Although thought of as "a natural enclosure" for a very wide variety of wildlife, up to 20% or more of the wildebeest ("Connochaetes taurinus") and half the zebra ("Equus burchelli") populations leave the Crater in the wet season. The Ngorongoro lions are significantly inbred, with many genetic problems passed from generation to generation. This is due to the very few new bloodlines that enter the local gene pool, because few migrating male lions enter the crater from the outside. Those who do enter the crater often cannot contribute to the gene pool. The crater's male lions, because of their large size (the result of an abundant food source), easily expel outside competitors.