The major concern of this paper is with the operational interpretation of animal distress and suffering – that is, with the issue of how we can measure the extent of an animal’s distress in practice. I shall probably make myself very unpopular by expressing the view right from the outset that I think it is neither possible nor desirable to try to discover universal ‘indicators’ of distress that will apply to all animals in all situations.
Dawkins, Marian Stamp, "Concepts of Distress, Suffering and their Operational Interpretation" (2009). Ethology of Nonhuman Animals. Paper 5.