Although it is possible to formulate stronger moral principles than "animals should not be made to suffer unnecessarily," there are significant grounds for doubting these stronger principles. But the principle that underlies the dictum regarding unnecessary suffering is generally recognized as valid, since denial of it implies that we can do whatever we want with animals, a conclusion that is usually considered unacceptable. A determination of whether any particular instance of suffering is necessary or unnecessary must be based on an analysis of both the seriousness of the purpose of the act that involves pain in animals, and its relative avoidability, as well as more concrete concerns like costs and availability of resources for a given community.
We can conclude, with reasonable certainty, that animals are suffering, by making observations of changes in physiological and behavioral factors that are similar to the changes that tell us other humans are in pain. Further, the conclusion that any animal is suffering is sound, according to scientific methodology, because this hypothesis is usually the best available explanation for the observed alterations in physiology or behavior.
Hurnik, F., & Lehman, H. (1982). Unnecessary suffering: Definition and evidence. International Journal for the Study of Animal Problems, 3(2), 131-137.