A We certainly don’t know the full answer, but I can tell you what we do know. Male horses, especially if not gelded, tend to defecate in particular areas, which in their stall tends to be in the same one or two spots. This is called elimination marking behavior, and it is believed to have evolved as a form of social communication among the herd members. As a herd moves along a trail to water, for example, each stallion who passes certain landmarks will defecate in the same place, on top of the feces of the ones that have gone before. We don’t really know the message or messages, but, for example, the feces of a harem stallion might be a message to other herd members that his group passed that way. The fecal marking behavior and the accumulation of stud piles, whether in a domestic or natural situation, is probably all done subconsciously. Meaning the animal is not aware of its behavior and the message, whatever it is.

Within a herd, defecation by males also involves some elaborate rituals of squealing and mini-fights and posturing repeatedly among the various pairs of stallions. So where there are harem stallions and bachelor stallions (stallions without a herd of mares), the defecation and rituals at a stud pile are one of the more conspicuous features of animal interaction, sometimes taking up a considerable amount of a stallion’s time.

Female horses do not have this same tendency; they just seem to defecate wherever and whenever the urge arises. Without the stallion hormones, many geldings also lose this tendency for defecating in the same place. In natural situations, harem stallions also spend some time and effort "covering" the urinations and defecations of the females in their band. So whenever a female defecates or urinates, the stallion approaches and exhibits a fairly fixed ritual sequence of investigating and covering the voided material with his own urine and feces. The sequence includes sniffing, then the flehmen response (the behavior where the horse raises and turns his head and curls his lip while inhaling). When the horse covers the voided feces or urine with his own, you can imagine that the social message to the next stallion passing by that way might be something like: "This mare is already taken