It's a stress behavioural thing.
I'm glad I did all the steps to rule out physical discomfort. I would have hated to think something was causing her pain, but now that I know it's not physical, I can look at it in a different way.
I've been really watching Kachina lately and trying to see if there are connections between the times she opens her mouth (is it when she's asked to collect? is it always when she goes to the right? is it when the halter is putting pressure on her face? etc). Turns out, when Kachina is relaxed, thinking, listening to me, and following directions successfully, she doesn't do it. When she's in a new place, tied away from her friends, being asked something she doesn't fully understand, or isn't been given enough direction, she does do it.
(This may seem obvious, but really, it isn't obvious until you figure it out)
As an example, one day recently I was working her in the indoor arena. There was nobody else inside and rain was making lots of noise on the roof so it was a little scary. When I was lunging her and clearly telling her what I wanted by giving aids for commands she knows, she was listening to me and relaxed, and her mouth was quiet. Then, my phone rang and I had to answer it (I generally ignore my phone when I'm at the barn, but this was an exception). As I was talking, I let her circle me a bit more aimlessly because I only had one hand and not my whole attention. Pretty quickly, Kachina realized she didn't have a solid leader to follow any more and she started getting more anxious. She kept staring off at different things and changing direction, and her mouth went into that characteristic gape.
|Mouth gaping while loose in the arena|
This is an excellent discovery. Kachina isn't super expressive with her ears or body, and since she has blue eyes, you always see the whites. That makes judging her mood tougher than with lots of other horses. Now that I know open mouth means stress, I can use it as a signal to adapt my training. Gaping her mouth doesn't let her get out of work, but if I see she's getting stressed, I can bring the exercise back to something she understands and can be successful at.
Also, now I can see that Kachina gets stressed out by not having a leader. Before, I was confused because Kachina would open her mouth even when I turned her loose in the arena to move around on her own terms. That's something that many horses love and so I didn't expect it would cause a negative reaction. Now, I see that if Kachina doesn't have a good idea of what exactly is expected of her in a situation, she gets stressed out. (The horse personality book also got me thinking about Kachina's needs as a submissive horse so that was helpful too)
|See mouth again, you can kind of see how she crosses her jaw a little when she does it|
Considering how often it happens, I have surprisingly few photos showing it
This also makes sense with how Kachina is better behaved under saddle than in ground work. When I am on her back, I am a clear leader and she is willing to relax and follow my direction. When I'm on the ground, I don't always have the same focus. (And she generally gapes her mouth less while I am riding compared to on the ground) Also, I don't have as big of a figurative toolbox on the ground compared to in the saddle (because I never needed it with past horses), so I'm not as confident and clear in my directions. Before, I would switch between different approaches on the ground because I didn't know which was the best method. Now, I am going to stick to one approach and be really really consistent with it so Kachina understands what the expectations are.
I'm excited to use this new discovery to try and cut down on stress for both of us and move forward with our training. :-) Maybe eventually, we'll build her confidence enough whereby the weird mouth thing goes away completely!