The function of strategic tree selectivity in the chemical signalling of brown bears

Large mammals select conspicuous objects on which to deposit their scent marks, which may function to supplement the olfactory signal, visually and/or chemically. Analysing marking sites is paramount to understanding whether signallers could mitigate potential fitness costs by placing scents strategically to reduce time and energy investment. The defining characteristics of marking sites are unclear across species, and variation in the literature concerning selectivity may be explained by behavioural plasticity. We took an evolutionary perspective on the selection and spatial distribution of marking trees by brown bears, Ursus arctos, to account for such variation. Our hypothesis, that brown bears would be selective in the trees used for scent marking, was supported; the trees chosen were located in regularly visited areas, where the defence of a resource is needed. The criteria of a marking tree appear to be primarily location and then about properties that facilitate their use as a conspicuous object; bears selected rarer species and trees of larger size than the average available. Other features, such as aromatic properties of the species, bark texture and the ability of the bark to hold scent, may act additionally to determine a tree’s marking potential. The energetic investment in manufacturing pungent volatile odours could be reduced if signallers utilize tree properties to attract receivers. Across mammalian taxa, whether a tree is selected for marking appears to vary based on environmental context; the principal function is to limit the energetic costs of producing scent marks by placing marks strategically to increase the likelihood of attracting potential receivers.