Habitat segregation in ungulates: are males forced into suboptimal foraging habitats through indirect competition by females?

Sex differences in habitat use (termed `habitat segregation') are widespread in sexually dimorphic ungulate species. They are a puzzling phenomenon, particularly when females use better foraging habitats than males. It has been suggested that males, owing to their larger body size and higher forage requirements, are inferior in indirect competition to females and are forced by female grazing pressure into marginal habitats (`indirect-competition hypothesis'). This hypothesis has been widely cited and has important theoretical and practical implications. However, evidence for it is inconclusive. The present paper presents the results of the first experimental test of the indirect-competition hypothesis. We manipulated female and male numbers of red deer (Cervus elaphus L.) on a large scale on the Isle of Rum, Scotland, and tested the influence of this manipulation on deer habitat use. We predicted that where female numbers were reduced, male use of preferred habitat should increase and sex differences in habitat use should decrease, while a reduction in male numbers should not have such effects. In contrast, we found that the manipulation of female and male numbers did not affect habitat use, and conclude that the indirect-competition hypothesis does not explain habitat segregation on Rum.