the historical range of the species in Namibia, Botswana, Zimbabwe, Uganda and Eswatini, while a small population survives in Mozambique. Populations have also been introduced outside of the former range of the species to Kenya and Zambia. Wild-caught southern whites will readily breed in captivity given appropriate amounts of space and food, as well as the presence of other female rhinos of breeding age. However, for reasons that are not currently understood, the rate of reproduction is extremely low among captive-born southern white females. Northern white rhinoceros The northern white rhinoceros or northern square-lipped rhinoceros (Ceratotherium simum cottoni) is considered critically endangered and possibly extinct in the wild. Formerly found in several countries in East and Central Africa south of the Sahara, this subspecies is a grazer in grasslands and savanna woodlands. Initially, six northern white rhinoceros lived in the Dvůr Králové Zoo in the Czech Republic. Four of the six rhinos (which were also the only reproductive animals of this subspecies) were transported to Ol Pejeta Conservancy in Kenya, where scientists hoped they would successfully breed and save this subspecies from extinction. One of the two remaining in the Czech Republic died in late May 2011. Both of the last two males capable of natural mating died in 2014 (one in Kenya on 18 October and one in San Diego on 15 December). In 2015, the Kenyan government placed the last remaining male of the subspecies at Ol Pejeta under 24-hour armed guard to deter poachers, but he was put down on 19 March 2018 due to multiple health problems caused by old age, leaving just two females alive which reside at the Ol Pejeta complex. Staff hope to inseminate the remaining females with the last male's semen, although the semen is not preferable due to the age of the rhino. Following the phylogenetic species concept, recent research has led to the hypothesis that the northern white rhinoceros is a different species, rather than a subspecies of white rhinoceros as was previously thought, in which case the correct scientific name for the former should be Ceratotherium cottoni. Distinct morphological and genetic differences suggest the two proposed species have been separated for at least a million years. However, the results of the research were not universally accepted by other scientists. Description The white rhinoceros is the largest of the five living species of rhinoceros. By mean body mass, the white