Orangutans' Comprehension of Zoo Keepers' Communicative Signals.

Zoological institutions often encourage cooperative interactions between keepers and animals so as to promote animals' welfare. One useful technique has been conditioning training, whereby animals learn to respond to keepers' requests, which facilitates a number of, otherwise sensitive, daily routines. As various media have been used to convey keepers' instructions, the question remains of which modality is best to promote mutual understanding. Here, we explored this question with two captive female orangutans. In the first experiment, we compared orangutans' understanding of previously acquired instructions when those were performed with verbal signals only, gazes only, gestures only, and when all those modalities were combined. Our results showed that gestures were sufficient for successful comprehension by these two apes. In the second experiment, we asked whether this preference could be driven by the non-arbitrary relationship that gestures bear to what they refer to, through iconicity or pointing. Our results revealed that neither iconicity nor pointing helped the subjects comprehend the keepers' instructions. Our results indicate a preference for instructions given through gestural signals in two captive female orangutans, although its cause remains elusive. Future practice may encourage the use of gestures in communication between keepers and orangutans in general or potentially other animals.