Japanese Shrews Shrink Their Brains but Enlarge Their Snouts in Winter

Long-clawed shrews in northern Japan appear to undergo an unusual seasonal transformation. As their braincases shrink during winter, their snouts grow larger—a combination researchers describe as the first known example of a “reverse Dehnel’s phenomenon” in shrews. The finding comes from a study published in the Proceedings of the Royal Society B on August 19. Yugo Ikeda of Japan’s Toyo University and colleagues examined 136 skulls collected in Hokkaido between 1948 and 1988. The specimens, held at the Botanic Garden of Hokkaido University, represented every month of the animals’ short, less-than-two-year lifespan. The researchers could not repeatedly measure live wild shrews without harming them, so they used the preserved skulls to track changes in 16 features as the animals aged. The skulls showed the braincase reductions and spring regrowth associated with Dehnel’s phenomenon, a seasonal adaptation documented in some small mammals since 1949. But the winter specimens also had snouts that were about 7 percent taller and 6 percent wider than those collected in spring. The result surprised the researchers, according to comments by Ikeda reported by the Guardian and Science News. The purpose of the enlarged snout is not established. One possibility is that it increases bite force and allows the shrews to eat tougher prey, although the study found no corresponding change in jaw shape. Another idea is that a larger snout could improve the animals’ ability to detect prey beneath Hokkaido’s deep winter snow. That explanation would require further research into whether scent-detecting tissue also expands. The researchers consider temperature regulation the most plausible explanation based on current evidence. A larger nasal cavity could provide more space for blood vessels, potentially warming freezing air before it reaches the brain, which becomes smaller during winter. However, this remains a hypothesis rather than a confirmed function. The study also highlights how little is known about shrews in northeastern Asia, despite the region’s diversity. Further research could determine whether long-clawed shrews in milder climates develop the same seasonal snout enlargement and clarify which environmental conditions drive the change.