‘Beware, I am big and non-dangerous!’ – Playfully growling dogs are perceived larger than their actual size by their canine audience

Nonhuman animals often use specific signals to initiate playful interactions. There is evidence also for different forms of play-maintenance. Playful encounters include out-of-context and exaggerated behavioural sequences. Scientists have already collected knowledge about virtual size modification via acoustic signalling in particular animal species during competitive/agonistic interactions, but the same was unknown in playful encounters. Using the cross-modal matching paradigm, we tested whether dogs prefer to look at the picture of a matching size dog when they are offered two differently sized projected pictures simultaneously with a playback of a playful or a food-guarding growl. We found that dogs looked at the matching picture when they heard the food-guarding growl, but they looked at rather the larger than the matching size dog when play growls were played back. These are the first results to show that dogs may communicate an exaggerated body size by the means of their growls during play, which may help in maintaining or enhancing the playful interaction. As agonistic dog growls were proven to be honest regarding their referential and size-related information content, our results gave evidence that exaggeration may work as play signal in the case of animal vocalizations.