The differences between us and other vertebrates are a matter of degree rather than kind. Not only do they closely resemble us anatomically and physiologically, but so too do they behave in ways which seem to convey meaning. They recoil from pain, appear to express fear of a tormentor, and appear to take pleasure in activities; a point clear to anyone who has observed the behaviour of a pet dog on hearing the word "walk". Our reasons for believing that our fellow humans are capable of experiencing feelings like ourselves can surely only be that they resemble us both in appearance and behaviour (we cannot read their minds). Thus any animal sharing our anatomical, physiological, and behavioural characteristics is surely likely to have feelings like us. If we accept as true for sake of argument, that all humans have a right not to be harmed, simply by virtue of existing as a being of moral worth, then we must ask what makes animals so different. If animals can feel what we feel, and suffer as we suffer, then to discriminate merely on the arbitrary difference of belonging to a different species, is analogous to discriminating on the basis of any other morally arbitrary characteristic, such as race or sex. If sexual and racial moral discrimination is wrong, then so too is specieism.
Animal research, by its very nature necessitates harm to the animals. Even if they are not made to suffer as part of the experiment, the vast majority of animals used, must be killed at the conclusion of the experiment. With 115 million animals being used in the status quo this is no small issue. Even if we were to vastly reduce animal experimentation, releasing domesticated animals into the wild, would be a death sentence, and it hardly seems realistic to think that many behaviourally abnormal animals, often mice or rats, might be readily moveable into the pet trade. It is prima fasciae obvious, that it is not in the interest of the animals involved to be killed, or harmed to such an extent that such killing might seem merciful.