HomeNew Rule Of Nature: Scientists Notice Something Unusual They Couldn’t Explain

New Rule Of Nature: Scientists Notice Something Unusual They Couldn’t Explain

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“There are a bunch of ecogeographic rules that scientists use to explain trends that we see again and again in nature,” but this time, they “have found a new one.”

Scientists examining mice in Patagonia’s Andes Mountains observed something strange: mice on the western side of the mountains were larger than those on the eastern side, yet DNA revealed that they were all from the same species.

The researchers investigated the skulls of 450 mice from the southernmost tip of South America and discovered that existing biological rules could not account for the size differences.

The authors instead proposed a different theory in a recent publication published in the Journal of Biogeography: the mice on the western slopes were bigger because more rain falls on that side of the mountain range, which results in more abundant food for the mice to eat.

New Rule Of Nature: Scientists Notice Something Unusual They Couldn’t Explain
New Rule Of Nature: Scientists Notice Something Unusual They Couldn’t Explain

“There are a bunch of ecogeographic rules that scientists use to explain trends that we see again and again in nature,” adds Noé de la Sancha, a research associate at Chicago’s Field Museum, an assistant professor of Environmental Science and Studies at DePaul University, and the paper’s corresponding author. “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.”

The shaggy soft-haired mice, Abrothrix hirta, were used in this study by de la Sancha and his associates. De la Sancha describes them as “very cute little buggers, they have soft white bellies.” They’re distinct since they reside in the mountains, but they can also be found at lower levels. They have not been extensively researched overall.

In order to complete his PhD thesis, Pablo Teta, a colleague of De la Sancha’s from the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Buenos Aires, Argentina, started researching the shaggy soft-haired mouse. He observed that different members of the species varied greatly in size. He believed they belonged to distinct species. Even though they are so distinct, their mitochondrial DNA indicated that they are one species, according to de la Sancha. 

New research explains how the rain shadow affects mice

“We wanted to explore why that is, to see if they were following some sort of rule.”

There are many “rules” of nature that explain patterns we see in the world. Bergmann’s rule, for instance, explains why larger animals of the same species are found in higher latitudes. Canada’s white-tailed deer are bigger and heavier than their leaner relatives in Florida. This is explained by Bergmann’s rule because, like how larger pieces of food take longer to cold than smaller bites, having a thicker body relative to your surface area helps you retain heat better.

The researchers compared measurements of 450 mouse skulls using statistical analyses to look for a trend to explain the variations in size. They then tried to map their discoveries onto several biological principles to see if any of them made sense. Bergmann’s rule didn’t hold true; there wasn’t a significant relationship between mouse size and latitude of the specimen. Other principles place more emphasis on the impact of temperature or precipitation, with varying outcomes for various groups and circumstances. This group did not find that latitude or any of the other 19 bioclimatic, temperature, or precipitation variables best explained the various forms and sizes of the mice. How far east or west the mice lived, though, did appear to follow a pattern.

De la Sancha and his coworkers came to the conclusion that this might be associated with the “resource rule,” as it is known in biology. According to de la Sancha, “this rule suggests that where there are more resources, individuals from the same species tend to be larger than where there are fewer resources.” For example, some deer mice located in deserts and other ecosystems tend to be smaller in those areas of their habitats that are drier. 
According to another theory, several species in North America’s mountains are often smaller than those in the nearby lowlands. 

“Our study found a mixed result of these rules.”

Although mouse sizes appeared to follow the resource rule, the reason why there were more resources on the western slopes of the southern Andes than on the eastern slopes remained a mystery. While instructing a class of students at Chicago State University, De la Sancha had a “Eureka!” moment.

“Believe it or not, when I was teaching ecology, one of the things that I was teaching about was the rain shadow effect,” adds de la Sancha.

The way water vapor moves over mountain ranges causes the rain shadow effect. Water vapor is absorbed by the air over the ocean, and as the ocean heats naturally, this water vapor rises. This air is transported from the ocean to the land by dominant winds, such as the jet stream, and as it crosses mountain ranges, it becomes colder as it rises in altitude. Cold air contains water vapor, which condenses and becomes rain. If the mountain is exceptionally high, the air will be dry by the time it reaches the far side of the top. In essence, the mountain will have cold, dry air on one side and humid, rainy air on the other. On some mountains, there is a big difference. One side may resemble a tropical rainforest, while the other may resemble a desert, according to de la Sancha. “There is a rain shadow effect in most mountains on the planet, we see this phenomenon all over the world.”

In the course of his talk, de la Sancha came to the conclusion that the rain shadow could account for the bigger mice on the western side of the Andes since there was more food available there. “That same day, I went home and wrote to Pablo,” he recalls. “I was like, ‘Dude, we need to talk about the rain shadow.’”

The rain shadow did, in fact, closely match up with the rodents’ sizes—the first time, according to de la Sancha, that anyone has proved the impacts of the rain shadow on animal size. Although it has only been demonstrated for one type of mouse so far, de la Sancha believes he and his colleagues have discovered a more general truth that could one day serve as the foundation for a new rule.

It’s intriguing because it might lead to something more widespread. According to de la Sancha, “We think it may be more of a rule than an anomaly. It’d be worthwhile to test it on lots of different taxa.”

The findings, however, may indicate that the shaggy soft-haired mice, as well as many other mammals, are in for a tough time. The frightening aspect is that, according to de la Sancha, “we show that, at least to some effect, climate patterns are important to determine the mice’s morphology— their shape and size, either directly or indirectly through the resources they can find.”

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

If the weather patterns change and have an impact on the plants that grow in the area, the mice may no longer be able to thrive as they once did.

De la Sancha adds that animals are already relocating to higher elevations to avoid the consequences of climate change. 

“At a certain point, you run out of mountain,” he adds. “There’s nowhere else to go. We don’t know what’s going to happen, but it doesn’t seem good.”

According to de la Sancha, studying typically overlooked creatures like mice is important because it will help us understand the uncertain future of these mice in the face of climate change. It’s crucial to realize how little we know about the majority of small mammals, he says. They may serve as reliable indicators of enduring alterations to our environment. 

“We need to study them more. Our findings also show why museum collections are so important. 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 writes, “This paper would not have been possible without museum collections and highlights the importance of museum- and collection-based research and its support worldwide.”

This kind of study advances our knowledge of the fundamental, universal principles governing Earthly life.

Image Credit: Teta et al, 2022

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