Ruminants are unevenly distributed across the range of body sizes observed in herbivorous mammals; among extant East African species they predominate, in numbers and species richness, in the medium body sizes (10-600 kg). The small and the large species are all hind-gut fermenters. Some medium-sized hind-gut fermenters, equid perissodactyls, coexist with the grazing ruminants, principally bovid artiodactyls, in grassland ecosystems. These patterns have been explained by two complementary models based on differences between the digestive physiology of ruminants and hind-gut fermenters. The Demment and Van Soest (1985) model accounts for the absence of ruminants among the small and large species, while the Bell/Janis/Foose model accounts both for the predominance of ruminants, and their co-existence with equids among the medium-sized species (Bell 1971; Janis 1976; Foose 1982). The latter model assumes that the rumen is competitively superior to the hind-gut system on medium quality forages, and that hind-gut fermenters persist because of their ability to eat more, and thus to extract more nutrients per day from high fibre, low quality forages. Data presented here demonstrate that compared to similarly sized grazing ruminants (bovids), hind-gut fermenters (equids) have higher rates of food intake which more than compensate for their lesser ability to digest plant material. As a consequence equids extract more nutrients per day than bovids not only from low quality foods, but from the whole range of forages eaten by animals of this size. Neither of the current nutritional models, nor refinements of them satisfactorily explain the preponderance of the bovids among medium-sized ungulates; alternative hypotheses are presented.

This paper summarizes the recent status of rhinoceros species, as provided by IUCN Species Survival Commission's Rhinoceros Specialist Groups, and describes some of the current conservation measures. At the time of writing there are c. 14 950 rhinoceros remaining in Africa and c. 2850 in Asia. During the last decade conservation initiatives have achieved notable successes; however, numbers of some species and subspecies have declined over this period and three subspecies are close to extinction. The illegal demand for rhinoceros horn and the subsequent poaching this generates continue to pose a serious threat to rhinoceros populations worldwide. However, experience indicates that where anti‐poaching efforts are concentrated above minimum threshold levels population losses as a result of poaching can be reduced to a low and sustainable level. However, not all populations receive sufficient protection and declining budgets of range‐state governments for field conservation are a major cause for concern. The role of donor support is, therefore, becoming increasingly important. For some subspecies lack of adequate habitat protection rather than lack of suitable habitat is a major constraint for population expansion and growth. Many rhinoceros populations in Africa are managed as part of bigger meta‐populations. However, sub‐optimal biological management is also reducing population growth rates in a number of populations.

Optimal plans to manage captive populations for propagation depend upon the goals of the program. Two basic goals have been proposed. The first and more frequent is preservation of genetic diversity in captivity for return to natural environments. The second is adapting a wild population to propagation in the captive environment. Each goal prescribes a general strategy for demographic and genetic management: a plan for return to natural environments and a plan for adaptation to the captive environment. These plans can be considered ends of a spectrum of possible management programs. Similarities and differences in the two plans are discussed. Practical contraints limit the implementation of the basic management plans.

Captive‐breeding programmes are important components of conservation strategies for rhinoceros. Rhinoceros in zoos can serve as (1) genetic and demographic reservoirs to reinforce wild populations as the need and opportunity occur, and (2) ambassadors to increase public awareness and support, especially financial, for conservation of wild populations. However, for these functions, rhinoceros in captivity must be managed scientifically and co‐operatively to produce viable populations. Population‐management programmes for Black rhinoceros Diceros bicornis, White rhinoceros Ceratotherium simum and Indian rhinoceros Rhinoceros unicornis are operating in various regions of the zoo world, especially North America [Species Survival Plans (SSP)] and Europe [European Endangered Species Programmes (EEP)]. Analyses indicate that rhinoceros populations in captivity are achieving variable levels of viability. In SSP and EEP populations Black rhinoceros and White rhinoceros are genetically but not demographically satisfactory, while Indian rhinoceros is healthy demographically but limited genetically. Improvement is needed and could be achieved through better management.

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