Profound reversible seasonal changes of individual skull size in a mammal.

Postnatal size changes in most vertebrates are unidirectional and finite once the individual reaches full size [1]. In rare cases, changes of body length may occur in response to harsh environmental conditions. Such reactionary changes are distinct from seasonal, often anticipatory morphological changes, such as the reversible size change of some adult bird brains [2]. A unique pattern of profound anatomical change known as Dehnel's phenomenon has been described for the body, skull and brain size of red-toothed shrews and some mustelids [3-5]. The seasonal 20% decrease and 15% re-growth of the most common proxy, braincase height, were documented at population level from extracted skulls post-mortem. Quantifying intra-individual change had so far been methodologically prohibitive. Here, we followed the intra-individual change in skull size and body mass throughout the full cycle in wild recaptured shrews (Sorex araneus). Using X-ray images we showed that individuals decreased the size of their braincases in anticipation of winter by an average of 15.3%. Braincases then partially regrew in spring by 9.3%. Body mass decreased by 17.6% and then dramatically increased by 83.4% in spring. Thus, we demonstrate that the dramatic changes incurred by Dehnel's phenomenon occur in the individual's bone and other tissues.