Geissmann, T. (2002). Duet-splitting and the evolution of gibbon songs. Biological Reviews 77: 57-76.
T. Geissmann
Institute of Zoology, Tierärztliche Hochschule Hannover, Germany
Key words: Hylobatidae, gibbon, song, duet,
call, vocalisation, evolution, Hylobates, Nomascus, Symphalangus,
Bunopithecus.
Abstract: Unlike the great apes and most other primates, all species of gibbons
are known to produce elaborate, species-specific and sex-specific patterns of vocalisation
usually referred to as ėsongsî. In most, but not all, species, mated pairs may characteristically
combine their songs in a relatively rigid pattern to produce coordinated duet songs.
Previous studies disagree on whether duetting or the absence of duetting represented
the primitive condition in gibbons. The present study compares singing behaviour
in all gibbon species. Various vocal characteristics were subjected to a phylogenetic
analysis using previously published phylogenetic trees of the gibbon radiation as
a framework. Variables included the degree of sex-specificity of the vocal repertoire,
the occurrence of solo songs, and the preference for a specific time of day for song-production.
The results suggest the following scenario for the evolution of gibbon songs: (1)
The last common ancestor of recent gibbons produced duet songs. (2) Gibbon duets
probably evolved from a song which was common to both sexes and which only later
became separated into male-specific and female-specific parts (song-splitting theory).
(3) A process tentatively called ėduet-splittingî is suggested to have led secondarily
from a duetting species to a non-duetting species, in that the contributions of the
pair-partners split into temporally segregated solo songs. This appears to be the
first time that a non-duetting animal can be shown to be derived from a duetting
form. (4) The return to exclusive solo singing may be related to the isolated island
distribution of the non-duetting species.
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