The ethical foundations of animal welfare are grounded in the scientific evidence for animal sentience - their capacity for subjective experience.
The ethical case for animal welfare ultimately rests on the question of whether animals can suffer. If they can - if there is something it is like to be a cow, a pig, a fish, or a shrimp - then causing unnecessary suffering is morally significant regardless of human convenience or economic interest. The scientific case for animal sentience has grown substantially: we know animals have nociceptors, pain-processing brain structures, anti-nociceptive systems (which would be pointless if pain did not serve a function), and behavioral responses consistent with aversive experience. We do not need certainty of human-equivalent suffering to justify welfare protections - we need only reasonable grounds for believing suffering is possible. The precautionary principle applied to animal sentience justifies meaningful welfare protection across the animal kingdom.