2. Physiology wise, as everyone has mentioned, the kidneys produce urine (a staggering quantity by the way) but you could remove the kidneys after filling the bladder, and the horse could still urinate. The bladder is safely inside the pelvis, where you could beat on the pelvic wings with a hammer (don’t), and the inches of solid bone plate (and the free floating nature of the bladder in the pelvis) prevent any external pressure from reaching it. And also remember, when you urinate (older men particularly) you bear down slightly (the Valsalva Maneuver), which is deliberatively putting external pressure on the bladder to force urine out.

3. Culture. I imagine that this is purely a cultural thing, Similar to being able to train a dog to only urinate out of the house, or on a “wee-wee pad” in an apartment, I am guessing that if the horse is taught by repetitive behavior that when it urinates the rider is going to stand, then heck, that will become the norm. Horses are not known for their deductive reasoning, so they learn pavlovian style. Riding western I never heard of this. I first encountered it in the highlands of Scotland, and have seen it only at places which have “English” riding as their primary style (this is anecdotal, I have not surveyed a wide number of stables)

4. The shifting of weight. Simple physics tells us that standing does not take any weight off the horse overall. However, depending on whether you are riding english or wester, you may be putting more of your weight on the chest wall with your grip of your knees, so standing actually seems to raise the weight directly on the back. That being said, squeezing from the side might be disturbing to a horse urinating (don’t know, I’m not a horse). As far as leaning going up/down, to me it’s more about not falling off (by association your horse would inherit your uncoolness if you fall off, so that’s a good thing for the horse) and staying in control (I can’t maintain a posture on a 17-20% grade without doing some serious leaning, which is easier if I can pivot by standing on the stirrups [western]). And for the record, I have urinated just fine with a large pack on my back, so I have to agree with above, although it is slightly more awkward.

I am basically saying, there doesn’t seem to be a physiologic explanation, and this is cultural training of the riders and horses.So do whatever your horse is used to (if the horse isn’t used to it, you may actually startle it when you stand)

I’m from Ireland and my boyfriend is from Australia.

thank you