Foraging in a Complex World: Do Mule Deer and White-Tailed Deer Use Olfactory Cues to Locate and Select Their Diets?

Lacey Rose

doi:10.7273/000003770

With recent increases in population densities of wild, large mammalian herbivores in some areas, the level of plant herbivory has reached a point that could be detrimental to the survival of plants preferred by herbivores if a new plant conservation strategy is not found. To better understand the foraging behavior of two species of predominant mammalian herbivores in North America that might shed light on potential methods for plant conservation, we examined the questions of whether mule deer (Odocoileus hemionus) and white-tailed deer (Odocoileus virginianus) use plant odor cues to locate and select their diet and, if so, from what distance these odor cues effect their decision. Based on recent research regarding mammalian herbivores and the effects of volatile organic compounds, metabolic products produced and released by all plants that produce unique smells, we hypothesized deer would use their nose to more efficiently locate and select their diets. We conducted two-option tests with 10 two-year-old deer (five mule deer and five white tailed deer) in a fenced, 1-acre pen. The two options we offered to the deer during the trials included two bowls with 100 g of nutritious pellets that either had an attractive scent (leaf acetate) wiped around the rim of the bowl, or a repellent scent (1,8 cineole) mixed into the pellets as 2% of the pellet weight to reinforce avoidance of that bowl. The leaf acetate was hypothesized as the attractive scent because it is normally produced by plants deer typically prefer, such as willow and grass, and cineole was hypothesized as repellent because it is a naturally occurring metabolic product produced by plants that typically causes indigestion and nausea when consumed in high concentrations by mammalian herbivores. These two bowls were then placed within separate, identical 10-gallon bins that had nine holes drilled into each of their four sides to allow for the equal detection of the two odors. The bins were also used to hide the contents of each bin from the deer until they had walked up to it, and to allow the observer to know what patch the deer were investigating at all times during the trials. These treatments or “food patches,” were then placed 30 m from the entrance of the pen and either 2 m or 12 m apart from each other. Each of the 10 deer completed these two-option choice trials at each of the two distances four times. The side on which the cineole patch was placed was switched each time to control for any side preference the individual deer may have had. The deer were allowed to enter the experimental area one at a time and they were observed on which bin the smelled first, which bin they ate from first, and the number of times they switched between the bins. When the deer lost interest in foraging, the remaining pellets were weighed at the end of each trial to record the proportion of the available food they consumed. Although we found that both mule and white-tailed deer initially approached and smelled the paired food patches randomly, they both ate first and ate more from the preferred patch, which smelled like leaf acetate, but tasted like plain food pellets, than from the cineole patch, that both smelled and tasted like cineole. These findings suggest that in our experiments, deer were either unable to detect the odors from a distance, or more likely were not using the odors to find their preferred food patch. Therefore, we propose that deer use a hierarchy of cues to establish foraging behaviors, where visual cues are primarily used to locate food from a distance but once they have found a potential food patch they use taste and smell to ultimately decide what they consume. This use of taste and smell to select their diet stems from food aversions acquired from previous encounters during which deer have learned to associate those particular smells and tastes with the presence or absence of negative consequences caused by consuming that food source. Future studies should find a way to decouple the visual and odor cues and to more tightly couple odor and taste cues to further test the mechanisms of diet selection.

Foraging in a Complex World: Do Mule Deer and White-Tailed Deer Use Olfactory Cues to Locate and Select Their Diets?

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