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Tiny ice mouse survived months of total darkness and sub-zero temperatures

In the harsh landscapes of northern Alaska, which are marked by frigid cold and stretches of unending darkness, a remarkable discovery has been made. This region, which sat even further north roughly 73 million years ago, was home to a tiny mammal adept at surviving in possibly some of Earth’s coldest conditions during the Late Cretaceous period.

The research was led by Jaelyn Eberle, a professor at the University of Colorado Boulder and curator of fossil vertebrates at the CU Museum of Natural History.

Focus of the study

Eberle explained that the name of the tiny ice mouse – Sikuomys mikros – is derived from the Iñupiaq word “Siku” for ice, combined with the Greek words “mys” and “mikros,” which translate to “mouse” and “little” respectively. 

Despite its name, however, the small mammal was not actually a mouse. Instead, it belonged to the extinct Gypsonictopidae family. This tiny fur-covered creature, with an appearance likely similar to today’s shrews, weighed only 11 grams – lighter than an empty soda can.

“These guys probably didn’t hibernate,” said Eberle. “They stayed active all year long, burrowing under leaf litter or underground and feeding on whatever they could sink their teeth into, probably insects and worms.”

This is a truly surprising and commendable feat, given the tough conditions they faced, which included months of darkness and sub-zero temperatures.

Identifying Sikuomys mikros

The fossil’s identification was based solely on a few minuscule teeth, each no bigger than a grain of sand. “I always like working at the ends of the Earth. You never know what you’re going to find, but you know it’s going to be new,” said Eberle.

The remains of the tiny ice mouse open up a fascinating window into ancient Alaska. “Seventy-three million years ago, northern Alaska was an ecosystem unlike any on Earth today. A polar forest bustling with dinosaurs, small mammals, and birds, all adapted to endure a highly seasonal climate,” said study co-author Patrick Druckenmiller, director of the University of Alaska Museum of the North.

How the research was conducted 

The dedicated team, which included paleontologists from institutions such as the University of Alaska Fairbanks and Florida State University, embarked on a challenging journey to the excavation site, located close to the Beaufort Sea. 

Their travels often involved navigating by snowmobile or bush plane from Deadhorse, Alaska. The fossils, mainly teeth and jaw fragments, required diligent scrutiny under microscopes. “You look under the microscope and see this perfect little tooth. It’s so tiny,” said Eberle.

Survival strategy 

One curious aspect of the ice mouse’s existence is its small size, especially when compared to related species found further south which were significantly larger. 

Drawing parallels with today’s shrews, Eberle hypothesizes that the ice mouse’s diminutive size could have been an evolutionary response to the scarcity of food during Alaskan winters. Being small would mean lower food and energy requirements, an invaluable advantage in such an environment.

The survival strategy of Sikuomys mikros possibly revolved around an underground existence during Alaska’s freezing months. This subterranean lifestyle might have been the saving grace for such mammals, especially after the cataclysmic meteorite crash that spelled doom for the dinosaurs 66 million years ago.

In a world dominated by towering dinosaurs, the tiny ice mouse and its incredible survival story stand as a testament to nature’s adaptability and resilience.

The study is published in the Journal of Systematic Palaeontology.


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