Pain assessment in animals depends on observable consequences of an experience that cannot be directly reported or measured. Yet behaviour is neither a transparent readout of pain nor a stable signal across time. Animals may preserve feeding, suppress conspicuous responses, redistribute activity, adopt economical postures, or reorganize social and self-maintenance behaviour while pain persists. Conversely, fear, weakness, nausea, sedation, confinement, handling, and environmental novelty may produce pain-like changes. This article develops an original non-empirical framework for interpreting such convergence and divergence across movement and gait, posture and guarding, facial expression, activity and rest, feeding and drinking, grooming, social interaction, vocalization, and context. It proposes that assessment should distinguish the underlying pain state, the animal’s capacity and motivation to express it, opportunities for observation, and the observer or technology that records the signal. No single domain is treated as compensatory for absent evidence elsewhere. Instead, domain-specific observations are interpreted jointly, longitudinally, and against within-animal baselines, plausible alternatives, and response to clinically justified intervention. Concordant change can strengthen concern without establishing pain specificity; discordance should trigger explanation rather than averaging. The framework therefore redirects attention from “Does this animal show pain behaviour?” to “Which pain-compatible changes are present, which expected signals are absent, and what could account for that pattern?” Its proposed classifications and decision logic require prospective, multispecies validation. They are not diagnostic thresholds and cannot substitute for clinical examination, etiological investigation, or welfare-sensitive action under uncertainty.
