These shrews have heads that shrink with the season

If any part of the body would seem ill equipped to shrink, it would probably be the head and skull. And, yet, researchers reporting in Current Biology on October 23 have found that the skulls of red-toothed shrews do shrink in anticipation of winter, by up to 20 percent. As spring approaches, their heads grow again to approach their previous size.

"We found that each shrew undergoes a dramatic decrease in braincase size from summer to winter," says Javier Lazaro of Max Planck Institute for Ornithology in Germany. "Then, in spring, the braincase regrows, almost reaching the original size in the second summer."

Earlier studies had suggested that the heads of these shrews might shrink seasonally. At the population level, the height of the braincase of shrews was shown to decline by 20 percent and then to increase again by 15 percent. There's even a name for it based on the first person to report the general observation: the Dehnel phenomenon. But the new study is the first to fully document the shrews' shrunken heads by following individual animals over the course of the seasons.

The researchers used live traps to capture shrews from the summer of 2014 into the fall of 2015. When a shrew was captured, the researchers anesthetized it before X-raying its skull and implanting a microchip under its skin for later identification.

Measurements taken using the X-rays confirmed that the shrews' heads shrunk over the course of the season. Twelve shrews were captured at all three stages, and all of them showed the same pattern: a peak head size in summer, a winter decline, and regrowth in the spring.

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