Terrestrial animals have historically played an important role in the production of certain therapeutic hormones and steroids. Progesterone was first isolated from pig ovaries in 1934. Animals were eclipsed by plants, however, as a source of steroidal precursors when in 1940 Russell Marker discovered that Mexican yams of the genus Dioscorea could be utilized to produce diosgenin. There has also been a long-term research interest in venoms produced by snakes as sources of pharmaceuticals. Ancrod, a drug used to treat circulatory diseases, is isolated from the venom of the Malayan pit viper Agkistrodon rhodostoma. Recent research, such as that conducted by Francis Markland at the University of Southern California, suggests that proteins such as contortrostatin found in certain snake venoms may be able to fight cancer tumors. The poisonous secretions of some toads and frogs contain bufotalin, which was traditionally used as a treatment for dropsy before Withering’s discovery of digitalis. Ethnozoologist Wade Davis has also found that secretions of species of Bufo were used as pyschotropic substances by indigenous peoples in Central America. Epibatidine, from the poison dart frog, is being developed as a possible analgesic. It is likely that other venoms and stings will produce new pharmacological insights.