A small brown and gray mouse, Abrothrix hirta, sits on green grass strewn with a few brown leaves. The mouse faces left and is photographed in profile.
The shaggy, soft-haired mouse Abrothrix hirta experiences changes in size based on which side of the Andes Mountains an individual lives. Credit: Pablo Teta
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Mice that live on the colder, rainier western slopes of the Andes mountain range have larger skulls than the same species of mice that live in the warmer, drier eastern steppes and grasslands. Researchers attribute these morphological differences to the rain shadow effect, a weather phenomenon common in coastal mountain ranges around the world that causes rain to preferentially fall on one side of the mountains.

“In this particular study, [longitude] mirrors the rain shadow effect from the Pacific Ocean over the Andes to the Patagonian grasslands on the other side,” said lead researcher Noé de la Sancha, a mammologist at DePaul University and The Field Museum of Natural History in Chicago.

In the Shadow of the Mountains

Environmental factors like temperature, precipitation, and elevation can influence the size and shape of mammalian species by controlling the availability of resources, namely, water and food. Generally speaking, de la Sancha explained, larger individuals within a species tend to live in environments that are at higher latitude, are at higher elevation, are colder, or receive more precipitation.

The researchers wanted to understand how the Andes, a mountain range that spans a long swath of western South America and contains a wide variety of ecosystems, might influence the morphology of the mammalian species that live there. “Nonflying small mammals, mainly represented by rodents…tend to be sensitive to ecological gradients, they tend to be abundant and diverse, [and] most have limited dispersal and thus live where they are captured,” de la Sancha said. Abrothrix hirta, a shaggy, soft-haired mouse, “is an excellent species for this type of study because it is widely distributed in a region that includes both sides of the Andes,” he said.

The researchers measured the sizes and shapes of 450 A. hirta skulls collected locally and also archived in museums. The mice represent 67 locations that span 19° latitude in southern South America and from the Pacific to Atlantic coasts in Patagonia and Tierra del Fuego. There were roughly even numbers of male and female specimens and specimens from west and east of the Andes. The team used 19 temperature- and precipitation-based metrics from a global climate database to quantify the mice’s environmental conditions and looked for any trends between the size and shape of the skulls and the environments in which the mice lived. Some of the mice skulls were significantly larger than others, but the researchers struggled to explain why.

“While teaching the principle of the rain shadow effect during ecology class one day, it occurred to me it correlated very nicely with our results.”

“None of the precipitation or temperature variables we tested best explained the size and shape of Abrothrix hirta,” de la Sancha said. “Originally, our analysis recovered longitude as a very important variable to explain size and shape. And while teaching the principle of the rain shadow effect during ecology class one day, it occurred to me it correlated very nicely with our results.”

The rain shadow effect describes how coastal mountain ranges shape regional airflow and precipitation. In the Andes, warm, humid air flows eastward from the Pacific Ocean. The mountains push the air upward, where it cools, condenses, and rains out on the western slopes. The now dry air continues to move eastward past the mountain peaks and descends over the steppes and grasslands, which then receive less precipitation than the western Andean forests.

An illustrative graphic explains the rain shadow effect in the Andes mountain range. From left to right on the graphic are the Pacific Ocean, forests with green and orange trees growing up the western slope of the mountains, three tall mountain peaks, a forest with green trees on flat land, and grasslands and steppes with small shrubs. A purple arrow indicates the flow of air from left to right from the Pacific, over the mountains, to the grasslands. On the western slopes, three graphical representations of clouds and rain show that areas west of the mountains receives more precipitation than areas east of the mountains, which has only one set of clouds and rain. At the top of the graphic are two mice: the mouse on the left/west side of the graphic is larger than that on the right/east, and labels indicate that the western mouse is larger because its environment is more humid and cool than the warm and arid environment in the east.
The west side of the Andes receives more rainfall than the east side as humid air from over the Pacific Ocean rises and cools. The eastern side of the mountain range is in the rain shadow. The eastern dryness and aridity lead to fewer resources for animals like A. hirta, which become consistently smaller than their western counterparts. Credit: Teta et al., 2022, https://doi.org/10.1111/jbi.14468

In their data, longitude is a rough proxy for the Andean rain shadow effect, de la Sancha explained. Water and vegetation on the western slopes of the Andes are more abundant than those in the east. The researchers suspect that the rain shadow cast over the eastern grasslands, which makes Patagonia warm and dry, provides fewer resources and helps keep eastern A. hirta smaller than their counterparts that live in the cooler, wetter, more resource-rich western Andean forests. The researchers published these results in the Journal of Biogeography in August.

A Changing Mountain Clime

Mountains cover about a quarter of Earth’s land area, and most create some degree of rain shadow effect. This study “adds to a growing literature that uses mountain systems for understanding the effects of climate and vegetation change on species evolution and adaptation,” said Anderson Feijó, a bioecologist at Beijing’s Institute of Zoology at the Chinese Academy of Sciences. “The rain shadow effect is one of several examples of how mountains shape the environment of a region and ultimately affect the animals and plants living there.” Feijó was not involved with this research.

Mice that adapted to the ecologies of their side of the mountain might soon find that their environments no longer meet their needs.

As with all environments, climate change will likely have significant impacts on the ecologies of mountain regions and thus on the evolution of mountainous species. Some climate models predict that the western Andean slopes will become wetter at higher elevations, resulting in larger mice farther up the mountains, de la Sancha said, whereas the Patagonian grasslands will become warmer and more arid, resulting in even smaller eastern mice. Mice that adapted to the ecologies of their side of the mountain might soon find that their environments no longer meet their needs.

“Their finding…reveals the strong connection between animals and their habitats, even across a short scale,” Feijó said. “Consequently, one may expect that environmental changes, such as those linked to global warming, might reduce the fitness of animals in their own natural habitats.”

—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer

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Citation: Cartier, K. M. S. (2022), Rain makes skulls bigger—in mice, Eos, 103, https://doi.org/10.1029/2022EO220475. Published on 4 October 2022.
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