Which person is my trainer? Spontaneous visual discrimination of human individuals by bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins are known to use signature whistles to identify conspecifics auditorily. However, the way in which they recognize individuals visually is less well known. We investigated their visual recognition of familiar human individuals under the spontaneous discrimination task. In each trial, the main trainer appeared from behind a panel. In test trials, two persons (one was the main trainer) appeared from the left and right sides of the panel and moved along the poolside in opposite directions. Three of the four dolphins spontaneously followed their main trainers significantly above the level of chance. Subsequent tests, however, revealed that when the two persons wore identical clothing, the following response deteriorated. This suggests that dolphins can spontaneously discriminate human individuals using visual cues, but they do not utilize facial cues, but body area for this discrimination.