Our understanding of animal sentience has expanded dramatically in recent decades, with evidence showing that pain, fear, pleasure, and social emotions are widespread across the animal kingdom far beyond what was previously recognized.
The science of animal sentience has undergone a revolution since the late 20th century, fundamentally changing our understanding of which animals matter morally and why. The discovery of opioid receptors and nociceptors in fish, evidence of emotional states in invertebrates including bees, and demonstration of complex cognition in cephalopods have all extended the circle of moral concern well beyond the mammals and birds that were the traditional focus of animal welfare science. This expanding knowledge base has practical implications: species previously considered unproblematic to use intensively, including fish and insects, are now understood to have at least the capacity for negative experiences, requiring application of the precautionary principle to their management. The direction of scientific progress is consistently toward greater recognition of animal experience, not less.