Yerkes, Robert Mearns, YUR keez, RO buhrt murnz (1876-1956), was an American psychologist known for his research on the behavior of apes. His most important book is The Great Apes: A Study of Anthropoid Life (1929), which his wife, the botanist Ada Watterson Yerkes, helped him write. The book describes the anatomy, behavior, and intelligence of chimpanzees, gorillas, and other apes. During 1923 and 1924, Yerkes raised two chimpanzees in his home. He wrote Almost Human (1925) and coauthored Chimpanzee Intelligence and Its Vocal Expressions (1925) based on this experience.