Among sexually dimorphic, polygynous mammals, adult females tend to outlive males and respond more strongly to predators than males. We asked whether a monomorphic, polygynous species virtually immune to predation due to large size (black rhinoceros, Diceros bicornis) conforms to this pattern. Data on 193 interactions with lions (Panthera leo) and spotted hyenas (Crocuta crocuta) in two nonhabituated populations in Namibia studied from 1991 to 1993 revealed that: (1) females were more vigilant or aggressive than males to either of the potential predators; and (2) whether solitary or with calves, females attacked more often than males. Although solitary females tended to be more aggressive to lions than to hyenas, neither females with calves or males seemed to discriminate between the two carnivores. We also simulated the behavior of human predators (poachers) during 69 encounters with rhinoceroses. While both sexes abandoned local sites because of our presence, females ran farther than males, covering up to 40 km in a day. These findings implicate a behavioral mechanism to explain why secondary sex ratios favor females - males are more prone to human predation, a prediction consistent with data from 12 populations throughout Africa. Black rhinoceroses appear to be an unanticipated exception to the well-established pattern of male-biased mortality in polygynous mammals; in the absence of intense human predation (a recent event), male mortality fails to exceed that of females, suggesting that intrasexual competition in a polygynous mammal: may not be the primary cause of unbalanced secondary sex ratios. Our results on the causes of sex differences in mortality and in responsiveness to different predators reinforce the relevance of behavioral ecology to conservation; such information is necessary for planning how best to minimize negative human influences on the few remaining wild African rhinos.