ally selected elements of the species-specific vocal repertoire into dif-
ferent positions of the call (Arcadi 1996).In this way a large number of
syntactically different call variants can be produced.
To discover the function of these variants,Mitani,Gros-Louis,and
Macedonia (1996) compared two types of calls,pant-hoots and pant-
grunts.The former,typically a long-distance call,shows greater interindi-
vidual variability than the latter,which usually is uttered within visual
distance of a conspecific.Moreover,individually distinguishable call
variants inform group members not only about the location of the caller
but also about his social status,and perhaps about his individual identity
(Clark and Wrangham 1994).
The function of long call variability is related to chimpanzee-bonobo
organizational patterns.In everyday activities,chimpanzees associate in
temporary parties that vary in size and composition.This is in contrast
to macaques or other group-living common monkeys,where members of
the whole group spend their time permanently together and travel
together.Chimpanzee group members and even favored partners are
often spatially separated (Mitani 1994).However,they have a strong
need to be assured of the permanent possibility of meeting and cooper-
ating.Long call vocalizations serve as an effective means of maintaining
social relations in such circumstances.In other words,the social struc-
ture of both chimpanzees and bonobos can be seen as a framework
favoring a type of vocalization composed of units making variants pos-
sible by changing the arrangements of the units,and serving for marking
individual animals.However,the origins of vocalizations with such
characteristics can be traced to another,perhaps more ancient,social
organization.
The composite nature of the long calls of present-day chimpanzees and
bonobos is not manifested strongly,and the source of their units is not
clear.However,in the compound territorial songs of gibbons,some ele-
ments (notes) also occur independent of song,such as in response to ter-
ritorial intrusions (Mitani and Marler 1989).The parallel use of vocal
elements,functioning both as communicative signals and as building
blocks for compound calls,is perhaps more definite in the simpler call of
capuchin monkeys (Robinson 1984),which sheds light on the evolution-
ary origins of this peculiar vocalization.
The syntactically variable compound call seems to evolve under the
constraint of labeling individual territories.Every territorial primate
species uses the acoustic channel for marking territory.Since primary
sounds are limited in number,the differences needed for marking terri-
tory individually can be achieved only by combining elementary sounds
and varying their arrangement.127 Social Organization