Albinos and leucistic specimens are seen with some frequency in the aquarium trade. White tigers cannot see as well as normal tigers and suffer from photophobia, like albinos. Partial melanism in badgers is known, and albinos are not uncommon. Fur of senescent river otters may become white-tipped, and rare albinos may occur. Occasionally, colour mutations such as albinos and white spots throughout the pelage occur. The epidermis is thinner in the albinos and contains fewer melanocytes. These three strains are called "tremper albinos", "rainwater albinos" and "bell albinos". Blue eyes and the eyes of albinos, however, lack pigment. Some albinos in Tanzania have been the victims of violence in recent years. Tanzania is thought to have the largest population of albinos in Africa. Occasionally, colour mutations, including albinos and erythrists, occur. The next mutations to occur were white-faces, silvers and albinos. Many color variations have been found, including albinos and white varieties. In the eyes of albinos, the cells of this layer contain no pigment.