Ontology is the development of an organism from zygote to end of life. In other words, how does a gibbon learn to sing?
It seems that singing is innate to gibbons, just as human babies will naturally babble. When being carried by their mothers, infants will just scream during the great call section of the duet. Juvenile females will sing along with her mother's great call, which seems to be necessary because it takes years to achieve excellence in performing a great call, and even adults sometimes make mistakes. Fathers will duet with their daughters to help her practice when she is ready to hold her half of the duet. For reasons unknown, juvenile males only rarely practice duetting with their parents, except in unusual circumstances such as an ailing father. The reason for this is unknown, but I speculate that there may be reasons a juvenile male should not advertise himself as it may lead to conflict (Raemaekers & Raekmaekers 1984).
It was found that a bred hybrid gibbon of the species Hylobates lar and Hylobates pileatus produces a song that did not match the songs of either parent species, but was rather a mix of both. This jumbled song appeared even if the infant gibbon was only exposed to the maternal song. The implications of this study are that the general patterns of the songs are hardwired in the gibbons. The study of the gibbon song has helped researchers understand the development of other primate vocalizations (Hammerschmidt 2001). However, there is incredible variation between individuals within a species, and every duet is unique to the couple. How does this variation come about? The duet is developed in the beginning of a pair-bond relationship in a period of intense practice. During this time, both gibbons have input in creating the final song (Geissman 1986). For more detail on how and why gibbons develop their unique songs with their partner, see the pair-bond strengthening theory under adaptive value.
It is fascinating to me that the general structure of the song is hardwired to the particular species of gibbon, but the individuals invent the actual notes and phrases themselves. It is as if one species that is only able to speak in sonnets, while another can only speak in the form of a ballad. The forms are rigid and distinctive, but both allow for great creativity within the form.