Absent are Giraffe, impala (Aepyceros melampus), topi (Damaliscus lunatus), oribi (Ourebia oribi), crocodile (Crocodylus niloticus). Cheetah (Acinonyx jubatus raineyi), East African wild dog (Lycaon pictus lupinus), and African leopard (Panthera pardus pardus) are rarely seen. Spotted hyenas (Crocuta crocuta) have been the subject of a long-term research study in the Ngorongoro Conservation Area since 1996. Although thought of as "a natural enclosure" for a very wide variety of wildlife, 20 percent or more of the wildebeest and half the zebra populations vacate the crater in the wet season, while Cape buffalo (Syncerus caffer) stay; their highest numbers are during the rainy season. Since 1986, the crater's wildebeest population has fallen from 14,677 to 7,250 (2003-2005). The numbers of eland and Thomson's gazelle also have declined while the buffalo population has increased greatly, probably due to the long prevention of fire which favors high-fibrous grasses over shorter, less fibrous types. Serval (Leptailurus serval) occurs widely in the crater. Lake Magadi, a large lake in the southwest of the crater, is often inhabited by thousands of mainly lesser flamingoes. The crater has one endemic species of mammal: Mduma's shrew (Crocidura mdumai), which is restricted to montane forests on the edge of the crater. This shrew is considered endangered due to deforestation from smallholder farming. Lions The crater has one of the densest known population of lions, numbering 62 in 2001. A side effect of the crater being a natural enclosure is that the lion population is significantly inbred. This is due to the very small amount of new bloodlines that enter the local gene pool, as very few migrating male lions enter the crater from the outside. Those who do enter the crater are often prevented from contributing to the gene pool by the crater's male lions, who expel any outside competitors. Long-term data imply that lions in the crater were struck by four deadly disease outbreaks between 1962 and 2002. Drought in 1961 and rains throughout the 1962 dry season caused a massive build-up of blood-sucking stable flies (Stomoxys calcitrans) by May 1962. They drained blood and caused painful skin sores that became infected, causing lion numbers to crash from 75-100 to 12. The population recovered to around 100 by 1975 and remained stable until 1983 when a persistent decline began. Numbers have generally remained below 60 animals since 1993, reaching a low of 29 in 1998. In 2001, 34 percent