Studies by Charles Otis Whitman, Oskar Heinroth and Julian Huxley set the tone in the 20th century. This work was mostly on bird behaviour, especially mating behaviour. The behaviour of birds played a big part in early ethology, but there was a start to studying our nearest relatives. Wolfgang Köhler's "Mentality of Apes" was a landmark in the study of primates. In 2020, Dr. Tobias Starzak and Professor Albert Newen from the Institute of Philosophy II at the Ruhr University Bochum proposed that animals may have beliefs despite these being more difficult to determine than humans.