The mare pasture is visible from the indoor arena, so I sometimes get to see interesting mare interactions. One of the things I like most about spending time with horses is getting to see their interactions with each other in the herd - our pastures are big and the herds have at least 15 horses in them.
I was standing in the indoor the other day, talking with another boarder. I noticed Dawn grazing in the mare pasture. A buckskin mare approached her, stopped about 20 feet from her, and stood there with her ears pricked, intently looking at Dawn. I said to the other boarder: "That mare is asking Dawn a question."
Sure enough, within a few seconds, they were grooming. I've seen that behavior before in herds - the subordinate mare will ask if it's OK to groom, and the more dominant horse has to accept the offer.
It's good to see Dawn grooming with another horse in her herd. Since she's moved to the barn - in 2012 - she really hasn't had a special mare friend, and this was the first time I've observed her grooming at this barn.
Dawn will occasionally call to Missy, both when she's in the pasture and I have Missy inside, or will nicker when Missy comes back to her stall - their stalls are next to one another - but I've never seen them grooming.