In a new meta-analysis published in PLOS One, researchers from Purdue, Stanford and the Canadian Council on Animal Care look at the different techniques used to induce laughter in rats in order to improve their wellbeing and capture their laughter, which is delightful.

Their conclusion: if you’re going to tickle a rat, use the "original" method, developed in 2000 by Jaak Panksepp and Jeff Burgdorf of Bowling Green State University, Ohio, which "mimics rat rough-and-tumble play through contact with the back of the neck and stomach. Each tickling session should last for two minutes, alternating between 15 seconds of rest and 15 seconds of active tickling, with daily sessions for at least five days."

Inter-individual differences are one of the most relevant moderators of outcomes of tickling for facility personnel. Rats repeatedly show a large range of 50-kHz vocalizations in response to tickling that impact experimental outcomes. In one study, only high-calling rats showed a positive cognitive bias after tickling [8]; thus low-calling rats may experience tickling differently than high-calling rats. Further investigation is needed to determine if low-calling rats indeed find the tickling intervention positive, neutral, or negative. Since you can also bi-directionally select rats for either a high- or low-calling rate and find similar differences in line bred rats, it seems to indicate these are trait differences. In future studies, we strongly recommend that investigators include calling rate as a continuous covariate in all statistical models and determine the calling rate of control rats after the termination of the experiment. Tickling can be used to determine behavioral traits and subsequently how a trait response to tickling affects response to stress [52]. Selectively bred high-calling rats could also be useful for understanding the importance of playful joy [39].

Tickling rats to investigate the effects of pharmacological substances on positive affect has been used repeatedly. All experiments found differences in 50-kHz vocalizations after application of some substances. One article attempted to use tickling combined with the application of the psychotomimetic drug phencyclidine to model the negative symptoms of schizophrenia, but concluded that it was unclear if this model would be valid or not [53]. Another article successfully concluded that 50-kHz calls in response to tickling were mediated by dopamine release as evidenced by a decrease in calls after application of dopamine agonists [28]. This article was using tickling to model the importance of play behavior during adolescence. Finally, an additional article has used tickling to evaluate calling rates and drug administration to further investigate the effects of trait differences on chronic variable stress as a model of depressions [34]. Overall, tickling is a promising method to evaluate pharmacological compounds designed to improve psychological measures of affect while also considering the individual differences in calling rates.

Rat tickling: A systematic review of applications, outcomes, and moderators [Megan R. LaFollette, Marguerite E. O’Haire, Sylvie Cloutier, Whitney B. Blankenberger and Brianna N. Gaskill/PLOS One]

What’s the best way to tickle a rat? [Tim Wallace/Cosmos]

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