Roberts made his discovery about negotiations in at age thirteen. He was hired to keep a herd of feral horses, mustangs, under observation and to report their location to his employers from time to time. So he had no duties other than to watch the horses. One day he observed that after a yearling colt had made a major pest of himself by biting and kicking other members of the herd without provocation, the alpha (lead) mare reached the limits of her tolerance for such misbehavior and drove the colt out. Once away from the protection of the herd, the colt felt insecure and wanted to get back among the other horses, but the lead mare kept a threatening eye on him and every time he tried to sneak back in she would forcefully drive him off again.
Finally, the colt that had been ostracized for insubordination seemed to realize that he could not sneak back in and that his position became progressively more perilous the longer he remained outside the protection of the herd. At that point he dropped his head close to the ground and moved along periodically sticking his tongue out to lick around his mouth and clashing his teeth together. The sound created by striking a horse's large teeth together is rather like the sound produced by hitting one fist-sized stone with another, and is quite audible at some distance. Once the lead mare perceived this message of submission being given by the colt, she signalled that she would allow him to reenter the herd by changing her body language from challenging to placid acceptance. After readmitting him to the herd she spent a great deal of time grooming him.