She began spontaneously whistling, mimicking an animal caretaker making the sound. The importance is that whistling is a sound that is in a human’s—but not an orangutan’s—repertoire. While some "lower primates" have been shown to make non-standard sounds for their species, it has always been the result of intense training, whereas Bonnie just picked up the new sound mechanism. Another interesting aspect is that she seems to whistle just because she likes the sound as opposed to producing this behavior in response to a potential food reward. Bonnie's whistling was documented in Primates by Dr. Serge Wich et al.
- "Orangutan". Smithsonian's National Zoo. 2016-06-06. Retrieved 2016-11-05.
- Wich, S. A.; Swartz, K. B.; Hardus, M. E.; Lameira, A. R.; Stromberg, E.; Shumaker, R. W. (2008). "A case of spontaneous acquisition of a human sound by an orangutan". Primates. 50 (1): 56–64. doi:10.1007/s10329-008-0117-y. PMID 19052691.