Category Archives: Animals

Most Mammals Are Colorblind orThe Evolution of Color Vision

Our color vision is based on three different types of visual pigments. This is called trichromacy and as a matter of fact, it is quite unusual in the animal kingdom.

The following information is a compilation of the article Color Vision: How Our Eyes Reflect Primate Evolution.

Animals have either only one visual pigment, like a few nocturnal mammals. Or they are dichromats, which means they have two different pigment types. This is true for almost all mammals—except the primates, which are most often trichromats. And some birds, fish and reptiles even have four different types which makes them sensitive also for ultraviolet light sources.

The questions arises now: How comes that we are trichromats?

1st step: It looks like that the short-wavelength (S) pigments are the most ancient ones, as they are found in almost all vertebrates.

2nd step: Similar forms to our medium- (M) and long-wavelength (L) pigments are also found quite often—and therefore very old. But only a few primates have both of them, so this has to be a recent evolution.

3rd step: Let’s say the M pigment existed (we actually don’t know). And then through some mutation of a few acids in the DNA the L pigment evolved. Now a first interesting step happened: Some female primates inherited one X chromosome encoding the M pigment and one X chromosome the L pigment and they became thrichromats! This way only female primates could be trichromats as only they have two X chromosomes (male = XY).

4th step: Now the New World primates evolved away from Old World primates. They still are carrying the information encoding color vision as described above. But in the lineage of Old World primates a next interesting development happened: A female primate merged the M and L encoding into one single X chromosome through mutation! This way also male primates became trichromats (thanks!).

5th step: This genetic mutation had such a strong advantage (seeing more colors = finding more good food) that X chromosomes encoding only one pigment were wiped out of the genetic pool.

This five steps of the evolution of color vision sound very interesting. But there are also some questions which arise when reading through, which make me think that we didn’t get to the bottom of it yet. Here are my questions:

  1. How comes that our ancestors suddenly could see the new colors only because of a new pigment type?
  2. Can really only one female primate be the source of our trichromatic vision?
  3. And why the hack are there still so many color blind people? Why is color vision deficiency in so many forms still that widespread?

The authors of Color Vision: How Our Eyes Reflect Primate Evolution have some answers ready. But I’m not sure if I can believe what they are writing.

And there is still no answer for my most important question. Maybe you have one? — Why is color blindness still such a common disease?

Can Dogs see Colors?

The question if dogs are colorblind, specifically red-green colorblind or if dogs only see shades of gray is raised among dog owners quite often. Even on the internet the question concerning dog vision is discussed controversial. But there is a simple answer to that question which I would like to share with you supported by some scientific resources.

Actually there are two distinct questions which I would like to ask, answer and set in relation hereafter:

  1. Can dogs see colors? — Answer: Yes
  2. Are dogs colorblind? — Answer: Yes

Let me explain this two answers to you a bit more in detail.

Dogs can see colors
Dogs not only see in shades of gray but also can see distinct colors contrary to what most people belief. About one hundred years ago some scientific tests were made to find out more about the color vision of dogs. But these tests weren’t that scientific as they thought and the researchers concluded only that color vision doesn’t play a part in the daily life of a dog.

Only about 90 years later distinct researches have shown that dogs can perceive colors. Neitz, Geist and Jacobs researched in 1989 the color vision of domestic dogs and found the following facts:

  1. Dogs have two different color receptors in their eyes and therefore are dichromats.
  2. One color receptor peaks at the blue-violet range, the other at the yellow-green range.
  3. Conclusion: Dogs are green-blind which is one form of red-green color blindness also called deuteranopia.

This results were support by later researches of Jacobs with colleagues in 1993 and Miller and Murphy in 1995.

Dogs are colorblind
This directly leads us to the second question concerning the color blindness of dogs. Colorblind doesn’t relate to not see any colors but describes the fact that you can’t see the same color range as somebody with normal vision. Because of that any kind of color vision deficiency is called color blindness. Therefore dogs are colorblind because of their dichromatic color vision.

Colors dogs can’t distinguish
Actually the color spectrum made up of wavelengths of light is the same to all of us. Only the perception of those colors can be quite different. As dogs only have two different color receptors in their eyes they have problems to distinguish certain colors:

  • Red — Orange — Green
  • Greenish Blue — Gray
  • Different shades of Purple

The list is not completed and there are a lot of different shades which can’t be differentiated if you are colorblind. And the conclusion is dogs can see colors but are at the same time colorblind.

More facts on the vision of dogs can be found in the book Handbook of Applied Dog Behavior and Training, Vol. 1: Adaptation and Learning by Steven R. Lindsay.

Camouflage of the Colorblind Cuttlefish

Cuttlefish have three hearts which pump blood with shades of green and blue through their veins. Cuttlefish have ink and they are caught for food. But the most fascinating fact is: Cuttlefish can rapidly change their skin color. Among others this is used to camouflage them from predators. They adjust their skin color to the ground below them even though they are colorblind!

Giant Cuttlefish

Giant Cuttlefish by Maynard Hogg

Recent researches about color blindness and contrast perception in cuttlefish at the Marine Biological Laboratory have approved this observation.

The next logical step for scientists now is to solve the puzzle how these creatures are nevertheless so good at comouflage themselves.

To find out more about the color blindness of cuttlefish the team around Lydia M. Mäthger used two different types of checkerboards to test their color vision in combination with a change of their body pattern.

  1. Gray – Green. One green shade, matched to the maximum absorption wavelength of cuttlefish, was combined with 16 different shades of gray. As supposed some shades of gray couldn’t be distinguished from the green by the cuttlefish, which didn’t adapt the body pattern. Experiment one was successful.
  2. Blue – Yellow. A checkerboard of blue and yellow matching in intensity was used for the second experiment. The skin color of the test species didn’t change either and therefore experiment two was also successful.

Even though cuttlefish have one of the most developed vision system and can even perceive the polarity of light, this experiments show that they are definitely suffering from color blindness.

The research paper closes with the following words about colorblind camouflage.

In shallow depths of water, broad-spectrum sunlight is available and consequently colored object in the natural environment will appear colorful. At greater depths, the composition of daylight becomes increasingly restricted to the bluegreen parts of the spectrum and the environment loses its colorful appearance. In this light environment, camouflage by intensity matching may be highly effective. Certainly, cuttlefish have broadband light reflectors that reflect the ambient wavelengths of light and may thus aid intensity matching at least at a localized level. However, the vexing question of how cuttlefish masters the task of camouflage in chromatically rich environments, such as those found at shallow depths of water, remains to be answered.

Further readings:
Cuttlefish Masters of Disguise Despite Colorblindness
Color blindness and contrast perception in cuttlefish (pdf)
Wikipedia: Cuttlefish

Are cats and dogs really colorblind?

As Cecil Adams observes in The Straight Dope cats and dogs are not completely colorblind but one can compare their vision with the vision of human being suffering from Deuteranope. The same is reported from Earth & Sky and as I read through my conclusion is: They are just not interested in colors. In their lifes there are much more important things to do then to distinguish different shades of color. Maybe it’s the same for us colorblind: Just more important things to do than spotting red flowers in a green field or learning that the sky is blue even if it is gray all day long these days. I would say we can concentrate much better on our work and don’t get abstracted by seeing our world full of colors. We get the work done!