There was a piece in our local news feed about a vet who, among other things, maintained that animals do not feel pain. This idea comes up surprisingly often, what is unusual is that this time it is from someone who has something to do with animals. As Mrs said when she read it, he clearly has not trodden on a cat's tail.
The proponents of this idea usually maintain that the pain-like responses we observe are something else, just a pain-free reaction to some kind of stimulus. It follows that animals do no have emotions either, our observation of those are just projecting our own feelings onto the animal. They also often add that even the higher animals don't really think as such, it is all just insinct, nothing like what we do at all.
It is an idea that can be made attractive to a certain extent. We see a cat or dog whose mouth corners are turned up and decide they are smiling, but are they? Do they have that facial expression? Do they use it the same way as we do? Maybe they do, but maybe not. It is a bit tricky to get them into an MRI to observe brain function and to get them happy and contented at the same time. Probably observing pain is more likely and I don't know if anyone has tried that, but smarter people than I have put a lot of effort into assessing animal pain. This paper covers some of the work, but it is quite technical..
I want to come at this from a different approach though. The denial of animal pain, emotions and thought is making an assumtpion about human exceptionalism. Our brains are smarter than any animal we know. Way smarter, right? We run rings around all of them. Except we keep finding things like crows that can solve multi-step puzzles that call for complex planning. Sure we can solve them too, and we can build computers and supermarkets so we are still ahead. Ahead, yes, but not exceptional. Animal brains aren't really so different to our own. They use the same basic apparatus: neurons fire and excit other neurons. We have more neurons than the crows, but it is more of the same not something totally different.
Since we are running a variation of the same hardware the people who argue that we are so very different from animals have to explain just where the difference is. You see it is they who are making the radical claim, not the rest of us, so they have to prove that animals do not feel pain etc. We have the default position here. Our brains are not so different, therefore unless demonstrated otherwise, we have to assume they work much the same way. Now plants really are different and most of us think plants don't feel pain. They don't have a brain, so no neurons etc. It is a reasonable assumption. I know some people claim to have evidence that they do feel pain, and that's worth arguing about. It isn't obvious that they must feel pain in the same way we do.
But your basic mammal seems to, and it is also obvious that it would be useful for them to, for the same reason it is useful to us humans. Someone stood on your tail? Screech like mad before they do any more damage. Why would you screech? Because it hurt, of course.
As for emotions, I've seen something that looked like grief when a cow died in a paddock and the rest of the herd discovered it. Sadness when my cat saw I had got my suitcase out and he knew I was travelling again (okay he didn't know I was travelling, but he knew I would be away). Excitement when I got back.
Thought is harder. The moment you call something thinking the response is that it is not really thinking, it is something else. Well, I'll define thinking as creative planning. Crows definitely do that. Cats and dogs do it too, especially those brilliant assistance dogs that give us so much help.
As for how smart we really are, well not as smart as we like to think. What we do really, really well is cooperate. Put a chimp by itself in a jungle and it will probably do just fine. Put a lone human there and they'll be dead in a month. Put 100 humans in the same place and they'll have a town. Some animals can cooperate, but none do it as well as we do.