By Jim Maxwell, Ph.D.
Whales are color blind. Why cetaceans are color blind remains somewhat of a mystery. Except for some nocturnal species, virtually all land mammals have color vision and, presumably, so did the ancient ancestors of whales from which they evolved (the closest living animal to the whale is the semi-aquatic hippopotamus). The lack of color vision in cetaceans must have occurred early in their evolutionary development because all cetaceans studied so far, whether toothed or baleen, have been found to be essentially color blind. Interestingly, pinnipeds (seals) are also color blind. Since seals and whales are not closely related, scientists think that the loss of color vision must have served some advantage millions of years ago when their ancestors lived in coastal environments.
Most land mammals have two types of color-sensitive light receptors (called cones) in their retinas. One type of cone is most sensitive to the red-green part of the color spectrum (long to medium wavelengths of light) and the other is most sensitive to the blue (short wavelength) part of the spectrum. Somewhere along the line, whales and seals lost the short-wavelength-sensitive receptor cells (called S-cones) leaving them with only the long to medium wavelength cones (L-cones). Because the brain relies on a comparison of the signals coming from different types of cones to produce the perception of color, having only one cone type leaves whales functionally color blind (monochromats, to be precise).
Incidentally, unlike most mammals, humans and many other primates have three types of cones which (except for color blind humans) allows us to distinguish red from green. We should not boast, however, of our color vision prowess. Some turtles, other reptiles, birds and many fish have four types of cones and can see into the ultraviolet part of the spectrum not visible to humans.
Of course, whales can see just fine (though not nearly as well as humans) it’s just that they rely on contrast more than color. Their visual acuity (ability to discern detail) has been studied in trained dolphins and has also been estimated by examining the density of photoreceptors within the retinas of several different species of whale. But that is a story for another blog.