section_path: Color vision/Nonstandard color perception/Tetrachromacy
section_name: Tetrachromacy
paragraph_text: Tetrachromacy
  
  Outside of humans, which are mostly trichromatic (having three types of cones), most mammals are dichromatic, possessing only two cones. However, outside of mammals, most vertebrates are tetrachromatic, having four types of cones. This includes most birds, reptiles, amphibians, and bony fish. An extra dimension of color vision means these vertebrates can see two distinct colors that a normal human would view as metamers. Some invertebrates, such as the mantis shrimp, have an even higher number of cones (12) that could lead to a richer color gamut than even imaginable by humans.
  
  The existence of human tetrachromats is a contentious notion. As many as half of all human females have 4 distinct cone classes, which could enable tetrachromacy. However, a distinction must be made between retinal (or weak) tetrachromats, which express four cone classes in the retina, and functional (or strong) tetrachromats, which are able to make the enhanced color discriminations expected of tetrachromats. In fact, there is only one peer-reviewed report of a functional tetrachromat. It is estimated that while the average person is able to see one million colors, someone with functional tetrachromacy could see a hundred million colors.