Crying With Laughter: Adapting the Tickling Protocol to Address Individual Differences Among Rats in Their Response to Playful Handling

It has been over 20 years since the first scientific papers on rat tickling were published (1, 2). Ratswere found to emit ultrasonic vocalizations in the 50-kHz range (hereafter referred to as USVs)when a human performed rapid manual stimulation on their dorsoventral region. Such tickling ofrats by a human hand is trying to imitate the rough-and-tumble play seen in young rats of bothsexes (3, 4). Emission of USVs by the rat indicates that it enjoys being tickled (5–7), as USVs havebeen linked with positive emotions (8, 9), are emitted in anticipation of, and during, social play(10, 11), and have been suggested to be homologous to human laughter (12).To date, more than 70 scientific articles on rat tickling have been published1, and the consensusis that tickling induces positive emotions in rats. Indeed, in a systematic review, LaFollette etal. (13) found that tickling increased USVs and human hand approach behavior, and decreasedmeasures of anxiety in rats. In this Opinion paper, we consider whether current methods of ticklingoveremphasize the use of pinning (Figure 1) to which there may be a wider response variation thancommonly acknowledged. We do not dispute that tickling can be a positive way to handle juvenilerats, but tickling may not always be perceived as a positive interaction by the rat, and we raise thepossibility that tickling methods need to be revised. In particular, we suggest incorporating moreaspects of play during tickling (increased diversity) and adapting the method to individual rats’responses (increased flexibility) to achieve positive emotions and increased welfare across a widercohort of rats.