We reviewed the literature on sexual segregation in polygynous ungulat
es in an effort to clarify terms and concepts, summarize recent inform
ation that supports or discredits three broadly defined hypotheses, an
d suggest directions for future research that should help resolve when
and why the sexes segregate in these large mammals. The hypotheses di
scussed include those based on intersexual differences in energetics a
nd security (reproductive-strategy hypothesis), body size dimorphism a
nd dietary requirements (sexual dimorphism-body size hypothesis), and
social mechanisms (social-factors hypothesis). These hypotheses repres
ent ecological, physiological, and social perspectives and are not mut
ually exclusive. Most evidence reviewed supported the reproductive-str
ategy hypothesis. Less support was available for either the sexual dim
orphism-body size hypothesis or the social-factors hypothesis. Nonethe
less, most available evidence is provided by field studies that conten
d with many confounding variables. We suggest several areas of future
study that may serve as critical tests and are likely to be productive
in resolving why sexual segregation occurs in polygynous ungulates.