Why do most dogs have brown eyes?

About the episode

Researchers say it's possible that we humans cause most dogs to have brown eyes.

It has been known for some time that the selective breeding of dogs, which we began tens of thousands of years ago, has affected their personality and appearance. Some of these things, such as large foreheads and short noses, are now causing health problems.

Now it also seems that their eye color is influenced by our preferences. While wolves, from which all dogs descend, have light-colored eyes (which likely helps them read each other's intentions), 90 percent of domestic dogs have dark eyes.

To find out why this happened, researchers from Japan showed participants pictures of dogs in an experiment in which the dogs' eye color was always slightly modified. The dogs then had to be classified based on characteristics such as friendliness, intelligence, and aggressiveness.

It turned out that dogs with dark eyes were friendlier. Now this was a small study, with a limited number of breeds (33) and mostly Japanese students, but the result fits with our preference to make dogs look more and more like our cute children. Children have larger pupils than adults, and dark eyes are closer to that.

But, say scientists who were not involved in this research, it is also possible that we have become accustomed to color and thus increasingly selected for it. The research also says nothing about when dark eyes became dominant.

So there's still a lot to be discovered, but it's an interesting theory.

Read more about the research here: Why do most dogs have brown eyes?