Do horses see colours at night?

Do horses see colours at night?  How well do horses distinguish colours at night? Light is detected by specialised receptors in the retina at the back of the eye. The receptors responsible for detecting colours are called "cones". At least two different types of cone are required to enable some form of colour vision. Each class of cone is particularly sensitive to light of a certain wavelength. The sensitivity is determined by the "photopigment" (a light-absorbing protein) inside of the cone. Most mammals possess two distinct classes of cone. Consequently, their eyes are particularly sensitive to two wavelengths of light and so are termed "dichromatic" (literally "two colours").

At dimmer light intensities the ability to detect movements from possible predators may be the most important visual task

Usually one photo-pigment is sensitive to middle- or long-wavelength light. A second one is sensitive at the short wavelength end of the spectrum. The normal human eye, in contrast, has three classes of cone and is described as "trichromatic".

The colours seen by the horse are likely to be similar to those seen by humans with red-green colour blindness, Normal human eyes detect four basic colours: red, green, yellow and blue. They can also differentiate up to 100 subtle variations in hue. Humans with red-green colour vision defects can only see two basic colours - yellow and blue.

Dr Lina  Roth, of the department of cell and organism biology at Lund University in Sweden, with colleagues Anna Balkenius and Almut Kelber conducted a behavioural experiment to determine whether horses were able to distinguish colours in dim light conditions.

They trained three horses to choose between two coloured doors (either blue or green) in return for a food reward. Two horses were trained to open the blue door;  the other was trained to choose the green door.  When the horses responded correctly at least 70% of the time, the investigators repeated the tests in ever decreasing amounts of illumination.

All the horses were able to differentiate blue from green correctly at levels of light  greater than or equal to 1.2cd/m2    (equivalent to the light at sunset. )  Two of the horses lost interest as the light levels were reduced further. 

However, one horse chose the blue stimulus a total of 31 of 40 times at a light level of 0.02 cd/m2   (equivalent to moonlight).  When the investigators reduced the light intensity further to 0.007 cd/m2 he failed to choose correctly. 

So these results suggest that at least some horses can discriminate colours at a light intensity similar to that of moonlight.

How does this compare with human vision?  The investigators subjected six people to the same test.  They found that at light intensities of 0.02cd/m2 ,  all but one individual was able to distinguish the difference between blue and green.

When  the light intensity was reduced to 0.007 cd/m2 ,  five of the six individuals failed to identify the colours correctly.  However one did manage to make the correct choices at 0.007 cd/m2 . Only when the light level was reduced further, did that individual fail to differentiate between green and blue. 

" Our study suggests that horses and humans have similar thresholds of colour vision and discriminate colours in moonlight intensities (0.02 cd/m2 )" the researchers report.

However, they note that only one horse performed well at such light levels. So "colour seems to be a less important stimulus for horses than for humans."

"At dimmer light intensities the ability to detect movements from possible predators may be the most important visual task, and achromatic vision may therefore be favoured."

For more details see:

The absolute threshold of colour vision in the horse.
LSV Roth, A Balkenius, A Kelber. PLos One (2008) 3, e3711.
doi:10.1371/journal.pone.0003711

Reproduced with kind permission of Mark Andrews BVM&S CertEP MRCVS
© Copyright Mark Andrews  - Equine Science Update 2008

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