The answer to this question is highly debated—even saying “besides humans” is bound to result in a heated discussion. Many lists of “the most intelligent” animals exist, and some of them include such vertebrates as rats and sheep. But according to the famous American behavioral biologist Edward O. Wilson (1929–), the ten most intelligent animals are as follows: 1) chimpanzee (two species); 2) gorilla; 3) orangutan; 4) baboon (seven species, including drill and mandrill); 5) gibbon (seven species); 6) monkey (many species, especially macaques, the patas, and the Celebes black ape); 7) smaller toothed whale (several species, especially killer whale); 8) dolphin (many of the approximately eighty species); 9) elephant (two species); 10) pig. Still another list based on a scientific study includes 1) dolphins; 2) chimps and orangutans; 3) elephants; 4) parrots; and 5) crows. Like all similar scientific studies, the list of animals not only changes as new information is learned, but also varies depending on the researcher’s criteria for “intelligent.”