Unlocking the secret to how shrews shrink their cognitive tissues in winter — only to grow back in spring — could help doctors treat neurodegenerative diseases in humans
To quickly solve a puzzle in Dina Dechmann’s lab, the shrew doesn’t just need to know where its food is hidden. There are other shocking things going on in its head. It had to regrow its own brain.
“It’s a crazy animal,” says Deschmann, a behavioral ecologist at the Max Planck Institute for Animal Behavior in Germany. “We can learn a lot from shrews.”
To prepare for harsh winters when food is scarce, many animals slow down, sleep in the cold or migrate to warmer areas.
Not your average shrew. To survive the colder months, the animal eats its own brain, reducing the organ by a quarter, only to regrow most of its brain matter in the spring.
The process by which the brain and other organs contract and expand with the seasons — known as the Dehnel phenomenon — allows animals to reduce the amount of tissue that consumes calories as temperatures drop. The researchers found seasonal shrinkage in the skulls of other small, high-metabolism mammals, including weasels and most recently moles.
The shrew’s incredibly shrunken brain isn’t just a biological curiosity. Understanding how these animals are able to restore brain power may help doctors treat Alzheimer’s, multiple sclerosis and other neurodegenerative diseases in humans.
“At first, I didn’t quite get it,” said John Dirk Nieland, an associate professor of health sciences and technology who is now working on drugs designed to mimic brain-altering chemicals in human shrews.
“The way they responded and the way they responded was really amazing,” he added.
The Untamable Shrew
For decades, few scientists understood the significance 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 research on European beavers and other mammals.
The young zoologist served in the Polish army, but during the war he remained devoted to his academic work. After being 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 Bialowieza Forest on the border of Poland and Belarus shrank and swelled with the seasons.
This high-metabolic mammal seems to be constantly chasing insects, spiders, slugs and worms to survive. From the Scottish Highlands to the Siberian tundra, it squeaks at pitches beyond human hearing, listening for echoes to navigate underground.
Unlike deer or bears, shrews are too small to migrate and too large to hibernate when winter arrives. They live fast and die young, averaging just over a year. “Their metabolism doesn’t slow down like that,” Dechmann said.
This makes the study of these neurotic creatures very challenging in captivity.
The common shrew, one of the few mammals that bites, emits a foul smell that potentially deters cats and other predators. To allow the shrews to adapt to the changing seasons, the team placed the cages outside.
Dehnel himself had struggled to cage the shrews, but he succeeded. And their metabolisms are so fast that Dechmann and her colleagues found it difficult to calm them down for the scans.
“We couldn’t put them to sleep,” she said. “There seems to be almost no knockout state built in because they can’t afford to lose consciousness because they’ll just starve to death.”
“They’re little bastards,” she added.
bigger isn’t always better
The shrews’ unorthodox strategy of reducing brain power may help them conserve energy in winter, but it comes at a cost.
Dechmann’s team found that shrews with larger brains in summer outperformed their smaller-brained counterparts in winter in a series of experiments that involved foraging for food in a sandbox.
“It’s a compromise,” she said. “You make your brain smaller, you conserve energy, but you become — I don’t want to say stupid, but you become less good at solving certain learning tasks.”
But it’s remarkable what happened next: In the spring, their brains grew back, and their ability to solve lab puzzles seemed to return. The team is now testing the shrew’s ability to navigate a maze made of Lego bricks.
“The beauty of shrews is that, yes, they shrink their brains, but we also see that in the spring, they can start growing their brains,” said Nieland, who is also co-founder of a biotech company called 2N Pharma. founder.
For some animals, the idea that smaller brains are better is a hard one for many, Dechmann said. She and colleagues received hate mail after publishing a study showing that some bats evolved smaller brains to fly faster. Their paper is titled “Bigger Is Not Always Better.”
“People at the time didn’t want to believe that brains were going to get smaller,” she said. “We have big brains, which means we’re smarter.”
The next step is to figure out how the shrews do this. Dechmann and Nieland, along with evolutionary biologist Liliana M. Dávalos of Stony Brook University in New York, received a grant from the French nonprofit Human Frontier Science Program to fund their shrew research.
For one, shrew brains don’t regenerate evenly. For example, the expansion of the hippocampus returned to normal, but the neocortex did not. Both parts of the brain help with memory.
Lipid-rich white matter scattered throughout the brain appears to be disappearing, suggesting that the small mammal’s body may be consuming part of its own brain to survive the winter.
Degeneration of the white matter that helps carry messages in the brain is a symptom of multiple sclerosis and other neurodegenerative diseases.
The researchers are now looking for proteins or other triggers that cause the shrews’ brains to shrink and regenerate. “We’re far from application results,” Dechmann cautions, although Nieland’s company is currently working on a drug.
If these chemicals are found, Nieland said, “we might be able to use these pathways to treat brain diseases as well.”
For Dávalos, the amazing ability to spot animals under the noses of European gardeners is remarkable in itself. The discovery suggests that there is much more to be found in the rainforests of the Amazon, the Congo and elsewhere.
“For how many centuries have people been studying European fauna?” she said. “How many scientists have been looking and they haven’t seen this?”
“Think of all the amazing stuff that’s been hidden because we’ve never seen it.”
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