New research has revealed a remarkable seasonal survival strategy in the masked shrew – a tiny, mole-like mammal found in the Appalachian Mountains of North America. Scientists discovered that these creatures shrink their bodies, skulls and even leg bones during the colder months to conserve energy.
The study, led by Dr Bryan McLean from the University of North Carolina Greensboro (UNCG), found that masked shrews (Sorex cinereus) lose around 13% of their body mass in winter.
When spring arrives and conditions improve, they grow larger again. Alongside weight loss, the researchers noticed changes in the height of the shrews’ braincases – the part of the skull that protects the brain – and the length of their femurs (thigh bones).
"Shrinking the body and its parts is in fact a clever survival strategy," says McLean. "And it's one that's important for us to understand as mammals face a constantly changing planet."
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Dehnel’s phenomenon
This seasonal shrinking is known as Dehnel’s phenomenon, a rare but fascinating trait seen in some small mammals – especially shrews. It’s an extreme form of phenotypic plasticity, where an animal can change its physical form in response to its environment.
"We don't know how common Dehnel's phenomenon is among mammals, but we know it is rarer in nature than other energy-saving strategies mammals use, like hibernation," McLean explains.
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Into the forest
To learn more, McLean and his team studied 125 masked shrews in Pisgah National Forest, North Carolina, between 2021 and 2023.
The animals were collected using small pitfall traps hidden in leaf litter. After being weighed, they were scanned at UNCG’s Joint School of Nanoscience and Nanotechnology using microCT technology to examine their bones.
"Our population of masked shrews is the southern-most yet studied for these multiple different traits," says McLean, "and the femur measurements we made are the first to show the magnitude of seasonal change in the long bones of the skeleton. This shows that shrews rapidly remodel much of their skeleton."
To put their findings in context, the team also looked at data from 74 other studies across the Northern Hemisphere. They developed models to see what factors drive these changes. The results showed a clear link: the colder the autumn and winter, the more the shrews shrink.
"This analysis reveals the generality of Dehnel's phenomenon in Sorex shrews," explains McLean.
"Across many populations of shrews on three continents, the degree of body mass and braincase height shrinkage is greatest in areas with the lowest cool-season temperatures.
As the climate continues to change, understanding these adaptations may help scientists predict how other small mammals might respond – or struggle – in a warming world, conclude the researchers.
Find out more about the study: Seasonal Body Size Plasticity and the Generality of Dehnel’s Phenomenon in Sorex Shrews
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