By Jim Maxwell, Ph.D.
ONE FISH, TWO FISH, RED FISH, BLUE FISH!
We don’t know if whales can count but we do know they can’t tell one color of fish from another. 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, scientist think that the loss of color vision must have served some advantage millions of years ago when their ancestors lived in coastal environments.
In order to understand color vision in whales, it helps to know a little bit about how the visual system works in humans and most other mammals. The retina in the back of the eye contains two types of light receptors, cones and rods, that convert light energy into electrical signals that can be further processed by the brain. Cones are active in the light and are responsible for color vision and the perception of detail. Rods are much more sensitive to light and take over for the cones in dim light but rods are typically not involved in color vision.
Most land mammals have two types of cones. 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, most odontocetes (toothed whales) and seals, through genetic mutations, lost the functionality of short-wavelength-sensitive receptors (S-cones) leaving them with only the long to medium wavelength cones (L-cones) and rods. 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 these marine mammals essentially color blind, or, “L-cone monochromats” in the jargon of vision science.
Studies within the past five years have indicated that many, and possibly even all, mysticetes (baleen whales) have not only lost the functionality of S-cones but the functionality of L-cones as well, making them totally reliant on rod photoreceptors for vision (and hence called “rod monochromats”). Rod monochromacy is rare in terrestrial mammals and results in very poor eyesight. By comparison, humans and many other primates have very good color vision having three types of cones. Most other mammals (and color-blind humans) have only two types of cones and consequently cannot distinguish red from green. We do not have the best color discrimination in the animal kingdom by any means. Some reptiles, including some species of turtles, birds and fish have four types of cones and can see into the ultraviolet part of the spectrum not normally visible to humans.
Needless to say, whales do have functional vision even if it is in shades of gray, it’s just that they rely more on contrast than on color. How well whales see is hard to determine with any precision but it is certainly much inferior to the average human. The acuity of cetaceans, that is, their ability to discern detail, has been studied in trained dolphins and has also been estimated in several other species of toothed and baleen whales by examining the anatomy of their eyes and retinas. But that is a story for another blog.