A Tiny Animal Shrinks Its Own Brain Every Winter — Scientists Want to Copy It 🐭 #shorts

This animal is one inch long and weighs less than a penny. It also shrinks its own brain every winter on purpose — and Alzheimer's researchers think it might hold a piece of the answer they have been looking for.

The animal is the Eurasian common shrew (Sorex araneus). At 5 to 12 grams (0.2 to 0.4 oz) of total body weight, shrews are the smallest mammals on Earth that maintain a constant body temperature. The trade-off is extreme: their metabolism is so fast that the heart beats 700 to 1,200 times per minute, and the animal must eat two to three times its body weight in food every single day or it starves. A shrew that goes six hours without eating can die.

Winter creates a problem this kind of animal cannot survive normally. Food scarcity, snow cover, frozen ground. Other small mammals solve this by hibernating. Shrews can't — their metabolism is too fast to slow down without dying.

So they evolved something almost no other mammal can do.

It is called Dehnel's phenomenon, named after Polish zoologist August Dehnel who first described it in 1949. As autumn approaches, the shrew's body begins reabsorbing itself. Body mass drops up to 18%. Liver, kidneys, and other organs shrink. The braincase shortens by about 13%. And most strikingly, the brain itself shrinks by 10 to 26% in mass, varying by region. The hippocampus (memory), the hypothalamus, the thalamus — all measurably smaller in winter than in summer. Bones in the skull literally retract.

Come spring, the entire process reverses. The skull regrows by about 10%, brain mass rebounds 9 to 17%, body weight climbs back up to 82% from its winter low. The shrew rebuilds itself.

Until recently, the mechanism was a mystery. A 2025 paper in Current Biology by Lazaro and colleagues at the Max Planck Institute, "Programmed seasonal brain shrinkage in the common shrew via water loss without cell death," provided the breakthrough. The researchers used MRI on living shrews across seasons and found the brain shrinkage is not destruction. It is dehydration. Brain cells lose water content, neuron soma volume decreases, dendrite length retracts — but the cells themselves do not die. In spring, the water comes back and the structures expand again. The brain is not being damaged. It is being temporarily compressed.

The medical implications are why this matters far beyond shrews.

Human brains shrink with age. In Alzheimer's disease and multiple sclerosis, neuronal loss and synaptic damage drive cognitive decline. Once human brain tissue is gone, it is gone. But the shrew demonstrates that mammalian brains can shrink and regrow without losing function. A 2025 study identified genes involved in shrew Dehnel's phenomenon that have parallels in humans, including genes linked to obesity and Alzheimer's. The hope, still distant but real, is that understanding how shrews protect neurons during shrinkage and trigger regrowth could eventually inform therapies for human neurodegenerative diseases.

A 5-gram animal that lives barely a year, with a heart that beats 700 times a minute, may be quietly demonstrating one of the most important neuroplasticity mechanisms scientists have ever found in a mammal. It does this because winter is coming and it cannot afford the calories to feed a full-size brain.

The shrew solved the problem by accident. We are trying to learn what it knows.
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