Deconstructing white-tailed deer forage selection: the role of plant chemistry and volatile organic compounds

When foraging optimally, mammalian herbivores should select food items that confer the greatest nutritional benefits (e.g., crude protein and non-structural carbohydrates) and impose minimal-to-no costs (e.g., plant secondary metabolites [PSMs]). PSMs, such as tannins, deter herbivores by providing post-ingestive feedback cues that the forager experiences after consumption, and are therefore difficult for herbivores to avoid altogether. However, some PSMs, such as terpenes, are also volatile organic compounds (VOCs) that become gaseous at ambient temperatures and can be detected by herbivores via scent prior to ingestion. Our objective was to discern if the foraging preferences of white-tailed deer, a near ubiquitous herbivore in North America, were influenced by the nutritional or anti-nutritional constituents of trees during summer and winter. We also examined VOCs emitted by trees during summer and tested for a potential relationship between VOC emissions and deer sniffing behavior or foraging preference. During summer, deer preferred trees with a high non-structural carbohydrate content. After sniffing trees that emitted a large proportion of terpenes, deer appeared to be averse to begin foraging. During winter, deer preferred trees with a low crude protein content and a high tannin content. When deer foraged from trees with high terpene contents, they consumed less than they did from low terpene trees. Our results show that white-tailed deer foraging behavior is influenced by plant nutritional constituents. Additionally, we suggest that differences in VOCs among tree species may provide deer with cues of plant quality that could play a role when selecting forage items.