A guinea pig couldn't become a dentist

This morning, I was at the dentist. Whilst in the chair, with my mouth numb, and one woman scraping at my teeth whilst another held a small hoover in my mouth I had some time to think. And I started to think about mirrors. Today I had a filling in one of my upper right molars. In order to see what she was doing, the dentist had to hold one of those little mirrors on a stick in my mouth and looking in that to see where she was poking.
We tend to take looking in mirrors for granted. Doing up a tie or buttoning up a shirt in a mirror is normal(*). If we have a bit of food around our mouth and look in the mirror, it's obvious what we need to do with our hands to achieve what we want, taking into account that we're looking at a reflection. But not all animals can do that.
There've been several experiments over the years which attempt to ascertain whether certain animals realise that seeing something in a mirror puts everything the other way around. The experiment, in its simplest form, runs like this. Put an animal in front of a mirror and blicker its view somehow so that the animal can only see in the mirror. Then place some food to one side of the animal or the other. Then remove the mirror and the blinkers and see whether the animal moves in the right direction to find the food. The experiment has been done on quite a few animals, and surprisingly few go straight to the food after seeing its location in the mirror. Apes (including people) have no trouble at all.
Elephants can do it it. As can dolphins (**) and pigs. But not cats, dogs or guinea pigs. It's not entirely obvious to me why these animals would've evolved this ability. Why would it convey any advantage for an animal to be able to just direction in a mirror.
The only mirrors to occur naturally are still pools of water, and by their nature they tend to occur flat on the ground. So anything looking into the pool will simply see a reflected view of itself and the sky and maybe a few trees. I guess it may be useful to be able to see any birds swooping down above which may want to prey on you, but I don't see this applying to elephants or gorillas. So the only animals in which I could see the evolutionary advantage of mirror awareness would be a small animal, preyed upon by birds, which spends a lot of time looking down into pools of water. Shrews, rats, that kind of thing. Except rodents can't do it.
Maybe Darwin was wrong after all (***)

So you see, a guinea pig couldn't be a dentist. At least not a dentist which did fillings in upper molars. However, an elephant would be fine.

(*) Human faces are generally not exactly symmetrical, and so what we see in a mirror is actually the reflection of how we actually look. That's why you never quite look the same in a mirror as you do in photographs.

(**) I have no idea how this experiment was conducted - in practical terms - on a dolphin. I presume with an underwater mirror...

(***) I am being sarcastic. Just in case anyone thought I was serious.

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