Animal Cognition

, Volume 22, Issue 3, pp 291–303 | Cite as

Guppies, Poecilia reticulata, perceive a reversed Delboeuf illusion

  • Tyrone Lucon-XiccatoEmail author
  • Maria Santacà
  • Maria Elena Miletto Petrazzini
  • Christian Agrillo
  • Marco Dadda
Original Paper


Animals are often required to estimate object sizes during several fitness-related activities, such as choosing mates, foraging, and competing for resources. Some species are susceptible to size illusions, i.e. the misperception of the size of an object based on the surrounding context, but other species are not. This interspecific variation might be adaptive, reflecting species-specific selective pressures; according to this hypothesis, it is important to test species in which size discrimination has a notable ecological relevance. We tested susceptibility to a size illusion in the guppy, Poecilia reticulata, a fish species required to accurately estimate sizes during mate choice, foraging, and antipredator behaviours. We focussed on the Delboeuf illusion, in which an object is typically perceived to be larger when surrounded by a smaller object. In experiment 1, we trained guppies to select the larger of two circles to obtain a food reward and then tested them using stimuli arranged in a Delboeuf-like pattern. In experiment 2, we tested guppies in a spontaneous food choice task to determine whether the subjective size perception of food items is affected by the surrounding context. Jointly, our experiments indicated that guppies perceived the Delboeuf illusion, but in a reverse direction relative to humans: guppies estimated as larger the stimulus that human perceived as smaller. Our results indicated susceptibility to size illusions also in a species required to perform accurate size discrimination and support previous evidence of variability in illusion susceptibility across vertebrates.


Comparative perception Fish cognition Size discrimination Visual illusion 



We thank the undergraduate students who helped in the project. We have no competing interests. Funding was provided by STARS 2017 Grant (Prot. 19215) and PRIN 2015 Grant (Prot. 2015FFATB7) from University of Padova.

Compliance with ethical standards

Conflict of interest

We have no conflicts of interest.

Ethical approval

We followed all applicable international, national, and/or institutional guidelines for the care and use of animals (Italy, D.L. 4 Marzo 2014, n. 26). All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted (Protocol n. 13/2018).


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Life Sciences and BiotechnologyUniversity of FerraraFerraraItaly
  2. 2.Department of General PsychologyUniversity of PadovaPaduaItaly
  3. 3.School of Biological and Chemical ScienceQueen Mary University of LondonLondonUK

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