Shrews Shrink Their Skulls and Brains for the Winter

The tiny animal have some surprising reactions to the changing seasons

Karol Zub

Shrews are all around weird. Found around the world, they are one of nature's few venomous mammals. And they are ferocious, dubbed the “the tigers of the small animal world” (although their prey is usually limited to worms and slugs).

Now, a new study heaps a little more shrew weirdness. As Bret Stetka at Scientific American reports, during the winter the skulls of the common shrew—Sorex araneus, which is found in Britain, Europe and parts of Asia—actually shrink with changing seasons.

As Stetka reports, Polish zoologist August Dehnel first noticed that the bodies of shrews seemed to shrink while studying them in the 1940s, something dubbed the “Dehnel phenomenon.” But the exact amount of shrinkage remained unknown. So researchers at the Max Planck Institute for Ornithology in Germany decided to investigate.

The researchers live-trapped 12 shrews in June 2014, X-raying the tiny insectivores and implanting them with microchips. Then they were released and trapped again for measurements during the summer, winter and again in spring.

Researchers found that over winter the braincases of the animals shrunk by an average of 15 percent in anticipation of winter, then rebounded almost, but not quite, to its former size in spring. The shrews also lost overall body mass: The brain mass shrunk some 20 to 30 percent in addition to mass loss in other major organs and even spine shrinkage. In total, the body mass dipped around 18 percent in winter and undergoes a dramatic 83 percent rebound in spring. ​

Though shrews only live about two years, the researchers looked at a number of older animals heading into their second winter and found the same shrinkage, indicating the change is seasonal and not just a function of age. The study appears in the journal Current Biology.

Now that the phenomenon has been confirmed and measured, it raises lots of unanswered questions. “This means every single individual undergoes this change every winter, which remains baffling to us," lead author Javier Lazaro, a PhD candidate at Max Planck Institute for Ornithology, says in a press release.

It's likely that the shrinkage is an attempt to conserve energy during the cold months. “These animals cannot hibernate [and] they cannot migrate and they live in a very seasonal environment – so they need some alternative strategy to deal with winter,” Lázaro tells  Nicola Davis at The Guardian. “If you shrink an organ like the brain which is disproportionally more ‘expensive’ than other kinds of tissue you might save energy.”

Other researchers agree. “Their hypothesis that brains shrink to reduce energy is very reasonable,” John Grady, an ecologist at the Michigan State University tells Stetka. “But one thing I wish they’d done is try to tease apart whether the shrews’ brains shrunk simply because they were too big for the newly shrunken body or if shrews are actually [able] to compromise brain function to save energy.”

That’s something the team hopes to find out soon and plan a follow-up study by monitoring shrews throughout the winter, seeing if the loss in brain mass dims their memory and ability to learn. And there’s something else that can be learned from the way the shrews absorb mass from their shrinking skulls—it could one day help researchers better understand skeletal diseases, Davis reports.

This is not the only weird shrew-bone news in recent years. In 2013, researchers discovered an African species called Thor’s hero shrew, related to a previously known spices called the hero shrew. As Richard Johnston at Nature reported at the time, it can supposedly support the weight of a full-grown man on its back, which  is the equivalent of human holding the Space Shuttle. This super-shrew strength is due to its unique interlocking spine, likely used to help it leverage up logs and other heavy debris to hunt for bugs and worms. That too could help researchers design new treatments or aids for spinal problems. 

Both of these studies underscore one thing: Never underestimate the tiny shrew.

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