1. IntroductionIn 2017, cat fans took to Twitter to document their cats’ attraction totight spaces by taping complete shape outlines on their floors andobserving their cats sit inside, spurring over eighty-two-thousandretweets and trending hashtag #CatSquare (Fig. 1). Affectionatelytermed “if I fits I sits,” the urge to inhabit enclosed spaces is well-knownto cat owners and has been documented to decrease stress in laboratorycats (Carlstead et al., 1993) and shelter cats given boxes in which to hide(Hawkins, 2005; Kry and Casey, 2007; Vinke et al., 2014). In fact, catsdeprived of shelter resources like boxes will attempt to manufacturetheir own by hiding behind or underneath box-like objects like litterpans (Gourkow and Fraser, 2006). The reason for this behavior is stillunknown but is clearly highly desirable.Regardless of the reason for their attraction to enclosed spaces, thisbehavioral phenomenon proves to be an excellent tool to study the vi-sual perception of shapes and contours in domestic cats. Neurologicalstudy of this phenomenon began in cats and found that these nonhumananimals’ (hereafter animals) retinal receptive fields are sensitive tocontours along a luminance gradient (Redies et al., 1986). Contourcomprehension is theorized to be evolutionarily critical in the under-standing of physical objects and boundary interpolation (Kellman,2003), and the study of visuo-cognitive phenomena such as illusionsusceptibility offers a fascinating perspective into the effects of envi-ronmental pressures and life experience on vision (Kelley and Kelley,2014).* Corresponding author at: Animal Behavior and Conservation Program, Department of Psychology, Hunter College, 695 Park Avenue, New York, NY, 10065,United States.E-mail address: gabriella.smith28@myhunter.cuny.edu (G.E. Smith).Contents lists available at ScienceDirectApplied Animal Behaviour Sciencejournal homepage: www.elsevier.com/locate/applanimhttps://doi.org/10.1016/j.applanim.2021.105338Received 1 September 2020; Received in revised form 20 April 2021; Accepted 26 April 2021

A well-known phenomenon to cat owners is the tendency of their cats to sit in enclosed spaces such as boxes,laundry baskets, and even shape outlines taped on the floor. This investigative study asks whether domestic cats(Felis silvestris catus) are also susceptible to sitting in enclosures that are illusory in nature, utilizing cats’attraction to box-like spaces to assess their perception of the Kanizsa square visual illusion. Carried out duringthe COVID-19 pandemic, this study randomly assigned citizen science participants Booklets of six randomized,counterbalanced daily stimuli to print out, prepare, and place on the floor in pairs. Owners observed and vid-eorecorded their cats’ behavior with the stimuli and reported findings from home over the course of the six dailytrials. This study ultimately reached over 500 pet cats and cat owners, and of those, 30 completed all of thestudy’s trials. Of these, nine cat subjects selected at least one stimulus by sitting within the contours (illusory orotherwise) with all limbs for at least three seconds. This study revealed that cats selected the Kanizsa illusion justas often as the square and more often than the control, indicating that domestic cats may treat the subjectiveKanizsa contours as they do real contours. Given the drawbacks of citizen science projects such as participantattrition, future research would benefit from replicating this study in controlled settings. To the best of ourknowledge, this investigation is the first of its kind in three regards: a citizen science study of cat cognition; aformal examination into cats’ attraction to 2D rather than 3D enclosures; and study into cats’ susceptibility toillusory contours in an ecologically relevant paradigm. This study demonstrates the potential of more ecologi-cally valid study of pet cats, and more broadly provides an interesting new perspective into cat visual perceptionresearch.

If I fits I sits: A citizen science investigation into illusory contoursusceptibility in domestic cats (Felis silvestris catus)

The focus of this class is the research process, not the product.