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This Ancient Shrew-Like Creature Weighed Next to Nothing and Survived the Arctic

The "ice mouse" eked out a living in the Cretaceous dirt, where it survived months of complete darkness every winter.

By Matt Hrodey
Aug 16, 2023 6:00 PM
Sikuomys mikros tooth
A microscopic scan of a Sikuomys mikros tooth. (Credit: Adrian Gestos/MIMIC)

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The so-called “ice mouse” lived on the fringes of ancient Alaska. Likely guided by a keen sense of smell, it burrowed under leaf cover or underground for worms and insects. Above grew conifer trees, ferns, ancient herbs and horsetail plants.

Some 73 million years ago, it would have lived alongside other small mammals, birds and much larger dinosaurs. During that time, northern Alaska would have extended even further north, into the arctic. And for four months every winter, the Earth’s orbit would have plunged the region into darkness.

What Was the Ice Mouse?

Amid such difficult conditions, Sikuomys mikros (“ice mouse”) weighed no more than about 11 grams, or two sheets of copy paper. Scientists discovered the miniscule mammal after finding several of its teeth, each of them about the size of a large grain of sand.

Despite the new species’ scientific name, the researchers say the animal more closely resembles a shrew or vole than a mouse. But what a consequential shrew-like creature it is: With its small size, it calls into question previous assumptions about arctic mammals.

Most commonly, animals grow larger at higher latitudes (e.g. polar bears) to reduce their surface area-to-volume ratio and conserve energy. Larger bodies also allow for greater fat stores, which can be used for hibernation.

J.P. Cavigelli of the Tate Geological Museum in Wyoming holds up an ice mouse tooth. (Credit: Jaelyn Eberle)

A Highly Efficient Animal

But the paper from the University of Colorado Museum of Natural History and other institutions concluded that S. mikros didn’t hibernate given its slender body size relative to other species. Neither did it fall into torpor, the process in which an animal falls into a state of reduced activity. Torpor relies on circadian rhythms and might not have worked during the long, dark winter.

How then did the ice mouse survive? It may have lived underground during the winter or under the leaf cover, eating what it could find. Its vanishingly small body size may have become an asset during the cold months as it required so little fuel to keep going.

“We see something similar in shrews today,” said Jaelyn Eberle, a professor of geological sciences at the University of Colorado at Boulder, in an interview with CU Boulder Today. “The idea is that if you’re really small, you have lower food and energy needs.”

How Did They Find the Ice Mouse's Teeth?

Finding S. mikros’ teeth involved sifting through ancient sediments and benefiting from more than a little luck. The team (which also included paleontologists from the University of Alaska at Fairbanks and Florida State University) removed dirt from the banks of the Colville River on Alaska’s northern coast, which is part of the Prince Creek Formation. To reach the site, the researchers traveled 75 miles from Deadhorse, Alaska, by bush plane or snowmobile.

The researchers transported literal buckets of sediment back to a lab, screen-washed away the mud, and examined any noteworthy objects under a microscope.

“You look under the microscope and see this perfect little tooth,” Eberle said. “It’s so tiny.”

The team found mostly molars that measured about two-thirds the size of those belonging to a related species, Gypsonictops hypoconus.


Read More: 9 of the Smallest Animals in the World and Where They Live


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