increased with the age of the calves. No other change occurred in the farmed common eland calves, but in the giant eland calves, the males were found to suckle more than female ones and shorter suckling bouts were marked in primiparous mothers than multiparous ones. The results suggest that Derby elands that lived in their natural habitat adjusted their maternal behaviour so as to be able to readily maintain a vigilant lookout for predators and other similar risks. In contrast, the farmed common elands behaved as in the conditions of captivity, without predators. Populations The eastern giant eland ranged from Nigeria, through Cameroon, Chad, the Central African Republic, and the Democratic Republic of the Congo (formerly Zaire) to Sudan and Uganda in 1980. But the rinderpest outbreak (1983–1984) caused a devastating 60–80% decline in the populations. The eastern giant eland is still found in extensive areas, though it has a decreasing population trend. Because of this, it is listed as 'Vulnerable' by the IUCN. It has many uninhabited habitats that are not expected to be occupied for human settlement, particularly in northern and eastern Central African Republic and south-western Sudan, where their population has notably increased. According to Rod East, 15,000 eastern giant elands existed as of 1999, of which 12,500 are in Central African Republic. The remaining areas are often disturbed by wars and conflicts—activities that can lead to a rapid decline in the eastern giant eland's numbers if not controlled. The western giant eland is in a more dangerous situation, being listed as 'Critically Endangered' by the IUCN. Today they mostly occur in Senegal. In 1990, populations were about 1000, of which 700 to 800 were found in the Niokolo-Koba National Park and the rest in the region around the Falémé River. As of 2008, a population of less than 200 individuals occur there, and only a few elands exist in neighboring countries. A study of the long-term conservation strategy of the western giant eland was done in the Bandia and Fathala reserves, using demographic and pedigree data based on continuous monitoring of reproduction during 2000 to 2009. In 2009, the semi-captive population was 54 individuals (26 males, 28 females). The female breeding probability was 84%, and the annual population growth was 1.36. With more population, the elands were divided into five groups for observation. Although the mean interbreeding level became 0.119, a potential gene diversity (GD) of