White bark in birch species as a warning signal for bark-stripping mammals

ABSTRACT Background Birch species such as Betula pendula have conspicuous white bark and the evolutionary drivers for this colouration remain unresolved. Aims We evaluated our hypothesis that the white bark is a visual warning signal to deter mammals from bark-stripping. Many species of deer (Cervidae) and multiple other mammals consume bark. White birch species’ bark contains betulin and other compounds that likely make the bark unprofitable for herbivores. The white bark has features consistent with a visual signal for mammalian herbivores and could act as a visual aposematic signal of chemical defence. Methods We compared deer bark-stripping between tree species in Scottish woodlands. For Betula pendula, we compared stripping of juvenile brown bark with mature white bark. We also reviewed existing literature to find the tree-species preference for a wide-range of bark-stripping mammals. Results In Scotland, we found that white-barked birch bark was less preferred. We also found mature white birch bark was avoided compared to juvenile brown bark. Existing literature for multiple herbivores showed that white birch species’ bark was often either not preferred or avoided. Conclusion We suggest that the conspicuous white colouration of birch bark may act as an aposematic visual signal to deter bark-stripping mammals.