A small proportion of people, and entire species of animals, also see only in shades of gray, not because of brain damage but because their retinas aren’t set up for color vision. They are called monochromats. Some, like sloths and armadillos, only have rod cells, which work well in dim light but aren’t geared toward color. Others, like raccoons and sharks, only have one cone, and since color vision depends on opponency, having one cone is effectively like having none. Whales have just one cone, too: To paraphrase the vision scientist Leo Peichl, for a blue whale, the ocean is not blue.
