Rosie, you rock like a Springsteen concert. Welcome to the Straight Dope Message Board.
Livin’ on Tums, vitamin E and Rogaine
Damn. Never thought that what seemed to be a rather inocuous question would raise such passions, such an ardent desire to get at The Truth (at one point, it nearly drove venerable Doctor Jackson to distraction – not sure he got over his frustration, actually).
If truth be told, I thought it might have been part of a mating ritual. Odd enough, given that there was little else action to be seen (mind you, in all cases I’ve seen this, I was driving and didn’t stop to look for…specifics).
Rosie, you went beyond the call of duty in sending e-mails to your expert friends. Much appreciated.
Doctor, I hope you keep digging, if only for your own peace of mind.
Manhattan, I guess congrats are in order re: moderator thing. Or are they…?
Okay Doctor Jackson, it definitely seems to be a tie. But if it is the tradition here for posters who end up in a tie to kiss their siblings, I will happily concede the point to you. g
A couple more replies:
From Dr. Katherine A. Houpt, Professor of Veterinary Physiology in the College of Veterinary Medicine at Cornell University, Director of the Animal Behavior Clinic:
“I think you are right because they do it mostly in summer when there are insects. If it were for predator protection they would do it all year. They need not be so close for predator protection. Mares will form circles around foals if they are threatened.”
From Dr. George Waring, Professor of Zoology at Southern Illinois University at Carbondale (http://www.science.siu.edu/zoology/waring/):
“The stance is a social arrangement often used by horses, especially among best-friend pairs. It allows for social interaction (such as touching and mutual grooming), tail flicking of insect pests, and the general monitoring of their environment while hanging out. Thus you and your friend have both identified part of the answer.”
And from Dr. Ted Friend, Professor of Animal Science at Texas A&M:
"This is one of those issues that no one knows for sure. I tend to favor keeping flies off their faces - because they do it in barns and other places during fly season where they could not see a predator approaching. I don’t know of anyone who looked for correlations between the behavior and numbers/types of flies. Sounds like and interesting study.
Elephants, for example, do stand head-to-tail when frightened. When you see them do it, it is obviously a predator defense behavior."
Finally, from Dr. Lee Boyd, Biologist-At-Large, Washburn University:
“I think you are both right. The two benefits aren’t mutually exclusive.”
(I have to confess, omni-not, that I tend to get a little obsessive about q’s like this. But I think I’m finished now. Oh, except that it is definitely not directly associated with mating - when a stallion gets that close to a mare in heat, he isn’t particularly interested in standing quietly by her side.)
And thanks for the welcome, Manhattan!
Rosie, you have to promise us that, in the future, you won’t try to smother us with useful information. This could create a dangerous precedent here 
Many thanks to all.
Doctor: Should the anxiety symptoms persist, can I humbly suggest a couple of Prozacs?
ROFL! Some kind of equine 69, omni? ROFL!
Doc: Would it surprise you to learn that THAT thought had also crossed my mind, although fugaciously?
Doctor Jackson:
(I apologize in advance for the hijack!!) This occured about 18 yrs ago. After we tied a rival high school in a soccer game, a teammate once told me , that a tie was like kissing your sister. I thought he made it up. Unless you’re him, that’s obviously not true!
Where did this expression come from??
That’s not a hijack, that’s a coup d’état. If you have searched the boards and http://www.google.com to see if your question is out there, you can start a thread about it.
Anyone wishing to discuss equine behavior is invited to start a thread in whatever forum is appropriate.
bibliophage
moderator, GQ