Are animals bigger on the rainy side of the mountain? •
Scientists studying a species of small mouse from the Andes Mountains in Patagonia think they may have come up with a new biological rule that explains the variation in size within populations of these mice.

Are animals bigger on the rainy side of the mountain?

The natural world is full of complex phenomena that are not easily understood at first glance. Over the centuries, naturalists have proposed numerous biological “rules” or “laws” to explain apparent patterns that are observed in nature, and then researchers have collected data that either supports or disproves the generalizability of these rules. Scientists studying a species of small mouse from the Andes Mountains in Patagonia think they may have come up with a new biological rule that explains the variation in size within populations of these mice. 

Ecologists are familiar with Gause’s law, for example, which states that two species competing for the same resource cannot coexist successfully together – one species will lose the competition.  And biologists who study patterns in animal morphology will be familiar with Allen’s rule stating that the body shapes and proportions of endotherms will be different in warm and cold climates in order to maximize or minimize heat loss respectively. There are numerous such biological rules, all of them helping biologists to identify and understand patterns that they observe in nature.  

The species of mouse that features in the current research is the shaggy soft-haired mouse, Abrothrix hirta. Biologist Pablo Teta of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Buenos Aires, Argentina, began studying the mice as part of his doctoral thesis. He noticed that the mice varied considerably in terms of body size, with some being much bigger than others. He was initially convinced that he was looking at two different species, but analysis of the mouse DNA confirmed that these all belonged to the same species. So began a quest to understand the cause of this variation in mouse body size. 

In their publication in the Journal of Biogeography, Teta and his colleagues propose that the mice on the western slopes of the Andes are bigger because that side of the mountain range gets more rain, which means there’s more plentiful food for the mice to eat. This represents a potentially brand new rule that may explain variations in animal sizes on different faces of the same mountain. 

“There are a bunch of ecogeographic rules that scientists use to explain trends that we see again and again in nature,” said 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.”

Initially, the researchers used statistical analyses to compare the measurements of 450 A. hirta skulls from museum collections, in order to identify a pattern in body size. They then tried to explain the variations using different biological rules. Bergmann’s rule, which states that larger individuals are found in colder environments, didn’t fit the data; there wasn’t a strong correlation between mouse size and how far north or south the specimen lived. Other rules that emphasize the importance of temperature, precipitation or latitude didn’t help explain the variation in the data either. 

The team applied 19 other, different biological rules that emphasize the roles of different bioclimatic factors, but none of them explained the observed data pattern. Until the researchers considered the effect of longitude, that is. It appeared that the variation in body size fitted an east-west pattern, with mice on the western slopes of the Andes being significantly bigger than those inhabiting the eastern slopes. 

De la Sancha and his colleagues realized this might be related to what biologists call the resource rule. “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 instance, some deer mice that are found in deserts and other habitats tend to be smaller in drier portions of their habitats. Another hypothesis suggests that some animals tend to be smaller in mountains versus adjacent plains in North America. Our study found a mixed result of these rules.”

The sizes of mice seemed to be following the resource rule, but the question still remained: why were there more resources on the western slopes of the southern Andes than on the eastern slopes? De la Sancha had a “Eureka!” moment while teaching a class of undergraduates at Chicago State University.

“Believe it or not, when I was teaching ecology, one of the things that I was teaching about was the rain shadow effect,” said de la Sancha. In the middle of his lecture, de la Sancha realized that the rain shadow could explain why there was more food on the western side of the Andes, and thus, why the mice there were bigger. “That same day, I went home and wrote to Pablo. I was like, ‘Dude, we need to talk about the rain shadow.’”

A rain shadow is an area of significantly drier land on the side of a mountain range that faces away from the prevailing wind. In the case of the Andes, air over the ocean gathers water vapor that rises and is carried onshore by the prevailing westerly wind. As the altitude increases, the air becomes colder and the water vapor condenses and forms rain. Rain therefore precipitates readily on the west-facing slopes of the mountains. However, the wind that continues to travel inland over the high peaks and onto the east-facing slopes will have lost all its moisture by the time it gets over the mountains. As a consequence, the eastern slopes fall within a rain shadow area and will be significantly drier. 

“Essentially, one side of the mountain will be humid and rainy, and the other will have cold, dry air. On some mountains, the difference is extreme. One face can be a tropical rainforest, and the other side will be almost desert-like,” said de la Sancha. “There is a rain shadow effect in most mountains on the planet, we see this phenomenon all over the world.”

The rain shadow indeed neatly matched up with the rodents’ body sizes – the first time, to de la Sancha’s knowledge, that anyone has demonstrated the effects of the rain shadow on mammal size. And while so far it’s only been shown for one species of mouse, de la Sancha suspects that he and his colleagues have hit on a larger truth which may perhaps even form the basis of a new biological rule. 

“It’s exciting, because it could potentially be something that’s more universal. We think it may be more of a rule than an anomaly,” said de la Sancha. “It’d be worthwhile to test it on lots of different taxa.”

The shaggy soft-haired mice may have guided the researchers to propose their new hypothesis, but they and many of their fellow mammals, may be in for a rough time ahead. “The scary part is that we show that, at least to some extent, climate patterns are important to determine the mice’s morphology – their shape and size – either directly or indirectly through the resources they can find,” said de la Sancha. 

“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 affect the plants that grow in the region, the mice might no longer be able to thrive as they once did. Plus, de la Sancha notes, animals are already moving up mountains to escape the effects of climate change. “At a certain point, you run out of mountain. There’s nowhere else to go. We don’t know what’s going to happen, but it doesn’t seem good.”

The unclear future of these mice in the face of climate change, according to de la Sancha, is a good reason to study animals like mice that often go unnoticed. “It’s important to understand how little we know about most small mammals. They can be good indicators of long-term changes in 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.”

“This paper would not have been possible without museum collections and highlights the importance of museum- and collection-based research and its support worldwide,” said Teta. “This type of research helps us better understand the big-picture; universal rules of how life on Earth works.”

By Alison Bosman, Staff Writer

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