What Colors Does Your Dog Actually See? And Why Did Color Vision Evolve?

I'm a bit late but recently there was a Tiktok trend where people would apply a filter that supposedly showed you what the world looked like to a dog. Reactions to the filter went semi-viral as some people sobbed that their dog couldn't enjoy the same vibrant colors that we humans do, which was pretty funny. This had me wondering though, how accurate was this filter? If a dog could see the same spectrums of light as us could it even "appreciate" these colors in the same way we do? What does THAT even mean? So, just like a dog that can't jump very high but wants to escape your backyard, I did a little bit of digging. 

I like this picture of this dog staring at me like an 18th century english orphan  

A little bit of background (long groan)...

To begin we have to understand what color vision is. Anybody that has taken a basic bio course knows about rods and cones, photoreceptor cells in your eyes that absorb light and trigger a complex reaction that allows us to see things. I remember these being described to me as "one helps you see shapes and the other helps you see colors," which is an oversimplification. In fact, both cells help you see colors, just slightly differently under different conditions. 

In the dark, both rods and cones release glutamate, an important chemical that sends signals to neurons called bipolar cells which act as an in-between to your other neurons, which lets the brain understand "vision". There are ON bipolar cells, which are called that because they are excited when the light is on, and OFF bipolar cells, which are excited when the light is off. Thus, OFF bipolar cells are excited by glutamate production during the dark and ON bipolar cells are excited by a complex chemical reaction that occurs when light shuts off your body's glutamate valve. The glutamate valve for rod cells are inhibited at lower levels of light and their bipolar cells can inhibit cone OFF bipolar cells, overtaking most of the cone pathway for vision at night and becoming our main way of seeing in the dark. 

But what about color? Thats where cones come in. Humans contain four types of light sensitive proteins called opsins. Cones have three classes of opsins: long (L), medium (M), and short (S),  that are excited by the corresponding wavelengths for red, yellow-green, and purple-blue. Rods only have one, which is why in the dark, when rods are dominant, everything appears more muted and grayish. 

    

A chart that every physics 101 class has seen

The possession of three cones for color vision is called trichromacy and its actually something that is specific to humans and other closely related primates. Most mammals (including your dog) have dichromatic vision, meaning they have some combination of only two types of opsins (usually S with M or L). Birds, amphibians, reptiles, and fish are often tetrachromatic (with 4 opsin proteins) and sometimes rarely pentachromatic, although having five types of opsins and being able to distinguish between the colors they provide are two different things. This is why although mantis shrimp have 55 different types of cones, the idea that they can "see more colors" than us is a bit of an oversimplification. A recent study found that mantis shrimp can not distinguish between wavelengths less than 25nm apart, contrasted by most humans who can distinguish between wavelengths by 1-5 nm. Its thought that this actually helps the shrimp out though. By not worrying itself with all these colors, they can reduce the amount of time it takes their brains to see contrasts between different organisms, helping its survival and making its responses faster. After all, seeing the world as a kaleidoscope of colors would probably get pretty distracting.

Mantis shrimps are however the only organism that can see circular polarization. What does that look like? I don't know, ask your local shrimp.

So what colors do dogs see? 

The answer is we really will never be able to know for sure (my least favorite answer that scientists love to give). But because they lack an L cone they probably see something like this:

The Tiktok filter was right all along! 

How did we get here?

The real reason I wrote this article wasn't because I cared too much about what colors dogs can and can't see. Most people already knew somewhat about rods and cones and that their dog is a dingus. The real reason is because I wanted to know more about opsin evolution. 

Like all proteins, opsins have genes that code them. It used to be that to study these proteins, we would have to isolate them directly from animal retinas, but with modern technologies scientists have opted instead to have cultured cells produce them for us in the lab. This is nice because it allows researchers, who are really just curious children at heart, to play around with the cells genes and see what happens to the opsins. In addition, we have sequenced the genes for about 1000 opsins, from humans to jellyfish, which has provided even more background to their history. 

Scientists theorize that around 500 million years ago, a jawless proto-vertebrate had already developed four opsins homologous to our modern day ones. Scientists dubbed the classes of these opsins as SWS1 and SWS2 (homologous to human S opsins), and RH2 and LWS (homologous to M human opsins). At some point, probably around 250 million years ago, mammals became more and more nocturnal in order to escape predators that hunted mostly in the daytime. In response, over time they lost RH2 and SWS2, which is why most mammals are dichromatic, like your dog. From what I've read, its not clear what the advantage of having SWS1 over SWS2 was however, since theres really not too much of a difference between the two except a little bit more UV-sensitivity in SWS1. 

Case in point, we as humans lost this UV-sensitivity in exchange for seeing blues and purples. Scientists found that exactly seven genetic mutations of the SWS1 gene changed the opsin wavelength sensitivity in primates as they switched from being nocturnal to foraging in the daytime. Seeing blues and purples might have allowed them to see berries and fruits better against green topiary, giving blue seeing primates a bit of an advantage.

Seeing more colors is useful!

But what about the LWS opsin? Overtime LWS slowly shifted its sensitivity to become M, allowing us to see the color green the way we do, and the L gene evolved from that. Again, this is probably because seeing contrasting colors in the daytime is helpful for finding food. Theres two hypotheses for what might have happened to create this two opsin gene system. The first is that there were two variants of this gene, one that was more sensitive to long wavelengths and one for medium ones. This means that at some point  our ancestors were probably running around seeing the world slightly differently from one another. Its theorized that this gene duplicated due to unequal crossing over during meiosis in a female primate. This meant that rather than one gene with two types (M and L) two distinct genes for M and L were created on a single X chromosome. Any children from this primate would now only need a single X chromosome with this mutation to attain trichromatic vision. 

The other hypothesis is that the M opsin gene duplicated. This would have allowed mutations to occur on one set of this gene whilst keeping the other intact. So while the original M gene remained, the duplicate could have had multiple mutations acting on it to eventually become L, allowing us to see the vibrant reds we know and love.

In the colorful history of our vision, it seems our ancestors were quite the sightseers, observing the world through slightly different lenses. Whether it was a tale of two gene variants, causing them to see greens and mediums with a quirky divergence, or the mischievous duplication of the M opsin gene, our vision has certainly evolved in an interesting manner. Keep those eyes wide open, my trichromats, and go hug your dog.