title: Rat tickling: A systematic review of applications, outcomes, and moderators.
abstract: BACKGROUND
  Rats initially fear humans which can increase stress and impact study results. Additionally, studying positive affective states in rats has proved challenging. Rat tickling is a promising habituation technique that can also be used to model and measure positive affect. However, current studies use a variety of methods to achieve differential results. Our objective was to systematically identify, summarize, and evaluate the research on tickling in rats to provide direction for future investigation. Our specific aims were to summarize current methods used in tickling experiments, outcomes from tickling, and moderating factors.
  
  
  METHODS
  We systematically evaluated all articles about tickling identified from PubMed, Scopus, Web of Science, and PsychInfo. Our inclusion criteria were publication in a peer-reviewed journal and collection of original, empirical data on rats using the handling method of tickling. One researcher extracted information from each article. Bias was assessed by 2 investigators using the SYRCLE bias assessment tool.
  
  
  RESULTS
  We identified 32 articles (56 experiments) published in peer-reviewed journals about rat tickling for inclusion. A wide variety of strains, sexes, and ages of rats were included. The most common method used for tickling was cycling through 15 seconds of tickling and 15 seconds of rest for 2 minutes for 3 to 5 days. Experiments with a control for tickling (N = 22) showed that tickling increases positive vocalization, approach behavior, decreases anxiety measures, improves handling, and in some cases decreases stress hormones. Tickling juvenile, individually housed rats with a trait predisposition to respond more positively to tickling, results in the most positive outcomes. Methods to reduce bias were insufficiently reported.
  
  
  CONCLUSIONS
  We conclude that tickling is a promising method for improving rat welfare and investigating positive affect. However, the establishment of tickling best practices is essential as the outcomes from tickling can be moderated by several factors.
authors.affiliation: Department of Animal Sciences, Center for Animal Welfare Science, College of Agriculture, Purdue University, West Lafayette, Indiana, United States of America.
authors.forename: Megan R
authors.identifier: http://orcid.org/0000-0003-0643-2210
authors.initials: MR
authors.lastname: LaFollette
authors.affiliation: Department of Comparative Pathobiology, Center for the Human-Animal Bond, College of Veterinary Medicine, Purdue University, West Lafayette, Indiana, United States of America.
authors.forename: Marguerite E
authors.initials: ME
authors.lastname: O'Haire
authors.affiliation: Canadian Council on Animal Care, Assessment and Accreditation Section, Ottawa, ON, Canada.
authors.forename: Sylvie
authors.initials: S
authors.lastname: Cloutier
authors.affiliation: Department of Comparative Medicine, Stanford University, Stanford, California, United States of America.
authors.forename: Whitney B
authors.initials: WB
authors.lastname: Blankenberger
authors.affiliation: Department of Animal Sciences, Center for Animal Welfare Science, College of Agriculture, Purdue University, West Lafayette, Indiana, United States of America.
authors.forename: Brianna N
authors.initials: BN
authors.lastname: Gaskill
mesh_terms: D000819:Animal Communication; D000818:Animals; D001007:Anxiety; D001522:Behavior, Animal; D005239:Fear; D005260:Female; D006801:Humans; D008297:Male; D051381:Rats; D014110:Touch