If a being suffers there can be no moral justification for refusing to take that suffering into consideration. So the limit of sentience is the only defensible boundary of concern for the interests of others. To mark this boundary with intellect, skin color, or gender is to mark it arbitrarily. The principle of moral equality states that equal consideration be given to all beings with the capacity to suffer.
All humans have the capacity to suffer.
Thus, racists and sexists violate the principle of moral equality. The principle of moral equality states that equal consideration be given to all beings with the capacity to suffer. Non-human nimals have the capacity to suffer. Thus, speciesists violate the principle of moral equality. Thus, if the principle of moral equality is true, then there is no way to justify regarding pain and pleasure that animals feel as less important than the pain the humans feels.