Shedding Light on Equine Night Vision

Do you ever worry about being out on the trail after dark? It's a legitimate concern; after all, if humans can't see well in low light, how well can a horse?

Researchers already know that horses have the physical equipment for scotopic (night) vision, with more rods than cones and a reflective structure in the retina. Experience tells us that they continue to graze, interact, and move about at night. Wild mustangs can even be seen running at full gallop over rough terrain while negotiating sagebrush, rocks, hills, and gullies with only starlight to guide them. 

night vision test

Bodie, a Paint/draft cross, correctly chooses the black circle instead of the triangle. (Note: this photo was taken in full light for enhanced photo quality, but all testing for scotopic vision was carried out in low-light conditions.)

Even so, the physics of the reflective structure could actually make it more difficult for horses to discriminate objects in dim light. As a result, humans have remained mostly “in the dark” about just how well our equine friends can differentiate shapes and objects in dimly lit settings. However, recent research at the Equine Research Foundation (ERF) in California, led by Evelyn B. Hanggi, MS, PhD, assisted by program director Jerry Ingersoll, is now shedding new light on this old question.

Through a series of positive-reinforcement tests with four horses in progressively darker situations, the ERF investigators discovered that horses could identify different shapes (circles or triangles) in almost all light levels. Lighting ranges were comparable to outdoor conditions of moonlight, starlight, and even dark, moonless nights in wooded areas. Only when light levels dropped to nearly pitch-dark were the horses unable to discriminate the given shapes. This research demonstrates, for the first time, how perfectly capable horses are of visually discriminating medium- to larger-sized objects in very dim light. 

The researchers further noted that even in extremely low light levels, when the horses could no longer distinguish precise shapes, they could still negotiate their way around the enclosure and testing equipment. Meanwhile, the human experimenters, during their occasional visits inside the testing area (they were only very rarely in the enclosure, for the purpose of conducting equipment maintenance), stumbled into walls, apparatus, pylons, and even the horse itself at times. 

So, most likely, you can trust your horse to follow that trail home at night and avoid many obstacles. Nevertheless, trail riding at night might not be completely safe: those low-hanging branches might be seen by your horse but not by you!--Christa Lesté-Lasserre and Evelyn B. Hanggi, MS, PhD

Reference: Hanggi, EB, Ingersoll, JF, (2009). Stimulus discrimination by horses under scotopic conditions. Behavioural Processes, 82(1), 45-50. DOI: 10.1016/j.beproc.2009.04.009

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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