The shrinking of the shrew

A common species of mammal found across the northern hemisphere shrinks its skull by an extraordinary 20% in winter.

Writing in the journal Current Biology, researchers led by Javier Lazaro of the Max Planck Institute for Ornithology in Germany report that the red-toothed shrew (Sorex araneus) reduces its head size on an annual basis. At the same time, its brain mass decreases by up to 30% and several major organs, as well as the spine, also grow smaller.

The researchers speculate that the bizarre phenomenon might be a survival strategy. The shrews have very high metabolisms, but neither migrate nor hibernate when temperatures fall and food becomes scarce.

“Reducing head size — and thus brain size — might save energy disproportionally as the brain is energetically so expensive,” Lazaro says.

The research has so far failed to uncover the cause of the head shrinkage, although the mechanism appears to be the absorption of tissue within cranial sutures.

The fact that the tiny mammals tended to have smaller heads in winter was first observed in 1949 by a Polish mammologist called August Dehnel – leading to the shrinkage being termed Dehnel’s phenomenon.

However, Lazaro and his colleagues are the first to document the reduction and expansion of individual shrews by following them around for an entire year.

The team captured 12 wild shrews, anaesthetised them, X-rayed their skulls, implanted microchips, and then released them. Using live traps, the rodents were periodically re-captured and X-rayed again. The scientists found that head size was smallest during winter, but regrew through spring.

Earlier studies had suggested that Dehnel’s phenomenon might be more apparent than real, the result of shrews with larger heads simply dying off during the colder months and leaving their better adapted little brethren to carry on.

Lazaro and his colleagues disprove the idea.

“This means every single individual undergoes this change every winter, which remains baffling to us,” he says.

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