In terms of ethics, the main issue in animal testing is simply that many experimental animals suffer in ways which are unnatural to them. Through the use of genetic manipulation, obese mice, diabetic mice, and mice with Huntington’s disease can be created. Surgical experiments can be performed on larger animals – such as pigs, sheep, and dogs, as “practice" for human surgery.
Normally, such things would not happen to these animals. Any suffering they might experience during such experiments is entirely the making of the researcher – and often these animals are purpose-bred and would not even exist if it were not for the research. These animals have been bred by us, for our use, and suffer on our behalf.
As humans—the dominant species on the planet—we can treat animals in any way we choose, and do with them what we please. The question is, is it moral, or ethical, to treat them in ways which cause suffering – even if it is to our benefit? To some opponents of animal experimentation there are no benefits which justify the use of animals; others believe that animal experimentation is acceptable providing that suffering to the animals is minimized.
Still others oppose animal testing selectively on the basis of the purpose of the tests, believing that animal experimentation for the advancement of medical science is acceptable, but cosmetic testing is not, for example.