Fig 1. Photos of the experimental set up and procedure.

(A) The generous experimenter is letting the dog eat from the baited bowl. (B) The selfish experimenter is eating the food in front of the dog. (C) Experimental set up in choice tests; the dog is approaching the experimenter on the right. All persons that appear in this figure have given written informed consent, as outlined in the PLOS consent form, to publication of their photograph.