The effect of food quality and relative abundance on food choice in fallow deer

The diet choice of large herbivores is influenced by the presence of nutrients and toxins in available plants. It is generally assumed that herbivores ingest food that contains higher amounts of nutrients and lower amounts of toxins than the average of the food available. We studied the effects of quality and relative abundance on food choice in fallow deer, Dama dama. Preferences for and aversions against different compounds were first examined in two-choice tests with food and water. We then did cafeteria (multiple-choice) tests with hydrolysable tannin and sucrose added to food to investigate the influence of the frequency of occurrence. We found that food choice was biased towards lower tannin content. Consumption with respect to tannin content was relative rather than absolute, however, in that the intake of a given concentration depended on which other concentrations were present. In addition, the relative preference for low-tannin food was weaker in the cafeteria test, where this food had a smaller frequency of occurrence, indicating that frequency of occurrence influences food choice. We discuss these results from the perspective of optimal foraging theory and suggest that a changing and complex distribution of food types in nature may make it advantageous for large herbivores to show intermediate selectivity, rather than attempting to ingest only the highest quality food possible.