Play behavior in pigs is a reliable positive welfare indicator - pigs that play are experiencing positive emotional states, and the presence of play in farm environments signals that welfare conditions are sufficiently good to support positive experiences.
The evolutionary function of play in young mammals - including pigs - is believed to involve development of social skills, motor coordination, and behavioral flexibility. The suppression of play by negative conditions makes its presence a sensitive indicator of welfare adequacy. Farms where young pigs rarely or never play are demonstrating welfare conditions that do not support positive emotional states. The practical use of play observation as a welfare metric requires training of farm staff to recognize and record play reliably, but this is achievable and provides information that physical health inspection alone cannot capture.