As large-bodied mammals become restricted to progressively smaller fragments of former habitat, competitive interactions and interspecies aggression are likely to intensify. Data on the outcomes of 159 encounters between endangered African pachyderms revealed that: (1) female elephants (Loxodonta africana) dominated both sexes of black rhinos (Diceros bicornis); and (2) rhino males but not females displaced elephant bulls. The results of an additional 127 interactions involving pacyderms and 12 additional mammals ranging in size from cheetahs (Acinonyx jubatus) to giraffe (Giraffa camelopardalis) indicates that females of either pachyderm deny immediate access to limited resources. Although the evolution of gender-specific asymmetries in interspecific dominance has received little formal study, it may best be explained by understanding patterns of parental investment; however. From a conservation perspective, one consequence of size-related dominance is that with continued containment of elephants and rhinos in managed reserves, the most handicapped species are likely to be the smaller ones.