I’ve asked my doctors that question and they don’t seem to know, or even care. But that only reinforces my opinion that Medicine is not a science, its more like engineering. Doctors and engineers need to know a lot of science, but they are more interested in practical results than in knowledge for its own sake.
But be that as it may, I have come up with a hypothesis. For any animal, it is better to piss in moving water, a stream, than in a stationary body of water, like a waterhole. Flowing water will carry the waste away, ensuring a cleaner source of drinking water, but it will also disperse scent away, making it harder for predators to locate you.
By urging an animal to preferentially empty its bladder where the waste will be carried away would have an evolutionary advantage; any species that did this would have a smaller likelihood of poisoning itself or its fellow herd/troop members so that behavioral trait would be selected for. I’m only speculating, of course, I have no way of proving or disproving this, but it seems reasonable.
Of course, there is another alternative, referring to Dan’s comments on religion in the post below. Perhaps God has programmed animals to preferentially piss in rivers, not ponds, for obscure reasons of His own, and of course, species hygiene is as good a reason as any.
And, of course, the religious explanation has one additional advantage. Once we adopt it, we don’t need to think about it any more.