There are indications that the chimpanzee’s long compound call may
originate in a territorial marking function similar to that of present-day
gibbon song.This possibility is supported in comparing gorilla and chim-
panzee calls.Gorillas also have a long call,the hoot,that is connected
with group movements.It may be a territorial call since it is emitted only
when encountering another group (Fossey 1972;Mori 1983).Gorillas
have also another call type,the wrah,with the function of cohesive
calling.A similar sound sometimes appears in the final part of the chim-
panzee pant-hoot.Thus a compound call might have existed in the
common ancestor of gorillas,chimpanzees,and humans,the function of
which was territorial.As species diverged,the call split into constituents
that acquired different functions in the separated lines,as the original
function itself ceased to exist.
As we have seen,territorial marking requires acoustically distinguish-
able signals.However,producing a compound call is not the only solu-
tion to this need.Other nonterritorial monkey species can also emit
acoustically variable calls, for example,vervets (Owren 1990) and
macaques (Hauser,Evans,and Marler 1993).These variants are pro-
duced by small articulatory differences in one basic call type.They are
produced through facial gestures modified by mandibular positioning or
lip configuration.That is,the ability to produce different facial expres-
sions also enables these animals to produce call variants.But elaborate
facial gestures evolved only in primate species living in large,complex,
intricate groups where modifiable faces play an immediate communica-
tive role.Territorial species,on the other hand,have another,simpler
form of social organization,namely monogamy.Since monogamous
sociality relies on a closed social unit with more limited social relations,
it does not promote evolution of facial gestures characteristic of group-
living species.
This limits the means available for producing call variants by articu-
latory means,and promotes production of variants through combining
available calls.Consequently,the emergence of compound calls was pro-
moted by the social structure of monogamous territoriality.
If the compound call of great apes derives from the territorial song of
monogamous ancestors,its retention after the breakdown of both terri-
toriality and monogamy presumably is related to its capacity to dis-
tinguish individuals.That is,survival of the compound call is closely
connected with the special way of life of both chimpanzees and bonobos.
Its specificity is based on the survival of an ancient vocalization capac-
ity that changed and evolved in new social circumstances,thus resulting
in an essential transformation of communication.
Whereas much lower-level animal vocalization is highly constrained in
its characteristics,the chimpanzee long call clearly shows elements of128 Maria Ujhelyi