homehome Home chatchat Notifications


Mice in the Andes grow bigger on the rainier side of the mountains -- and this could be a law of nature

More rain means more plentiful food for the mice to eat, the researchers explain.

Fermin Koop
September 2, 2022 @ 2:02 pm

share Share

The shaggy soft-haired mice (Abrothrix hirta) grow bigger on the western slopes of the mountains than on the eastern sides. As weird as this seems, researchers also believe they know the reason and it’s all because of the rain. The researchers found that the west side of the mountain gets more rain, which means there’s more food for the mice to eat, a phenomenon called the rain shadow effect.

Image credit: Pablo Teta.

Researchers from Chicago’s Field Museum studied the skulls of 450 of these mice and found the ones living on the western side of the mountains were consistently larger, despite being genetically identical to the ones on the east. They linked this to the rain and food combination and suggested this could be replicated in other animals and species.

“There are a bunch of ecogeographic rules that scientists use to explain trends that we see again and again in nature,” Noé de la Sancha, study author and researcher at Chicago’s Field Museum, said in a statement. “With this paper, I think we might have found a new one: the rain shadow effect can cause changes of size and shape in mammals.”

Pablo Teta, a researcher at the Museo Argentino de Ciencias Naturales in Buenos Aires, Argentina, started studying the shaggy soft-haired mice as part of his doctoral thesis. He saw that some individuals of the species were very big and others really small. At first, he thought they were different species, but DNA analysis showed they were the same.

Rainfall, food, and size

There are many “rules” of nature that explain patterns we see in life. For example, Bergman’s rule explains why animals of the same species are bigger in higher latitudes. The white-tailed deer in Canada, for example, is larger than its Floridian cousins, and this is because having a thicker body in relation to your surface area helps you retain heat better. This new finding could very well pave the way for a new rule.

The researchers used statistical analyses to compare measurements of 450 mouse skulls, seeking to find a pattern to explain the differences in size. Then they tried to map their findings onto different biological rules, but this didn’t lead anywhere. There simply wasn’t a strong link between mouse size and how far north or south the specimen lived, for example.

However, there was a pattern with longitude– how far east or west the mice lived. The researchers believed this could be related to the ‘resource rule’. This suggests that where there are more resources individuals from the same species tend to be bigger than where there are fewer. The size of the mice in the Andes seemed to be following this rule.

However, there was still the open question of why there were more resources on the western slope of the Andes than on the eastern slopes. This is when the rain shadow effect enters – the result of the way that water vapor travels over mountain ranges. Humid air rising up one side of a mountain gradually loses its moisture, leaving the other side in the rain’s “shadow.”

The rain shadow effect matched up perfectly with the rodent’s sizes. The researchers argued it’s the first time that anyone has proven the effects of rain shadow effect on mammals. While exciting for them, this isn’t necessarily good news for the mice, which are in for a rough ride because of expected climate change in the Andes mountain range, de la Sacha said.

“With climate change, we know we’re going to see dramatic changes in temperature throughout the year, and changes in precipitation,” de la Sancha said in a statement. “While they might not be the most important variables affecting the mice’s well-being, they are important in determining available food sources.”

The study was published in the journal of Biogeography.

share Share

A Brain Implant Just Turned a Woman’s Thoughts Into Speech in Near Real Time

This tech restores speech in real time for people who can’t talk, using only brain signals.

Using screens in bed increases insomnia risk by 59% — but social media isn’t the worst offender

Forget blue light, the real reason screens disrupt sleep may be simpler than experts thought.

Beetles Conquered Earth by Evolving a Tiny Chemical Factory

There are around 66,000 species of rove beetles and one researcher proposes it's because of one special gland.

We Should Start Worrying About Space Piracy. Here's Why This Could be A Big Deal

“We are arguing that it’s already started," say experts.

An Experimental Drug Just Slashed Genetic Heart Risk by 94%

One in 10 people carry this genetic heart risk. There's never been a treatment — until now.

We’re Getting Very Close to a Birth Control Pill for Men

Scientists may have just cracked the code for male birth control.

A New Antibiotic Was Hiding in Backyard Dirt and It Might Save Millions

A new antibiotic works when others fail.

Researchers Wake Up Algae That Went Dormant Before the First Pyramids

Scientists have revived 7,000-year-old algae from Baltic Sea sediments, pushing the limits of resurrection ecology.

A Fossil So Strange Scientists Think It’s From a Completely New Form of Life

This towering mystery fossil baffled scientists for 180 Years and it just got weirder.

ChatGPT Seems To Be Shifting to the Right. What Does That Even Mean?

ChatGPT doesn't have any political agenda but some unknown factor is causing a subtle shift in its responses.