Scientists working in the Andes mountains in Patagonia have been presented with a puzzle: mice of the same species grow to larger sizes on the western side of the mountains compared to the eastern side.
Now a new study suggests a solution to the mystery.
It seems that thanks to the rain shadow effect – where the clouds are pushed higher as they pass over the mountains, causing rain on the first side they hit – the mice on the western slopes have more food to eat , causing additional growth.
The rain shadow effect is a common phenomenon that occurs on many mountain ranges, leaving one side drier than the other.
However, scientists are still learning how this leads to changes in the environment and local wildlife.
“There are a lot of ecogeographical rules that scientists use to explain trends that we see over and over again in nature,” says mammalogist Noé de la Sancha of DePaul University in Chicago.
“With this paper, I think we’ve found a new one: the rain shadow effect can cause size and shape changes in mammals.”
The mice in question are of the furry, soft-haired species Abrothrix hirta, and the team analyzed 450 mouse skulls, using a roughly even split between male and female skulls, to assess size differences.
After that, they set about trying to find an explanation, looking at possible correlations between size and latitude, altitude, temperature or precipitation.
In the end, it was the longitude, how far east or west the mice were, that matched the size variations. Add to that what is known as the resource rule, where more resources tend to lead to larger animals, and the researchers almost had their answer.
(Teta et al., Journal of Biogeography, 2022)
One question remained: why were there more resources in the west?
The team eventually realized it was the rain shadow. Water is taken from the ocean and carried towards the mountains, and as the air rises it becomes colder. This causes rain, which can mean a lot more on one side of the range.
“Essentially, one side of the mountain will be wet and rainy, and the other will have cold and dry air,” says de la Sancha.
“On some mountains, the difference is extreme. One side may be rainforest, and the other side will be almost desert.”
There are other natural rules like this. Take Bergmann’s rule, for example, which links larger animals of the same species to colder environments; in this case, it’s because having a thicker body helps retain heat better.
The newly established link brings new concerns related to climate change. Changes in temperature can lead to changes in precipitation levels, which scientists now know affects the morphology of these mice and potentially other animals. Changes in weather patterns could affect the amount of food available to eat.
While the future is still unclear, it’s important to gather as much data as possible so we can understand the current and potential implications of climate change, and that data now includes how mountain locations relate to animal size.
“It’s exciting, because it could potentially be something more universal,” says de la Sancha. “We think it may be more of a rule than an anomaly. It would be worth testing in many different taxa.”
The research has been published in the Journal of Biogeography.