Play behavior and exploratory activity are reliable positive welfare indicators in pigs — their presence indicates animals in good affective states, and their absence indicates welfare deficiency even when negative indicators are absent.
Pig play behavior represents the clearest evidence of positive emotional states in farmed animals — animals at play are by definition not experiencing fear, pain, or significant negative affect. The presence of abundant play in a pig population is evidence that the system is meeting both negative welfare (absence of suffering) and positive welfare (presence of good states) criteria. Conversely, the absence of play in young pigs is a welfare warning sign warranting investigation of pain, illness, or environmental deficiency. The practical implications for farm management are significant: enrichment that promotes play is measurably welfare-positive; housing that eliminates play demonstrates welfare deficiency. Moving from "no suffering" to "evidence of good states" as a welfare standard requires this kind of positive indicator measurement.