Mice grow larger on the rainy side of the Andes than on the drier side, and the difference may reveal a previously unseen law of nature.

When researchers studied the skulls of 450 mice Abrothrix hirta, they thought the western ones were so big because they represented different species. Instead, what they discovered was a likely new biological law.

Carl Bergmann came up with the observation that animals in higher latitudes are larger than those in equatorial regions.

“But what about very tropical elephants and hippos?” you might ask; but Bergmann was referring to differences occurring within the same species, not between two different species.

Nearly two centuries of research have confirmed what is now known as Bermann’s Rule, and we know why. Smaller animals have a greater surface area relative to their body mass, which means they lose energy more quickly, a benefit in warm climates and a hindrance in cold climates.

A possible new law of nature

Dr. Noé de la Sancha of the Field Museum and co-authors believe they may have found a new version, one that applies not to temperature but to precipitation, which they presented in Journal of Biogeography.

“There are a bunch of ecogeographical rules that scientists use to explain the trends we see repeating themselves in nature. With this work, I think we found a new one: the rain shadow effect can cause size and shape changes in mammals,” said de la Sancha.

Rain shadows occur when winds over mountains consistently come from one direction, capturing a lot of water vapor from oceans or rainforests. Mountain slopes push clouds up into cooler air, causing them to shed most of the water they carry.

When the clouds passed the mountains, there was very little water vapor left. Consequently, the windward side of the mountains can be very wet, while the other side is dry.

This is such a widespread geographic feature that it is often taught in high school, but the effect on animals there is less well known.

Mice grow larger in the rainy parts of the mountains

Dr. Pablo Teta, of the Museo Argentino de Ciencias Naturales, did not seek to discover a broad biogeographical principle; he was trying to study some overlooked mice.

“They are very cute little animals, they have white and soft bellies. They live in the mountains, which makes them unique, but they are also found at lower altitudes. In general, they are not very well studied,” said de la Sancha.

Teta observed, using 450 skulls, that mice grow larger in certain places, but mitochondrial DNA indicated that multiple species were unlikely. Although the skulls were taken from over 19 degrees latitude, Bergmann’s rule had only a minor effect on size. The authors attempted to investigate a number of other variables in the locations from which the skulls were collected; by far the most important was longitude.

While teaching an ecology lesson on the rain shadow effect, de la Sancha remembered the Andes as one such example and realized that mice grow larger on the rainy side. “On some mountains, the difference is extreme. On one side it can be a rainforest and the other side is almost desert,” said de la Sancha.

Why do mice only grow bigger on one side of the mountain?

Further investigation confirmed that his suspicion was an application of the “resource rule,” which states that members of a species grow larger where food and other resources are more abundant. More rain on the west side of the mountains means more plant life, which leads to bigger mice. It might seem obvious, but according to Sancha, no one has yet found a link between rain shadows and mammal size, he writes IFL Science.

If mice turn out to be the only species in which rain shadows have this effect, the team will have discovered a curiosity, but if further studies produce a pattern, it could one day be considered a rule of equal importance to Bergmann.

“Mountains cover about 22 percent of the planet’s land surface,” so there’s a lot of territory that can be influenced by this effect, the paper says.

The impact of climate change

But all geography today is cast in another shadow: that of climate change. No one knows what will happen to these mice, but rain patterns are changing and the animals could suffer. This is in addition to the fact that the animals could be forced to keep climbing due to the heat. “At some point, you run out of mountains,” says Sancha.

The Andes are high enough to give the mice plenty of room, but without more studies we won’t know if they’re still a threatened species.

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