FORAGING AT DIFFERENT SPATIAL SCALES - DORCAS GAZELLES FORAGING FOR LILIES IN THE NEGEV DESERT

We examined the foraging strategies of dorcas gazelles (Gazella dorcas ) at different spatial scales. When in sand dunes in the Negev desert of Israel, the gazelles fed on a single plant species, the madonna lil y (Pancratium sickenbergeri). In summer, when all live plant material is subterranean, the gazelles dug holes in the sand to remove the stem and bulb. After the winter rains, only the tips of the leaves that ha d emerged above ground were eaten. We examined search patterns and pat ch choice on a large scale using both null hypotheses generated by a r andom walk model and ''rules of thumb'' that may approximate an optima l foraging strategy. At a smaller scale, we examined the selection of parts of individual lilies, again making predictions about ''rules of thumb'' that the gazelles should use. Gazelle search paths deviated co nsiderably from a random walk. Gazelles made shorter move lengths in a reas of high plant density. Bouts of concentrated feeding in small are as were interspersed with long moves to new foraging areas, suggesting that these animals were repeatedly sampling their environment. Congru ent with optimal foraging predictions, the gazelles selected plants wi th more and larger leaves than randomly available, and concentrated th eir foraging activity in areas of highest lily densities. In summer, g azelles altered the depth of digs in response to differences in sand c ompaction. There was a surprising negative correlation between the siz e of the plant and the amount eaten. This selection of small plants ap pears to be due to the increased probability of reaching the bulb, whi ch contains most of the plant's volume. Summer foraging by gazelles ha d a negative impact on the lilies. There is strong selection on the li lies to grow to a sufficient depth that damage of the bulb by herbivor y is minimized. Conversely, the gazelles must select plants with bulbs close to the surface to maximize energy intake and minimize the energ y cost of digging.