This experiment, done on lab rats, isn't terribly cruel. It will still probably keep you up at night when you're thinking about hiring an exterminator.
Rats are nearly universally disliked, but they are useful — their physiology isn't that different from our own, so studying how their bodies react to drugs gives us clues to how human bodies would react to the same drugs. We've known this for a long time. What we're learning is that a rat's emotional state might also not be that different from a human's, and that's troubling.
Rats got a lot of press for responding with apparent happiness when they were tickled, much like humans do. But hey don't just experience positive emotions. One experiment proved this by injecting rats with acetic acid, a.k.a. vinegar. This isn't as horrible as it sounds; provided the concentration isn't strong, the dose doesn't do more than cause discomfort. Such injections are often used to test the efficacy of painkillers.
During this experiment, the injected rat shared a cage with a second rat, which watched the first rat writhing in pain after the injection. The scientists then injected the second rat with acetic acid. It writhed as well, but it writhed even more than the first one, having watched what its fellow went through. This response was increased if the rats had spent a couple of weeks together prior to the experiment. Researchers believed that the degree to which the rats responded showed empathy. Rats could understand each other's pain, and it caused them more suffering. Rats can experience happiness and joy, but also dread and sympathetic agony. They can understand the pain of their fellows, and apply it to their own situation. In other words, when we trap them or bait them, we're not just inducing pain, but we might be inducing suffering as well.
Image: Janet Stephens.