Socially meaningful vocal plasticity in Campbell's monkeys

Alban Lemasson 1, Martine Hausberger 1 & Klaus Zuberbuhler 2

1 Ethologie-Ecologie-Evolution, Universite de Rennes; 2 School of Psychology, University of St Andrews

Email: kz3@st-and.ac.uk

Compared to other primates, human vocal behaviour is exceptionally
flexible, generating somewhat of an evolutionary conundrum.  What are
the evolutionary origins of the human vocal flexibility in the primate
lineage? We present data on Campbell's monkey vocal behaviour, which
show that, although very subtle, important acoustic flexibility is
present and determined by social factors.  Previous work has shown that
in this species, females frequently exchange particular vocalisations,
the combined-harmonic calls, during social interactions.  These calls
can be classified into distinct subtypes and one of them, the CH6
subtype, is highly variable in its acoustic fine structure.  Females
differ in the number and types of CH6 variants they produce at any given
time.  Some variants can be produced by more than one female, indicating
that females share variants.  A female's pattern of variant production
and variant sharing can change in response to important social events,
even during her adult life.  Here, we provide observational and
experimental evidence to further explore the communicative function of
this intriguing vocalisation system.  First, our analyses showed that
older individuals were more likely to elicit vocal responses from other
group members than younger ones.  Responses typically occurred within
less than a second, suggesting that individuals answered each other's
calls.  In some cases, responding individuals matched their own call
subtype to that of the initiating animal.  Second, we conducted a
playback experiment to investigate whether the CH6 variants used in
vocal sharing were meaningful to recipients.  We compared the group's
response between currently and formerly used variants, i.e.  calls
produced by the same individuals at present or several years ago.  We
found that current, but not former, variants reliably triggered a
natural exchange of calls in recipients.  Instead, former variants
caused a temporary decrease in calling rates, demonstrating that the
monkeys discriminated these exceedingly subtle acoustic differences and
that they were socially meaningful to them.