We investigate the social significance of the within group ('close') calls of
gorillas by examining correlates of calling with dominance rank and with
relatedness of adults, and by examining whether the outcomes of interactions
between adults differ depending on the calls given during the interactions. In
two wild gorilla groups, the majority of adults give most of their calls when
near another adult and fully in sight of them. Thus gorillas' use 'close' calls
as more than mere contact calls. An adult gorilla's use of 'close' calls
correlates with its own and with its partner's dominance rank, with effects
being most obvious for the most dominant animals, the fully adult males. Thus
the proportion of 'double grunts', the most common 'close' call, in an
individual's repertoire correlated consistently with dominance rank; all
non-silverback adults gave a higher percent of double grunts in the presence of
subordinates than they did near dominant animals; individuals were most likely
to give 'non-syllabled' grunts in the adult male's presence; and they
consistently exchanged calls at a higher rate with the adult males than with
other group members. An adult's type of calls did not obviously differ depending
on whether their neighbour was kin or non-kin, but kin were overall more likely
to give calls in the presence of kin, and to exchange calls with kin than with
non-kin, although the association was not consistent throughout the two years of
the study. With regard to the consequences of calling, subordinates were less
likely to be feeding one minute after an approach by a fully adult male during
which calls were exchanged than during silent approaches. In contrast to
findings from studies of some other species, calling did not correlate with
duration of grooming. We suggest that, among other functions, gorillas' 'close'
calls mediate social interactions. One form of calls, 'non-syllabled' calls, are
interpretable as appeasement signals. The broadest interpretation of the 'double
grunt' is that it is an exaggerated announcement of presence, whose function is
to attract attention to the caller, and to signal conditional future
activity.