Data from: Profound reversible seasonal changes of individual skull size in a mammal

Animals typically cope with seasonal environments through changes in anatomy, physiology and behaviour. Such anatomical adaptations rarely include size changes of the skeleton once animals are fully grown. Here, we document extensive individual seasonal size changes in the braincase of free-ranging common shrews (Sorex araneus), previously only documented post-mortem and at population level. Using x-ray images of recaptured shrews we show that individuals shrink their braincases in anticipation of winter by an average -15.3% (individual maximum -20.1%). They then partially regrow their braincases in spring by 9.3% (individual maximum 13.2 %). Thus, this pattern is not caused or enhanced by seasonal size-dependent mortality, but reflects profound individual changes in bone anatomy. We hypothesize that this is linked to an alternative wintering strategy to hibernation or migration. Our findings are important for understanding the evolution of adaptations to extreme conditions in a small high-metabolic animal and have tremendous potential for applied research on mature mammalian bones and nervous systems.