Crater lions. 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. The lion population is also influenced to some extent by the takeover of prides by incoming males, which typically kill small cubs. The biggest influence, however, appears to be disease, particularly canine distemper.