Thursday, September 23, 2010

How Do Horses Recognize Each Other? Part One: Color Vision

I've always been interested in how horses see, and in particular how they recognize other horses, and distinguish, often at a distance, horses they know from horses they don't know. Horses also clearly have a strong ability to recognize different people, although I believe that horses recognize people by using a subset of the recognition methods they use to tell familiar horses from unfamiliar horses.

But before we get to that, how horses perceive color is important.

One thing that's clear to anyone who's spent time around horses is that they have very acute vision, even for things at a far distance - this would make sense for an animal whose native habitat is open grasslands.  People often wonder how they see color, and the answer's pretty interesting.  First, how do people perceive color?

The average human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight). Normally, there are three kinds of cones, each containing a different pigment, which are activated when the pigments absorb light. The technical names for these receptors are S-cones, M-cones, and L-cones, but they are also often referred to as blue cones, green cones, and red cones, respectively. The absorption spectra of the cones differ; one is maximally sensitive to short wavelengths, one to medium wavelengths, and the third to long wavelengths, with their peak sensitivities in the blue, yellowish-green, and yellow regions of the spectrum, respectively. The absorption spectra of all three systems cover much of the visible spectrum. Although these receptors are often referred to as "blue, green and red" receptors, this is not entirely accurate, especially as the "red" receptor actually has its peak sensitivity in the yellow region. The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different degrees. Red light, for example, stimulates the long wavelength cones much more than either of the others, and reducing the wavelength causes the other two cone systems to be increasingly stimulated, causing a gradual change in hue. (from Wikipedia article on human color blindness)
Here's a diagram of the color sensitivity of the three types of cones humans have in the eye - it's hard to make out the details in this diagram but it gives you the idea:

Horses have only two sets of cones, the S-cones and the M-cones - the two to the left in the diagram above - which means that they have the same type of "color blindness" as humans with a fairly rare type of color blindness, affecting 1% of the male population (the more common type of color blindness, affecting 5% of the male population, is caused by the M-cones being missing and the L-cones (the far right-hand ones) being present - this is the "normal" type of red-green color blindness)(Side note: most varieties of color blindness are much more common in men than women since the genes for the cones are on the X chromosome).

For comparison, here is the color spectrum as seen by humans with all three sets of cones:

Here is the color spectrum as seen by humans with the more common red-green color blindness (missing the middle set of cones):

And here is the color spectrum as seen by horses, most other mammals and humans with the rarer form of color blindness (missing the right-hand set of cones):

So those orange traffic cones or red caution tapes? Horses see them as drab brown, or yellowish-brown, so not much alerting value there.

Contrast, on the other hand, is a big deal for horses in my experience, as they are well-supplied with the rods that make up black/white vision capabilities. White objects are common sources of spooking, and situations with high contrast - such as crossing stripes on pavement or the contrast between an area that is in shadow and one in sunlight, or between two parts of a path made of different materials, can provoke a lot of anxiety in horses. Horses have much better low-light vision than people as well.

I think the way horses see color, and also the way they react to high contrast, have a good bit to do with how they recognize other horses, and also people - the next post will talk about my (entirely hypothetical) thoughts on this, in part based on some interesting experiences involving Lily.  Stay tuned . . .


  1. Very interesting! can't wait for part 2!

  2. Wow! I started reading and got sucked in... It's like a black hole!! I kept reading and reading... and found it very interesting:o}. I wonder - do markings and/or special tints of color such as roan and silver dapple confuse horses? What if two horses have the same color (like, exactly the same - such as my horse and his brother, who are both jet black with white dots on their backs and small, teeny white stars)?

  3. I may be jumping the gun, so I'll wait and share my story when part 2 comes out, but I do think horses are able to recognize each other from a distance. :)

  4. Where did you get the information about horses' vision? There was an interesting scientific paper that I linked to a long time ago on the subject, but it's no longer there. I ask because your analysis has one big difference -- this paper showed that red, bright orange, and bright bright green all showed up as the brighter yellows in a horse's vision -- not drab at all. And that would make sense, if you think about the fact that these are danger colors in nature.

  5. My boy has no problem stepping in muddy water, but if it's bright and reflective there's no way he's getting near it and I think it's 100% about contrast.

    I wonder if they look like holes in the ground.

    He had a massive spook last weekend when we came up on a deer in the trail. She was smack dab in the middle of the it and I could see her just fine, but I knew he couldn't because she wasn't moving. He probably thought she was a rock or something until she took off.

  6. I have a feeling you may get to this as well, but in my experience a lot of how they recognize each other has to do with their sense of smell. Tucker and I frequently run into horses that he knew as babies, and they quite clearly know each other instantly, sometimes even before they are in sight of each other, which I assume is based on smell. The horses that Tucker "recognizes" look very different now than they did as yearlings, so I'm convinced that they must just smell the same. I've always been curious as to how their sense of smell compares to other animals, like dogs for example. Interesting post -- looking forward to Part 2!

  7. Marrissa has a good point, often when out, where other horses have pooped, she stops to smell, or lowers her head to test the smell! From that she will whinny, or not, havent quite decided what the means yet. As I am colour blind, that makes me an my horse almost the same.

  8. Katharine - there isn't all that much out there that I've been able to find. Here's an article from The Journal of Vision:

    There are other articles, but this is the best one I've found.

    The basic facts are that horses have two sets of cones. No one knows the subjective experience of horses seeing objects in color, but the best analogy is to humans who are missing the right (L) set of cones. Keep in mind that brightness depends on how close a color is to the peak sensitivity of a set of cones - so colors to the right of the peak sensitivity will be dimmer/duller.

  9. I know that Gilly LOVES blue! The blue of a Pepsi can is his favorite color. If I have a blue glass, blue can he WANTS it and will drink anything you are drinking!

  10. So bright purple would be more visible to the horse than red or orange. Interesting. And green grass would be a shade of yellow?

  11. Well, chartreuse is one of my favourite colours...looks like I will have to swap out Gem's red saddle pads, etc....:-) Can't wait for part two!

  12. I've really noticed how horses have a stronger reaction to palamino's. This post would explain that.

  13. I knew about the cone thing; actually we painted our pedestals blue for that very reason :o) I would agree with Marissa though, that the sense of smell plays a huge part in their memory process (and in ours too). Isn't that why they breathe into each other's nostrils; to get the scent?
    Hearing also plays a role, I think. When I had my mastectomy and couldn't go outside for a week, I said something to my daughter while I was standing near a window watching the horses. Bella heard me speak and started calling and calling; she was looking everywhere for me (made me cry, actually :o)

  14. So interesting, Kate! I, too, cannot wait for the next post on this!

  15. What a VERY interesting post! Thanks for taking the time to write it.


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