Night Vision In Horses Better Than Previously Thought

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29th September 2009

Research shows that horses see better at night than we do.

Dr Evelyn Hanggi and Dr Jerry Ingersoll, of the Equine Research Foundation in California, conducted a stud to see if horses could distinguish simple geometrical shapes under low light conditions.

Firstly, they trained the horses to choose between two shapes - a circle and a triangle. If the horse chose the correct shape, by touching it with its nose, it received a reward. Two horses were trained to choose the circle and two the triangle.

If a horse could not differentiate between the shapes, it would be expected to choose the correct card 50 per cent of the time purely by chance.

"In reality, chance is higher still, given the number of trials run and the significance level" Dr Hanggi explains.

"For a run of 20 trials, horses could still respond by chance at 75 per cent, given an alpha of 0.01. For alpha equal to 1.05, chance would occur at 65 per cent."

So, to be sure that horses' response was not down to chance, the researchers set a target of 80 per cent correct tests before the would conclude that the horse could distinguish the shapes.

Once the horses were rained, the next step was to test if they could still distinguish the shapes under dimly lit conditions.

The lighting was controlled to give conditions ranging from the equivalent of twilight to a dark moonless night in a dense forest.

Hanggi and Ingersoll noticed during early training that it took the horses some time to adapt to dark conditions. When the lights were dimmed quickly, the horses tended to bump into objects or stand still. So they were allowed to stand in the dark for at least 15 minutes before the start of each test.

The result showed that horses were able to see down to very low light levels. They could differentiate between to two shapes in almost complete darkness - something that the human experimenters were unable to do. Only in conditions similar to a dense forest with minimal visible sky did the horses lose the ability to distinguish the shapes.

In even darker conditions they could still navigate their way to and from the start point of the test - even when it was so dark that the light meter was off the scale.

It should come as no surprise that horses see better at night than we do. Indeed, Dr Hanggi points out that wild mustangs are able to run over rough terrain, dodging sagebrush, rocks hills and gullies, their way illuminated only by the starts.

The horses eye is well suited for seeing in the dark, Rods, the light-sensitive cells that are responsive in dim light, outnumber the colour-sensitive cones by about 9:1.

The eye also has a reflective layer, the tapetum lucidum, which reflects light back through the retina increasing the light available to the light responsive cells. While increasing the sensitivity to dim light, the tapetum lucidum also reduces discrimination ability in some species due to light scattering.

Dr Hanggi adds, "Our research not only provided scientific evidence that horses see very well in the dark (which many horse people already know), it also showed that they can discriminate individual objects in very low light conditions (which most people didn't know)."

For more details see:

Stimulus discrimination by horses under scotopic conditions.
EB Hanggi, JF Ingersoll
Behavioural Processes. (2009) 82, 45-50. et feature