The following statement is quoted from the comments about the article The function of zebra stripes published in the journal Nature on 1 April 2014.

Eric Faure said:

Equid stripes: do not forget the horses

There have been many explanations for the zebra's stripes. In this study, Caro and his colleagues 1 suggested that biting flies, including horseflies and tsetse flies, are the evolutionary drivers for zebra's stripes. In their article, the authors considered that Przewalski's horses have no stripes and they have excluded domestic and feral horses because their coat colour is labile and they have been subject to intense selection through domestication. In fact, there are supporting evidences that since at least Palaeolithic, horses have worn stripes. So, the relationship between striping on equids and presence of biting flies needs to be explored more accurately in future research.

Striping is observed among very diverse breeds of domestic horses 2. For example, one of the native horse breeds (Sorraia) of the Iberian Peninsula generally exhibits in adulthood stripes on the neck and chest, whereas newborn foals can appear to have stripes all over 3. In addition, some of the feral "mustangs" in the American West have "vestigial" leg stripes and one or two vertical shoulder stripes, and stripes are also found on the pelage of some Przewalki's horse individuals 4. Moreover, Eurasian Palaeolithic engravings and carvings show that horses which seem similar to present-day Przewalski's horses are often portrayed with stripes on their face, neck, shoulder, trunk and upper-legs in various patterns and combinations 4. Striping on the trunk is particularly common among Palaeolithic horse representations in the Southernmost European areas and even if striping appears to be highly variable in a same area, its frequency seems to be significantly higher in Spain, less important in Perigord (a French area particularly rich in Palaeolithic art) and rare in all the regions north of Perigord. It must be emphasized that Palaeolithic cave paintings are remarkable naturalistic representations of the animals shown [4,5]. Indeed, the prehistorical artists had also well depicted horses with "leopard spots" and several occurrences of corresponding allele have been found in Pleistocene horse DNA samples 5. In addition, these striped horses could be associated with depictions of individual of species belonging rather to the boreal fauna, suggesting that temperatures were frequently outside of the optimal range for tabanid flies. Interestingly, Equus burchelli which has the greatest geographical range among the zebras seems to exhibit a clinal pattern of increasing stripe loss toward the south in the Southern hemisphere analogous of that of the Palaeolithic horses in the Eurasian area 4. All these facts suggest that horses must be taken into account in the studies on adaptative significance of stripes and that the hypothesis of the implication of biting flies needs further investigations.

Quoted from the comments about the article The function of zebra stripes published in Nature on 1 April 2014.