Prominent Dutch zoologist and pioneer cryptozoologist best remembered for his study of sea serpents. Oude-mans was born into a family known for its scientific and intellectual accomplishments. He developed an early interest in the study of animals. He completed his doctorate in zoology in 1885 and shortly thereafter became the director of the Royal Zoological and Botanical Gardens at The Hague. A decade later, he moved to the University of Sneek before beginning a 27-year tenure at the University of Arnheim.
As a youth, Oudemans became interested in the question of sea serpents and began to assemble a collection of reports. He reached an early tentative conclusion that the reports referred to a variety of primitive whale only known from fossil records. His ever growing interest in the sea serpent question led to a book, The Great Sea Serpent, published in 1892. By that time, he had concluded that the sea serpents were probably a form of long-necked seals. The book met mixed reviews, but although it was denounced by some, it succeeded in renewing consideration of the reports of a giant ocean animal, which had persisted through the century in spite of the general consensus among his colleagues that no such beast existed. In 1933, a decade after his retirement, Oudemans suggested that the Loch ness monster, which had at that time first become well known internationally, was also a variety of long-necked seal. He expressed hope that its identification would solve the sea-serpent question once and for all.