You just finished giving your horse a bath. She finally is clean of whatever mystery gunk was stuck in her mane, she smells good, and dang is she going to look shiny when's she's finally dry. You lead her out to the field, fingers crossed, take off the halter, and.... DOWN she goes. You watch in horror as your beautifully bathed white mare plunks herself into the first patch of dirt she can find and goes to town, rolling back and forth until she looks (at best) like a dun - or worse if it's really muddy, as dark as a bay. *Face/palm*
Chances are, you know this scenario all too well. I always get a chuckle out of the photos my friends post on Facebook of their horses after they ruined the results of their baths. It's a long-standing joke, especially among owners of white horses: your horse won't stay white for long.
But despite the comedic timing that makes us imagine it's some kind of scheme that horses make behind our backs ("Oh yeah, she thinks she can get me nice and clean? Watch this!") this common situation has a fascinatingly scientific explanation. Because, as research has proven, horses really can't think logically enough to devise plans to get under our skin. So it's gotta be for another reason ("It'd better be", you're thinking).
Sure enough, if we look at the ethology, we can find some very logical explanations that support not only why our horses roll, but also why we could all probably just chill out a little bit about the desire to keep our horses spotlessly clean.
So here's the down-and-dirty on rolling:
It's actually in important self-grooming behavior for horses. If you really think about it, it's a genius, practical solution for a large animal that wouldn't normally have humans to brush it in the wild. Rolling can help get rid of shedding hair and dead skin, and it helps maintain a healthy coat. Horses generally roll after getting wet - whether from a bath or from a swim in a pond, and so far behaviorists guess that that's generally just because it feels good (like drying or combing your hair after a shower). Some venture to say that the dirt and mud act as natural insect repellent, but it's hard to say if that's just coincidence or if horses are aware of the benefit.
Rolling is also a herd behavior, and it has been surmised that a major function of rolling is depositing scents over the body. Research shows that over 80% of rolling occurs where another horse has rolled (which explains those giant bare patches in your paddock). The general assumption is that the most dominant horse rolls last in order to leave their scent last, although I haven't yet discovered any research confirming that.
Rolling has also been concluded to be a comfort behavior, at least according to one study.
So you see, regardless of how proud you are of your horse's clean, shiny, fluffy, nice-smelling coat, your horse is naturally driven to instantly get it dirty again with a nice relaxing roll in the dirt.
Well, at least they're happy... right?