I teach ecology, one of the things I teach is the rain shadow effect," says De la Sancha

The rain shadow effect is a product of water vapour flowing through mountains

Halfway through his undergraduate teaching, De la Sancha realized that rain shadows could explain why there is more food on the west side of the Andes and, therefore, why the rats are bigger there

As far as De la Sancha knows, rain shadows do exactly match rodent body size—this is the first time anyone has demonstrated the effects of rain shadows on mammalian body size

"It's exciting because it might be something more general

However, the findings could mean that these furry soft-haired mice, and many of their mammalian companions, are having a tough time

Additionally, De la Sancha points out, animals have begun migrating up the mountains to escape the effects of climate change

De la Sancha believes that the future of these mice is unclear in the face of climate change, which is a good reason to study animals as often overlooked as mice

Teta noted: "This paper would not be possible without museum collections and underscores the importance of museum- and collection-based research and its support around the world

Andean rain shadow effect drives phenotypic variation in a widely distributed Austral rodent.
Journal of Biogeography, 2022; DOI