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Scientists baffled that mountain beaver evolved a bigger body, smaller brain

By Brooks Hays
Scientists baffled that mountain beaver evolved a bigger body, smaller brain
The mountain beaver doesn't chop down trees or build dams. Instead, it chews on saplings and lives in underground burrows. Photo by John M. Regan/Northwest Wildlife Online

June 28 (UPI) -- A bigger brain isn't always a winning evolutionary strategy. The mountain beaver offers the latest proof that sometimes it makes sense to sacrifice brain power for other advantages.

Today, the mountain beaver, Aplodontia rufa, spends most of its time in underground tunnels. But new research suggests its most closely related ancestor lived in trees.

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The rodent's 30-million-year-old ancestor also had a smaller body and bigger brain. Over time, fossil evidence now suggests, the beaver became nocturnal and took to underground layers. The mammal's eyesight became less important, and as a result, the mountain beaver's neocortex shrank.

Mountain beavers still boast bigger brains than their relatives, but they are significantly smaller relative to the beaver's increased body size.

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"The brain is metabolically expensive, meaning it needs a lot of food energy to function," Ornella Bertrand, a postdoctoral fellow in the department of anthropology at University of Toronto Scarborough, said in a news release. "So the parts of the brain that are not crucial for survival might have been selected against."

Mountain beavers can still climb trees, and do so on occasion, but without the brain and vision power, they're not as capable as their relatives.

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"There appears to be a relationship between being arboreal -- that is living in trees -- the size of the neocortex and strong vision," Bertrand said.

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Scientists have previously observed a relative decrease in the size of brains in domesticated animals, including dogs, pigs and chickens, but the phenomenon is rare among wild animals.

Mountain beavers are found in the Pacific Northwest and parts of southern British Columbia. Like their more famous cousins, the North American beaver, mountain beavers eat wood. However, they don't chop down trees or build dams. Instead, they chew on small saplings and live in underground burrows, like groundhogs.

Researchers detailed their analysis of the mountain beaver's brain in the journal Palaeontology.

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