Along with his wife Maria, whom he met at the University of Kiel and later married in Peru, he spent much of his life studying the Peruvian and South American fauna. He co-authored with Maria many scientific publications, mostly ornithological. His greatest individual academic accomplishment was the publication (in German) of the 1,684 page two volume opus entitled Die Lebensformen: Grundlagen zu einer universell gültigen biologischen Theorie (Life Forms: The basis for a universally valid biological theory), in 1971 and 1973. According to François Vuilleumier, curator of the Department of Ornithology, American Museum of Natural History in New York City:The number of topics covered in this monumental work (volume 1, pages 1–789; volume 2, pages 790–1,684) is
