While using animals in trials can often be a dicey object, where the line is drawn in terms of what animals can be used is not so often a hard set one. Like many other areas the answer tends to be “it depends”. What exactly is being tested? If it’s something that can easily be tested on a smaller “less significant” animal, then the line is drawn there. At that point it would be inhumane to do the study on a “more significant” animal. However, if it is something that requires more human-like features (apes for similar anatomy, pigs for similar skin qualities, etc.), then it is not reasonable to draw the line at “less significant” animals, for reasons of practicality. All in all, whatever causes the least harm and is the most appropriate choice for the experiment is where the line should be drawn. Like many, I myself also don’t like the idea of testing on animals, but it is a necessary practice, at least until technology can progress to the point where animal testing is not needed, as someone mentioned earlier. Right now, if it is not done, the net harm done to many if it wasn’t tested would be greater than the harm done from testing on the animal(s).