These mice grow larger on one side of their mountain house. Now we know why: ScienceAlert

A puzzle has been presented to scientists working in the Patagonian Andes: mice of the same species grow to larger sizes on the western side of the mountains than on the eastern side.

Now a new study suggests a solution to the puzzle.

It seems that thanks rain shadow effect – as clouds are pushed upward as they pass over the mountains, causing rain on the first side they hit – rats on the western slopes have more food to eat, causing more growth.

The rain shadow effect is a common phenomenon that occurs across many mountain ranges, causing one side to dry out more than the other.

However, scientists are still learning how this leads to changes in the environment and local wildlife.

“There is a set of environmental rules that scientists use to explain the trends we see over and over again in nature,” Mammologist Noe de la Sancha says:from DePaul University in Chicago.

“With this paper, I think we may have found a new one: the rain shadow effect can cause changes in size and shape in mammals.”

The mice in question are shaggy and fine-haired Abroothrix Herta The team analyzed 450 mouse skulls – using a roughly equal split between male and female skulls – to assess differences in size.

Next, they set out to try to find an explanation, looking at possible correlations between volume and latitude, altitude, temperature, or precipitation.

In the end, it was the longitude—how east or west the rats live—that corresponded to the differences in size. Add what is known as the resource base, where more resources tend to be larger animals, and the researchers almost got their answers.

(Tita et al., Journal of Biogeography2022)

One question remained: why were there more resources in the West?

The team eventually realized it was the shadow of the rain. The water is picked up from the ocean and directed toward the mountains, and as the air rises it gets cooler. This leads to precipitation, which can mean more of it on one side of the mountain range.

“Essentially, one side of the mountain will be wet and rainy, and the other will have cool, dry air,” Sancha says.

“The difference is huge in some mountains. One side could be a tropical rainforest, while the other side would be semi-desert.”

There are other natural rules like this. Takes Bergmann’s rulefor example, which associates larger animals in the same species with cooler environments—in this case, it’s because a thicker body helps retain heat better.

The newly created association brings with it new concerns regarding Climate change. Changes in temperature can cause changes in precipitation levels, which scientists now know affect the morphology of these mice and possibly other animals. Changes in weather patterns can affect the amount of food available to eat.

While the future remains unclear, it is important to collect as much data as possible so we can understand the current and potential effects of climate change – and this data now includes how locations on the mountain relate to animal size.

“It’s exciting, because it could potentially be something more universal,” Sancha says. “We think it might be more of a rule than an anomaly. It would be useful to test it on many different varieties.”

The search was published in Journal of Biogeography.