It was long believed that dogs could only see in black and white, and it is unquestionably true that dogs don't react to colors the same way humans do. However, modern science has given us a window into how dogs experience the world, and it has blown the old idea of dogs only seeing in black and white out of the water. Dogs cannot see the full range of colors that humans can, but their view isn't monochromatic, either. Dogs can see color, but they see the world very differently than humans do.
The structure of the eye determines the amount of colors a creature can perceive. There is a specific type of cell, called a cone, which functions as a color receptor. Humans typically have three types of cones, which pick up red, blue and green. Dogs only have two types of cones, so they can perceive blue and yellow. Some animals have even more cones. For example, the mantis shrimp has 12 cones, which means it can probably recognize colors that humans cannot.
Because dogs do have one fewer cone than humans do, they would fit the human definition of colorblindness. If a human saw the world as dogs do, they'd be red-green colorblind. However, this is the normal state for dogs. Dogs do not seem to experience colorblindness like humans do, as there has never been a recorded case of a dog with fewer color receptors than normal.
In practical terms, dogs do have limited color vision. They can generally just see blue, yellow, and shades of gray. This is called dichromatic vision. So for your dog, that vivid blue sky might stand out strongly, but that bright green grass will fade into the background as a dull gray hue.
Color vision isn't a big thing for dogs in most senses, but it can crop up sometimes. For example, that bright orange toy may pop out to you, but for your dog, it will probably fade into the background since they can't see red. On the other hand, a somewhat dull blue toy will stand out strongly to your dog in most situations, even if it is easy for you to overlook. Some modern toys are designed with a mix of bright orange and blue sections so that they stand out to both humans and dogs.
Since humans tend to place a lot of emphasis on color perception, it is easy to believe that dogs have poor vision because they can't see as many colors. However, dogs have strengths in other areas. Their eyes have more rods, which is a type of cell that perceives motion. If you've ever been walking a dog and had it spot an approaching dog before you could see a thing, those extra rods may be part of the reason.
The structure of a dog's eye can vary significantly depending on the breed. Some breeds, such as bloodhounds and basset hounds, have notoriously poor eyesight. However, all dogs generally have less visual acuity than humans. This means that they can't see fine details as well as humans can. This is particularly true for close-up objects. Your dog may not be able to read the fine print, but luckily that doesn't come up much in a dog's life.
In addition to seeing motion better than humans, dogs also blow humankind out of the water when it comes to low-light vision. They can routinely perceive motion and details better in low-light and dark conditions than humans can. This may be related to their wolf ancestry since wolves, and other wild canines typically hunt between dusk and dawn.
Humans rely heavily on vision, so it's hard for many people to understand how dogs can get by without such acute eyesight. However, dogs rely primarily on their sense of smell, which is around 40 times better than what the average human experiences. Their sense of hearing is also very strong, and they can move their ears to help them pinpoint the direction the sound is coming from.
Although it's not strictly related to color vision, one lingering myth about canine eyesight is that they can't watch TV. According to this myth, TV shows appear as a series of still images to dogs. There is some truth to this with older TVs. Since dogs have better motion perception to humans, slow frame rates that looked like moving images to humans could sometimes be slow enough to look like a slide show to dogs. However, modern TVs have such a fast frame rate that even dogs perceive it as seamless motion.
Dogs' eyes aren't that different from human ones, but it can have some big applications in training. If you want to create a barrier or point that stands out to your dog, try painting it a nice vivid blue. By the same token, if your dog seems to be ignoring something that seems obvious to you, he might just not be able to see it as well as you can. Understanding how dogs see the world is the key to successful training.
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