HomeWorldMeet the mammals that eat their own brains -- shrews

Meet the mammals that eat their own brains — shrews

Unraveling the shrew’s secret to shrinking its own cognitive tissue in the winter – only to regrow it in the spring – could help doctors treat neurodegenerative diseases in humans

(Video: Illustration Washington Post; iStock)


This article is part of Animalia, a column that explores the strange and fascinating world of animals and the ways we value, endanger, and depend on them.

The shrew ran across the sand, swishing its small, velvety body right, left, right, left.

Within seconds he found the prize hidden in the sandbox: a tasty mix of earthwormsmealworms and other meats.

To quickly solve the puzzle in Dina Dechmann’s lab, the shrew didn’t just need to find out where his meal was hidden. Another amazing thing happened in his mind. It had to regrow its own brain.

“It’s a crazy animal,” said Dechmann, a behavioral ecologist at the Max Planck Institute for Animal Behavior in Germany. “We can learn a lot from the shrews.”

To prepare for the depths of winter when food is scarce, many animals slow down, sleeping through the cold or migrating to warmer areas.

Not the common shrew. To survive the colder months, the animal eats away its own brain, shrinking the organ by as much as a quarter, only to regrow a lot of brain material in the spring.

Researchers at the Max Planck Institute of Animal Behavior in Radolfzell, Germany, used mazes made of Lego to test shrew cognition. (Video: Max Planck Institute for Animal Behavior)

The process of shrinking and expanding the brain and other organs with seasons — called Dehnel’s phenomenon — allows animals to reduce calorie-consuming tissue when temperatures drop. Researchers have discovered seasonal shrinkage in the skulls of other small mammals with high metabolisms, including weasels and, most recentmoles.

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The shrew’s incredible shrinking brain is more than just a biological curiosity. Understanding how these animals can restore their brain power can help doctors treat Alzheimer’s diseasemultiple sclerosis and other neurodegenerative diseases in humans.

“I couldn’t quite grasp it at first,” said John Dirk Nieland, an associate professor of health sciences and technology who now researches drugs designed to mimic the brain-altering chemistry of shrews in humans.

“It’s really amazing how they react and how they respond,” he added.

A shrew that cannot be tamed

For decades, few scientists understood the implications of August Dehnel’s discovery in 1949.

Born in Warsaw, Dehnel spent his early career studying bird eggs before the Nazi invasion of Poland interrupted his work on European beavers and other mammals.

The young zoologist served in the Polish army, although he remained devoted to his academic work during the war. Captured by the Germans, he taught biology in a POW camp.

Back in the lab after the war, he noticed that shrew skulls collected from the forest of Białowieza on the border of Poland and Belarus contracted and expanded with the seasons.

The high-metabolic mammal pursues insects, spiders, snails and worms seemingly non-stop to survive. Ranging from the Scottish highlands to the Siberian tundrait beeps in places beyond human hearing, listening for reverberations to navigate underground.

Unlike deer or bears, shrews are too small to migrate and too hyper to hibernate when winter arrives. They live fast and die young, with an average lifespan of just over a year. “Their metabolism is not set up to slow down like that,” Dechmann said.

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That makes the nervous creatures very challenging to study in captivity.

The shrew is one of the few mammals with a venomous bite and emits a foul odor that may deter cats and other predators. To acclimate the shrews to the seasons, the team keeps its cages outside.

Dehnel himself struggled to cage and breed shrews, though he eventually succeeded. And their metabolism is so high that Dechmann and her colleagues find it difficult to sedate them for scans.

“We can’t get them to sleep,” she said. “It almost seems like a knockout state isn’t built in because they can’t afford to pass out because they’re just going to starve.”

“They’re little bastards,” she added.

Bigger is not always better

Shrews’ unorthodox strategy of reducing their brain power may help them conserve energy during the winter, but this comes at a cost.

In a series of experiments involving finding food in a sandbox, shrews with larger brains in summer outperformed their smaller-brained counterparts in winter, Dechmann’s team found.

“It’s a compromise,” she said. “You make your brain smaller, you save energy, but you become – I don’t want to say stupid, but you become less good at solving certain learning tasks.”

But what happens next is remarkable: In the spring, their brains grow back and their ability to solve lab puzzles seems to return as well. The team is now testing the shrew’s ability to navigate a labyrinth of LEGO pieces.

“The beauty of the shrew is that they, yes, shrink the brain, but what we’re also seeing is that in the spring they can grow the brain,” said Nieland, who also co-founded a biotech company called 2N Pharma.

The idea that a smaller brain is better for some animals is a hard idea for many people to accept, Dechmann said. She and colleagues received hate mail after publishing a study that showed some bats developed smaller brains to fly faster. Their paper was titled “Bigger Isn’t Always Better.”

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“People at the time didn’t want to believe that the brain would get smaller,” she said. “We have a big brain and that means we are more intelligent.”

Figuring out exactly how shrews do this is the next step. Dechmann and Nieland — along with Liliana M. Dávalos, an evolutionary biologist at Stony Brook University in New York — receive a grant from the French non-profit Human Frontier Science Program to fund their research on shrews.

First, the shrew’s brain doesn’t grow back evenly. For example, the hippocampus expands back to normal, while the neocortex does not. Both parts of the brain help with memory.

And it’s the high-fat white matter that appears to be scattered throughout the brain, suggesting the tiny mammal’s body could be consuming parts of its own brain to get through the winter.

The deterioration of white matter, which helps transmit information in the brain, is a symptom of multiple sclerosis and other neurodegenerative diseases.

Researchers are now looking for the proteins or other triggers responsible for the shrinkage and regrowth in the shrews’ noggins. “We’re far from applied results,” Dechmann warned, though Nieland’s company is currently working on one drug.

If those chemicals are found, Nieland said, “we may also be able to use these pathways to treat brain diseases.”

To Dávalos, it is remarkable in itself to find such amazing ability in an animal right under the noses of European gardeners. The discovery suggests that there is so much more to be found in rainforests in the Amazon, Congo and elsewhere.

“How many centuries have humans studied European fauna?” she said. “And how many thousands and thousands of scientists have looked and they hadn’t seen this?”

“Think of all the amazing things that are hidden there because we’ve never looked.”

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