Short video clip with an  Observer  animal (right side of the cage) displaying a yawn.
Carrillo M, Migliorati F, Bruls R, Han Y, Heinemans M, Pruis I, Gazzola V, Keysers C
CC BY 4.0
<a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4565705/bin/pone.0136979.s003.mp4">S1 Video</a> from <cite style="font-style:normal">Carrillo M, Migliorati F, Bruls R, Han Y, Heinemans M, Pruis I, Gazzola V, Keysers C (2015). "<a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4565705">Repeated Witnessing of Conspecifics in Pain: Effects on Emotional Contagion</a>". <i>PLOS ONE</i>. <a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="w:Digital object identifier">DOI</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1371/journal.pone.0136979">10.1371/journal.pone.0136979</a>. <a href="https://en.wikipedia.org/wiki/PMID" class="extiw" title="w:PMID">PMID</a> <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pubmed/26356506">26356506</a>. <a href="https://en.wikipedia.org/wiki/PubMed_Central" class="extiw" title="w:PubMed Central">PMC</a>: <a rel="nofollow" class="external text" href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4565705">4565705</a>.</cite>