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The short version

  • Long-clawed shrews in Hokkaido, Japan, increase snout size by roughly seven percent during winter months while simultaneously reducing brain volume.
  • Researchers propose that the enlarged nasal cavity facilitates heat exchange, warming freezing air before it reaches the shrunken brain and preserving cognitive function.
  • The findings represent the first documented case of this reverse seasonal growth pattern, expanding understanding beyond European species previously studied.

A recent study published in the Proceedings of the Royal Society B has identified a novel physiological adaptation in long-clawed shrews that challenges conventional understanding of how small mammals survive extreme cold. While scientists have long documented the seasonal shrinking of brains and other organs in certain species, this research reveals that these animals also temporarily enlarge their snouts during winter months. This dual transformation offers new insights into the complex mechanisms these creatures employ to maintain survival in frigid environments.

The phenomenon of seasonal organ reduction, known as Dehnel’s phenomenon, was first described by Polish biologist August Dehnel in 1949. It involves small mammals such as shrews, moles, and weasels reducing the size of their brains and other body parts by more than twenty percent during winter, only to regrow them in the spring. This adaptation is widely believed to help animals conserve energy and survive harsh conditions. However, the newly reported expansion of the snout represents a significant deviation from this established pattern, marking what researchers term a reverse Dehnel’s phenomenon.

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The study focused on long-clawed shrews found in Hokkaido, the northernmost main island of Japan. Northeast Asia is recognized as a hotspot for shrew diversity, yet species in this region remain significantly understudied compared to their European counterparts. To address this gap, Yugo Ikeda, a mammalogist at Toyo University, and his colleagues analyzed 136 skulls collected between 1948 and 1988. These specimens were housed at the Botanic Garden of Hokkaido University and covered every month of the shrews’ short lifespans, which typically last less than two years.

The decision to rely on historical museum specimens was driven by practical constraints. Studying live wild shrews repeatedly without causing harm is considered practically impossible due to their small size and elusive nature. By examining sixteen different traits across the collected skulls, the researchers were able to track structural changes as the animals aged and seasons changed. The data confirmed the expected reduction in braincase size during winter and subsequent regrowth in spring, but also revealed unexpected changes in facial structure.

Analysis showed that shrews collected in winter had snouts that were approximately seven percent taller and six percent wider than those collected in spring. This finding surprised the research team, who noted that the results were completely unexpected. The discovery highlights a previously unknown aspect of seasonal adaptation in these mammals. While similar size reductions have been documented in European species, this specific combination of brain shrinkage and snout expansion had not been observed before.

Researchers have proposed several hypotheses to explain why shrews might develop larger snouts during colder months. One possibility is that an enlarged snout increases bite force, allowing the animals to consume tougher prey when food sources are scarce. However, the study authors noted that jaw shape did not exhibit similar size changes, providing little direct evidence to support this theory. Another suggestion is that a bigger nose enhances the sense of smell, aiding in locating prey buried under deep snow. This idea requires further investigation to determine if scent-detecting tissue also expands during winter.

The most plausible explanation currently supported by the data involves thermoregulation. A larger nasal cavity may provide more space for a dense network of blood vessels. When shrews inhale freezing air, this vascular network can efficiently warm the air before it reaches the brain. This process helps protect the delicate, winter-shrunken brain from cold damage and maintains core cognitive functions necessary for survival. This mechanism underscores the intricate balance these animals must maintain between energy conservation and physiological protection.

The study builds on recent work exploring the genetic and molecular changes underlying Dehnel’s phenomenon in other species, such as the Eurasian common shrew. However, questions remain regarding the environmental factors driving this adaptation. Jan R. E. Taylor, an evolutionary ecologist at the University of Białystok who was not involved in the study, suggested that future research should examine whether long-clawed shrews in milder climates exhibit the same snout enlargement. Such investigations could clarify how different environmental pressures shape these remarkable physiological responses.

With over 385 known species of shrew worldwide, there is still much to learn about their diverse adaptations. This research not only deepens understanding of a specific species in Northeast Asia but also prompts a reevaluation of how seasonal changes affect mammalian anatomy. As climate patterns shift and habitats change, understanding these survival mechanisms becomes increasingly important for conservation efforts and broader ecological studies.

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  • Smithsonian Magazine↗These Tiny Shrews Grow Bigger Snouts and Shrink Their Brains to Survive Harsh Winters, Study Suggests