Meet the mammals that eat their own brains — shrews

Unlocking the secret of how a fish shrinks its cognitive tissue in winter – only to regrow it in spring – could help doctors treat neurodegenerative diseases in humans

(Video: Washington Post image; iStock)


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

The ringer scurried across the sand, twisting her velvety little body right, left, right, left.

Within seconds, it found the prize hidden in the sandbox: a delicious mixture of earthworms, mealworms and other meat.

In order to quickly solve the puzzle in Dina Dechman’s laboratory, it was necessary to learn more than just where its food was hidden. Another strange thing happened. It has to regenerate its 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 researchers.”

To prepare for the dead of winter, when food is scarce, many animals slow down, hibernate in the cold, or migrate to warmer areas.

Not a simple revolution. In order to survive the cold months, the animal eats its brain, reducing the organ to a quarter, only to grow most of the brain matter again in the spring.

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

The process of shrinking and expanding the brain and other organs with the seasons, called the Dehnel phenomenon, allows animals to reduce calorie intake when temperatures drop. Researchers have discovered seasonal shrinkage in the skulls of other small mammals with high metabolisms, including moles and, more recently, moles.

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

John Dirk Niland, an associate professor of health science and technology who is now studying drugs to mimic brain chemistry in humans, “was not completely clear to me at first.”

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

A hunt that cannot be tamed

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

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

The young zoologist served in the Polish army, although he remained devoted to his scientific work during the war. Captured by the Germans, he gave lectures on biology in the POW camp.

In the laboratory after the war, he noticed that skulls collected from the Beloveza Forest on the Polish-Belarusian border closed and expanded with the seasons.

A mammal with a high metabolism seems to eat insects, spiders, slugs and worms in order to survive. From the Highlands of Scotland to the tundra of Siberia, it hums at heights beyond human hearing and listens for sound to move underground.

Unlike deer or bears, moose are too small to migrate and are too hyperactive to hibernate when winter comes. 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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This makes these pesky creatures very difficult to study in captivity.

The venomous snake is one of the few mammals that has a venomous bite, and it can emit an unpleasant odor to deter cats and other predators. To accommodate the seasons, the team keeps their cages outside.

Dehnel himself struggled to cage and breed shrews, although he eventually succeeded. And their metabolism is so high that Dechman and his colleagues have a hard time keeping them quiet for the scan.

“We can’t put them to sleep,” he said. “It doesn’t seem like the knock-out situation has occurred because they can’t pass out because they’ll just starve.”

“They are small though,” he added.

Bigger is not always better

Shrews’ unusual strategy of reducing brain power may help them conserve energy during winter, but it comes at a cost.

In a series of experiments with finding food in a sandbox, larger-brained cats in summer outperformed their smaller-brained counterparts in winter, Dechmann’s group.

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

But what happens next is amazing: in the spring, their brains grow again and their ability to solve laboratory puzzles returns. Now the team is testing tourists’ ability to navigate a maze of LEGO pieces.

“The beauty of shrews is, yes, they shrink the brain, but what we’re also seeing is they can start growing brains in the spring,” said Niland, who co-founded a biotech company called 2N Pharma. .

Dechman said the idea that some animals are better off with smaller brains is a difficult idea for many to accept. He and colleagues received hate mail after publishing a study showing that some bats have evolved smaller brains to fly faster. Their article was titled “Bigger Is Not Always Better.”

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“People at the time didn’t want to believe that the brain was getting smaller,” he said. “We have bigger brains, and that means we’re smarter.”

Figuring out how the creatures do this is the next step. Dechman and Niland – together with Liliana M. Davalos, an evolutionary biologist at Stony Brook University in New York, received a grant from the French non-profit Program for Human Frontier Sciences to fund their groundbreaking research.

First, the brain does not regenerate uniformly. The hippocampus returns to normal, for example, while the neocortex does not. Both of these parts of the brain help with memory.

And it’s the lipid-rich white matter spread throughout the brain that appears to be disappearing, suggesting that the tiny mammal’s body may be consuming parts of its own brain to get it through the winter.

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

Researchers are now looking for proteins or other triggers responsible for shrinking and regrowth in shrews’ noggins. “We are far from applied results,” warned Dechman, although Niland’s company is currently working on a drug.

If those chemicals are found, Niland said, “we can use these pathways to treat brain diseases as well.”

For Davalos, finding such an amazing ability in an animal right under the noses of European gardeners is amazing in itself. The discovery suggests that many more can be found in the rainforests of the Amazon, Congo and elsewhere.

– How many centuries have people been studying European fauna? he said. “And how many thousands and thousands of scientists have looked and not seen it?”

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

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