This video says about itself:
Ferocious Shrews Fight For Mating Rights – Life Of Mammals – BBC Earth
11 October 2017
In the undergrowth, out of sight for most of us, shrews are battling for mating rights!
If any part of the body would seem ill equipped to shrink, it would probably be the head and skull. And, yet, researchers 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: here.