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. 2020 Jan 16;10(1):148. doi: 10.3390/ani10010148

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© 2020 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Examples of qualitative assessment of behaviour as an externally observable animal-based indicator of positive mental experiences associated with: (a) Play, and; (b) Affectionate sociability. Images A.M. Harvey.