quality.In this sense,rhesus food-associated calls appear to be function-
ally referential.
With these observations in hand,we turned to field experiments.Pro-
duction experiments were designed to provide a more rigorous test of
our hypotheses concerning the acoustic correlates of affective and ref-
erential information.Perception experiments were designed to assess
how rhesus classify food-associated calls.Field Experiments:Further Dissection of Affective and Referential Components
When a rhesus monkey finds food,what determines whether or not it
produces a food-associated call,and if it does,at what rate and which
kind? We combed the island for lone individuals (targeted “discoverers”)
visually isolated from all other group members.Once located,we set up
our experiments.Some individuals were tested before chow was placed
in the dispensers (early morning,hungry group) and others late in the
afternoon (satiated group).Our target discoverers were adult males and
females,from social groups or peripheral to them (only males),and of
high and low dominance ranks.Some discoverers were presented with
fifteen pieces of coconut (high-quality rare food) and others with fifteen
pieces of chow (low-quality common food).
Only 50% of subjects called on discovering the food cache.Of those
who did,females called more often than males,and call rate was highest
early in the morning and in response to coconut;dominance rank was
not significantly correlated with any aspect of calling behavior.Further
paralleling the natural observations,only coos and grunts were given to
chow,whereas warbles,harmonic arches,and chirps were given to the
coconut.Peripheral males (individuals who had yet to join a social
group) never called.Together,these results support our earlier conclu-
sions:call rate covaries with hunger level and acoustical structure
covaries with food type or quality.
Interesting functional consequences arose for those who called as
opposed to those who remained silent.For discoverers who were
members of a social group,those who remained silent and were caught
at the food source received significantly higher rates of aggression from
other group members than those who called;among females,those who
called obtained more food than those who were silent.The story has two
further twists.First, discoverers who failed to call and were never
detected obtained more food than any other discoverer. Second,
although peripheral males never called on discovery,they were never
attacked when caught at the food source.These results raise two intrigu-
ing ideas with respect to vocal communication and the emergence and
maintenance of a convention.One,given the targeted aggression toward83 Primate Vocalizations in Emotion and Thought