Sex differences in the enjoyment of enrichment: affective bias testing confirms female rats prefer more playful and less vigorous tickling

There is increasing interest in positive animal welfare which emphasizes that animals should experience predominantly positive mental states. Positive animal welfare raises challenging biological questions, including how we assess positive experiences. Rat tickling mimics rough and tumble play; it is applied to improve welfare and study positive affect, often using ultrasonic vocalizations (USVs) to assess rats' affective state. Standard tickling protocols involve frequent pinning where the supine rat is vigorously tickled on its belly, which we have argued may not be positive for all rats. Here, using an affective bias test, we show that both sexes experience positive affective states when tickled, but females, unlike males, prefer playful tickling with minimal pinning. This result corresponds with our previous work using USVs to assess rats' affective response to tickling, further validating USV recording as a marker of positive affective states. We believe this to be the first example where a refinement aimed at enhancing positive, rather than minimizing negative, mental states in animals has been independently validated; this should provide a model for similar studies in other species. In rats, this refinement should enhance welfare and more generally demonstrate the importance of considering sex differences when designing welfare interventions.