Long-clawed shrews experience a remarkable physiological transformation during winter, where their snouts physically grow in size while their brain cases shrink by 12 percent. Documented by researchers studying Hokkaido specimens published in the Proceedings of the Royal Society B, this reverse Dehnel’s phenomenon helps these high-metabolism mammals warm freezing air before it reaches delicate neural tissue.
Anatomy of Survival: Dehnel’s Phenomenon and the Reverse Trend
Humans reach for thermal layers and heated blankets when winter temperatures plunge. Long-clawed shrews, however, execute an extreme anatomical reshaping to survive freezing conditions without hibernating or storing food.
Scientists have long tracked Dehnel’s phenomenon, a biological trait where small mammals like the common shrew and European mole reversibly shrink their body mass, skull size, and brain mass as winter sets in. Spring brings a corresponding regrowth of these tissues.
Yet, a research team led by Dr. Yugo Ikeda at Toyo University in Japan documented a surprising twist in the long-clawed shrew. Examining 136 skulls collected in Hokkaido between 1948 and 1988, the team tracked 16 physical measurements across monthly intervals. They discovered that while the brain case height dropped by 12 percent in winter specimens, the height of the snout—known as the rostrum—increased by 7 percent, and its width grew by 6 percent.
“This is precisely why we termed it the ‘reverse Dehnel’s phenomenon’,” said Dr. Yugo Ikeda, first author of the research from Toyo University in Japan.
“This is a ‘true change’ where the bone structure of the rostrum itself physically alters its size,” Ikeda noted, explaining that the rostrum expands while other parts of the skull shrink during winter.
In Plain English: The Clinical Takeaway
- Reversible Tissue Remodeling: Unlike permanent skeletal growth, these tiny mammals physically expand and contract bone and organ structures seasonally.
- Thermal Protection Mechanism: Enlarging the nasal cavities creates additional space for a dense network of blood vessels to warm frigid air prior to neural contact.
- Metabolic Extremes: Operating with heart rates between 800 and 1,500 beats per minute, shrews require constant foraging and cannot afford prolonged winter starvation periods.
Metabolic Demands and Physiological Stressors
Shrews belong to the Soricidae family, comprising over 400 species distributed throughout Eurasia, Africa, and North America. They maintain an exceptionally fast metabolism, moving at speeds requiring up to 12 body movements per second. Their hearts beat at 800 to 1,000 times per minute, while the Etruscan shrew—the smallest terrestrial mammal on earth—reaches up to 1,500 beats per minute, outpacing hummingbirds.
Because common shrews do not hibernate or store provisions, they must eat their own body weight daily. The common shrew (Sorex araneus) starves if it goes without food for more than two to three hours. To locate prey such as earthworms or finches beneath leaf litter, several species in the genera Sorex and Blarina utilize ultrasonic echolocation calls at approximately 50 kHz for close-range spatial orientation.
When winter arrives, these animals lose between 10 and 20 percent of their body weight as fat, muscle, and internal organs diminish. According to the research published in the Proceedings of the Royal Society B, the expansion of the nasal cavity during this lean period serves clear physiological purposes. “Expanding the nasal cavity creates more room for a rich vascular network,” Ikeda explained. When the animal inhales freezing air, this vascular network warms the air before it reaches the winter-shrunken brain, preserving core cognitive functions.
| Anatomical Feature | Winter Measurement Trend | Biological Mechanism |
|---|---|---|
| Brain Case Height | Decreased by 12% | Energy conservation via neural mass reduction (Dehnel’s phenomenon) |
| Snout (Rostrum) Height | Increased by 7% | Bone structure expansion to accommodate warming vascular networks |
| Snout (Rostrum) Width | Increased by 6% | Enlargement of nasal passages for enhanced airflow conditioning |
Future Research Directions in Mammalian Adaptability
Hokkaido alone hosts three additional shrew species, prompting researchers to plan further investigations into how widespread these alternating skull modifications might be.
“We are very excited about investigating the presence, absence, and degree of both Dehnel’s and reverse Dehnel’s phenomena among these species in future research,” Ikeda stated.
References
- Ikeda, Y., et al. Proceedings of the Royal Society B: Biological Sciences. Research published regarding seasonal skull morphology and reverse Dehnel’s phenomenon in long-clawed shrews.
- Botanical Garden of Hokkaido University. Historical specimen archives supporting longitudinal craniometric analysis of Soricidae populations (1948–1988).
