Correlation between dichromatic colour vision and jumping performance in horses Julie Spaas a , Werner F. Helsen a , Maurits Adriaenssens b , Sarah Broeckx c , Luc Duchateau d , Jan H. Spaas c, * a Research Centre for Movement Control and Neuroplasticity, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, B-3100 Heverlee, Belgium b Elscolab, Hogenakkerhoekstraat 14, B-9150, Kruibeke, Belgium c Global Stem Cell Technology, Geeneindestraat 1, B-3560, Meldert-Lummen, Belgium d Department of Comparative Physiology and Biometrics, Faculty of Veterinary Medicine, Ghent University, B-9820 Merelbeke, Belgium ARTICLE INFO Article history: Accepted 21 July 2014 Keywords: Equine Colour vision Performance Show jumping A B ST R AC T There is general agreement that horses have dichromatic colour vision with similar capabilities to human beings with red–green colour deficiencies. However, whether colour perception has an impact on equine jumping performance and how pronounced the colour stimulus might be for a horse is unknown. The present study investigated the relationship between the colour of the fences (blue or green) and the show jumping performance of 20 horses ridden by two riders using an indoor and outdoor set of green and blue fences. In the indoor arena, significantly more touches and faults were made on blue fences in comparison to green fences (median difference of 2.5 bars). When only touched bars were included, a significant median difference of one bar was found. Mares (n = 4) demonstrated more faults and had a significantly greater difference in touches and faults between the two colours than male horses (n = 16). Repeating the same experiment with eight horses in an outdoor grass arena revealed no significant differences between the two colours. In order to draw any definite conclusions, more research concerning the colour perception, influence of contrast with the arena surface and sex of horse is required. © 2014 Elsevier Ltd. All rights reserved. Introduction In mammalian retina, colour discrimination is mediated by colour-opponent neurones that respond with opposite polarity to signals from short (S, blue) and longer wavelength (M, green or L, red) cones (Mills et al., 2014). Different spectral cone classes are found in the visual system of most evolved vertebrates (Bowmaker and Hunt, 2006; Bowmaker, 2008). Among the vertebrates, there are monochromic, dichromic, trichromatic and tetrachromatic types of chromatic vision, depending on the different number of photopigments (Jacobs, 2009). Several cone classes appeared very early in vertebrate evolu- tion (at least 540 million years ago), prior to the separation of the jawed (Gnathostomata) and jawless (Agnatha) vertebrate lineages (Xian-Guang et al., 2002; Shu et al., 2003). The recently supported ‘contrast theory’ (Sabbah and Hawryshyn, 2013) proposed that mul- tiple cone classes evolved in shallow water fish to maximise the visual contrast between objects and their background (McFarland and Munz, 1975). This was an interesting evolutionary feature, since primates with better colour discrimination are able to detect preda- tors earlier against a green foliage background (Pessoa et al., 2014). Mammals have an ancestor with achromatic vision in dim light and this is probably why most non-primate mammals have been left with only two of the four ancient vertebrate cone pigments and a high rod-to-cone ratio in their retina (Jacobs and Rowe, 2004). In addition, it has been postulated that dichromatic vision is superi- or to trichromatic vision at detecting camouflage when there is a colour match between the target and the background (Morgan et al., 1992) and dichromatic animals would also outperform trichromat- ic ones when foraging in shade (Caine et al., 2010). The horse (Equus caballus) is classified as a perissodactyl mammal or odd-toed ungulate. In this order, there are still conflicting data as to which wavelengths of light (i.e. which exact colours) the animals can discriminate (Jacobs, 1993; Farrall and Handscombe, 1999; Hall and Cassaday, 2006; Hall et al., 2006). Previous studies using different methods indicated that the horse has two cone pig- ments (Sandmann et al., 1996; Carroll et al., 2001; Hall et al., 2006). Carroll et al. (2001) reported that the cone spectral sensitivity for the M/L photopigment of the horses has a spectral peak (λmax) of 539 nm and that the best fitting S cone pigment curve had a λmax * Corresponding author. Tel.: +32 13 556106. E-mail address: janspaas@gst.be (J.H. Spaas). http://dx.doi.org/10.1016/j.tvjl.2014.07.016 1090-0233/© 2014 Elsevier Ltd. All rights reserved. The Veterinary Journal 202 (2014) 166–171 Contents lists available at ScienceDirect The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl