The single worst thing you can do to an animal emotionally is to make it feel afraid. Fear is so bad for animals I think it's worse than pain. I always get surprised looks when I say this. If you gave most people a choice between intense pain and intense fear, they'd probably pick fear.
I think that's because humans have a lot more power to control fear than animals do. My guess is that animals and normal humans are opposites when it comes to fear and pain, and for roughly the same reason: different levels of frontal lobe functioning. This idea first popped out at me when I read two studies back-to-back on the frontal lobes in pain and in fear. What struck me was that while an active prefrontal cortex was associated with increased pain, it was also associated with reduced fear (though not with reduced anxiety). Pain and fear, at least in these studies, were opposites.
The story isn't that simple, of course, but it's close enough that, until we learn more, I believe animals have lower pain and higher fear than people do. My other reason for believing this at least provisionally is that it's the same with autistic people. As a general rule, we have lower pain, higher fear, and lower frontal lobe control of the rest of our brain than nonautistic people. Those three things go together. (I'm not saying that autistic people have no pain at all and don't need painkillers. I don't want to give that impression.)
You almost have to work with animals to see what a terrible emotion fear is for them. From the outside, fear seems much more punishing than pain. Even an animal who's completely alone and giving full expression to severe pain acts less incapacitated than an animal who's scared half out of his wits. Animals in terrible pain can still function; they can function so well they can act as if nothing in the world is wrong. An animal in a state of panic can't function at all.
I also think intense fear is an easier state for animals to get into than it is for normal human beings -- a lot easier. Animals feel intense fear when they're threatened in any way, regardless of whether they're predators or prey.
While all animals can be overwhelmed by terror, prey animals like cows, deer, horses, and rabbits spend a lot more time being scared than predators do. You've heard the expression "like a deer caught in the headlights" -- that pretty much sums up the prey animal's psyche. They are very nervous animals, because the only way a prey animal can survive in the wild is to run. Since a prey animal has to start running before the lion does, that means it has to be hyper-alert all the time, keeping a watch out for danger.
We know animals feel pain thanks both to behavioral observation and to some excellent research on animals' use of painkillers. Starting with behavior, dogs, cats, rats, and horses all limp after they've hurt their legs, and they'll avoid putting weight on the injured limb. That's called painguarding. They limit their use of the injured body part to guard it from further injury. Chickens who've just had their beaks trimmed peck much less, another obvious form of pain guarding. (Ranchers trim chickens' beaks because chickens get in horrible fights and will peck each other to death. The vet trims off the sharp point so the chicken can't use it as a knife blade.)
We have more evidence that animals feel pain from the experiments Francis C. Colpaert did on animals and pain medication in the early 1980s. He injected rats with bacteria that produce a temporary bout of arthritis we know is painful in humans, then gave them a choice between a bad-tasting liquid analgesic and a sweet, sugarytasting liquid rats normally like. The rats chose the bad-tasting painkiller over the sugar solution, a pretty good sign they were choosing it for its painkilling properties. They definitely weren't choosing it for its taste.
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