Do ungulates exhibit a food density threshold? A field study of optimal foraging and movement patterns

1. The behaviour of ungulates foraging in non-patchy habitats is studied with the help of a new model of optimal foraging based on a time minimization utility function. The model predicts that an animal uses a foraging station only if the biomass exceeds a prescribed threshold level. Moreover, the forager does not consume all the available food but it gives up the station when food density reaches the threshold level. 2. The assumptions of the model were tested using 36 foraging paths of adult female fallow deer (Dama dama L.) in two different study areas (one wild and one fenced) and seasons (summer and autumn, respectively). Despite the ecological differences between the two sites, the deer exhibited the same pattern of search behaviour which can be described as a first-order biased random walk. Neither herd seemed able to detect and exploit food patches. 3. Partial consumption of food was regularly observed in both study areas, There is strong evidence that the fenced-in animals behaved in agreement with the model's prediction, abandoning the foraging stations when the biomass density fell below 10 g m(-2) of dry matter. On the other hand, the deer living in the wild exhibited a more variable foraging behaviour. 4. The theoretical analysis shows that in comparison to the fenced-in deer those in the wild require more information to correctly evaluate the threshold level of their more heterogenous environment. 5. It is argued that the foraging behaviour of fallow deer consists of two basic components: a random search for food coupled with a threshold in the selection of foraging stations.