Etruscan Shrews Shrink Their Own Skulls to Survive Winter
To endure winter cold when food is scarce, the tiny Etruscan shrew reduces its head size by shrinking its skull and brain tissue by up to 20%. Known as Dehnel's phenomenon, this dramatic bone resorption lowers the mammal's metabolic energy needs. When spring arrives and food becomes plentiful, the shrew regenerates its skull bones and brain volume back to normal.
The Winter Energy Crisis of Tiny Mammals
For small warm-blooded animals living in temperate and cold climates, winter presents a severe thermodynamic challenge. Shrews, among the smallest living mammals, possess exceptionally high metabolic rates and lose heat rapidly due to their high surface-area-to-volume ratio. Unlike bears or ground squirrels, shrews cannot hibernate through the winter, nor do they carry thick reserves of insulating body fat. Their physiology demands a continuous supply of food, requiring them to forage almost around the clock just to maintain their internal body temperature.
When winter sets in, their primary food source—soil-dwelling invertebrates such as insects, worms, and spiders—becomes scarce or buried beneath frozen ground. Faced with diminishing calories and heightened heat loss, small insectivores must find ways to lower their baseline energy expenditure. While many species adjust their fur density or enter brief periods of torpor, certain shrews employ an entirely different evolutionary adaptation: they dramatically shrink the physical size of their bodies, including their internal organs and skeletal structures.
August Dehnel and the Discovery of Skull Changes
This biological process is named Dehnel's phenomenon after the Polish zoologist August Dehnel, who first documented it in 1949. While studying morphological variations in wild populations of the common shrew (Sorex araneus) and related species in Poland, Dehnel noticed a striking seasonal pattern in their skull measurements. Skulls collected from shrews trapped in the late autumn and winter were consistently smaller and flatter than those collected in late summer.
For many years, some biologists questioned whether this represented true shrinkage within living individuals or merely a demographic shift, such as larger individuals dying off earlier in the season. Subsequent longitudinal research and modern imaging technologies proved that the changes happen within the exact same individuals over time. As autumn transitions into winter, a single shrew's skull height and braincase volume visibly decrease, only to expand again when conditions improve the following spring.