The influence of rainfall on range in a female desert ungulate: the Arabian oryx (Oryx leucoryx) in the Sultanate of Oman

In this study we investigated the effect of rainfall on the home range (cal culated over 90 days) of adult female Arabian oryx (Oryx leucoryx) in a rei ntroduced population in central Oman. We looked at response to rainfall for six rain events in the period June 1986 to January 1992; rainfall was loca lized on five occasions and on one occasion covered the study area. Respons e, in terms of range and core area, showed considerable variation between r ainfall events, perhaps as a result of variation in ranging behaviour betwe en individual oryx. However, in the first 90 days after localized rainfall the home range and core area of oryx outside rain areas increased before de creasing significantly 91-180 days after rain. In comparison, range and cor e area of oryx inside areas of localized rainfall either did not change or decreased after rainfall as animals took advantage of improved conditions l ocally rather than investigating grazing elsewhere. When rain fell over the whole study area, range size did not change, but there was an immediate in crease in core area and a reduction in range overlap indicating movement to new areas. During extended drought very localized rainfall caused oryx to move quickly to areas of new rain and by 28 days 70% of all adult females h ad found fresh grazing. Thereafter numbers declined to around 30% by 225 da ys, before oryx again moved back in to the new rain areas. We attributed th is pattern of movement to changes in the nutritional quality, measured as c rude protein, of available forage. Data for single animals revealed conside rable individual variation in response to rainfall; some animals moved quic kly to rain whilst others failed to reach it. This variation had consequenc es for reproductive success and survival and illustrated that, for the oryx , response to rainfall is a key to desert success.