Most of us are aware that cats, dogs, and many different wild mammals, birds, and reptiles have better night vision than we humans do. However, we might forget to include domestic horses, cattle, and sheep in this group of animals that possess eyes adapted for sensitivity and acuity in low-light conditions. The large eye of the domestic horse, Equus caballus, has a mobile pupil that can dilate extensively at night to increase sensitivity to photons. In addition, the choroidal, or vascular, layer of the equine eye has a reflective modification called the tapetum lucidum, which bounces photons back for a second chance at capture by the photoreceptors of the retina (reviewed by Ollivier et al., 2004).
Photographs representing the trichromatic appearance for a human (above), and modified to represent the appearance to a dichromat, such as the horse (below). From Carroll et al. (2001)
Horses are active during the day and night, and at dawn and dusk, and are thus considered to be arrhythmic. Their eyes have evolved to achieve high sensitivity in dim light, as well as acuity and color vision capacity in brighter light conditions. Like other ungulates, such as cows, sheep, and deer, horses have two cone types (S = short-wavelength-sensitive; L = long-wavelength-sensitive) that allow for dichromatic color vision (Carroll et al., 2001). Cone pigments in various ungulate species differ in their spectral sensitivities. Humans, as trichromats, possess three cone types; the photos above simulate dichromat vision in daylight, in comparison with our trichromat view.
Experimental apparatus for testing horse responses to colors. The wooden divider forces the horse to choose a door at a distance of 40 cm from the stimulus. From Roth et al., (2007)
Molecular and biophysical studies of cone pigments have allowed characterization of the spectral sensitivities and modeling of the visual experience, but behavioral experiments are key to understanding how the horse eye works under different environmental conditions. In 2007, the research group of Roth and colleagues, at Lund University in Sweden, trained horses to choose between rewarded and unrewarded colors, in an experiment that revealed how dichromats perceive their chromatic space. Horses were first trained to positive stimuli on both sides, with carrots behind both unlocked doors (which the horse could open), in the apparatus shown above; next, a negative stimulus was introduced on one side, with the door locked so that the horse could not reach the carrots if it chose incorrectly.
Dual choice apparatus wall, with the door for the positive stimulus left unlocked, so that the horse can reach the carrots. From Roth et al. 2008
Recently, the Lund University group used this experimental paradigm, to compare the absolute threshold of color vision in horses and in humans, under dim light conditions corresponding to a moonlit night (Roth et al., 2008 ). Three horses were initially trained in the dual-choice paradigm shown above: Chap, a 14 yo half blood gelding, Rex, an 11 yo Thoroughbred gelding, and Rosett, a 33 yo Shetland pony mare. All three horses reached the required “learning criterion”, and performed well at higher light intensities, equivalent to or higher than those typical of sunset. However, both Rex and Rosett eventually lost motivation for the experiment after the trials at the lower light intensities; only Chap persisted throughout the lowest light intensity (0.007 cd/m2) tested. As shown in the figure below, Chap was able to discriminate between the positive and negative stimulus colors at the light intensity equivalent to moonlight. However, at the lowest light intensity, equivalent to a moon-less (starlit only) night, Chap could no longer correctly identify the positive stimulus color.
Choice frequencies at different light intensities for Chap (black bars) and Rosett (gray bars). From Roth et al. 2008
This paper suffers a bit from the very low number of equine subjects included, especially as compared to the previous study, in which 7 horses were used. However, there is an interesting comparison with six human subjects in this report, and all but one could distinguish between the test colors under the moonlight illumination conditions. One human subject even outperformed Chap, and was able to discriminate between the colors at the lowest light intensity. In addition, the researchers measured focal length and cone dimensions in sections of fresh-frozen horse eyes, taken from two animals euthanized for humane reasons. Based on their results, Roth and colleagues conclude that although horses and humans have similar thresholds of color vision, color may be less important as a stimulus or signal for horses at low light intensities. This might explain why Rex and Rosett lost interest in the experiment as the light intensity was lowered in the testing area. In dim light, achromatic rod vision is favored in the horse eye, and would still allow detection of movement, as well as avoidance of most obstacles.
Carroll J, Murphy CJ, Neitz M, Ver Hoeve LN, and Neitz J (2001) Photopigment basis for dichromatic color vision in the horse. J. Vision 1, 80-87
Ollivier FJ, Samuelson DA, Brooks DE, Lewis PA, Kallberg ME, and Komaromy AM (2004) Comparative morphology of the tapetm lucidum (among selected species).Vet. Ophthal. 7, 11-22
Roth LSV, Balkenius A, and Kelber A (2007) Colour perception in a dichromat. J. Exp. Zool. 210, 2795-2800.
Lina S. V. Roth, Anna Balkenius, Almut Kelber (2008). The Absolute Threshold of Colour Vision in the Horse PLoS ONE, 3 (11) DOI: 10.1371/journal.pone.0003711