Each of the vervets’ escape responses is fine-tuned to the hunting skills
of the predator.Natural selection favors an acoustic division of labor:
different calls for different predators.A general-purpose alarm call
would fail because there is no general-purpose escape response.Simi-
larly,a system requiring vervets to find out what the caller was alarmed
about would fail because approaching by ground would leave one vul-
nerable to each predator type,whereas approaching by tree would leave
one vulnerable to eagles and tree mambas,and leopards in the lower
branches of the tree.
Building on these initial results,Cheney and Seyfarth (1990) provided
an increasingly sophisticated description of the function and meaning of
vervet monkey vocalizations.It is clear that some of these vocalizations
are functionally referential (Marler,Evans,and Hauser 1992;Marler,this
volume) in the sense that they appear to map onto salient objects and
events in the environment.Similar kinds of claims for referentiality
have been made for other primate species (e.g.,ring-tailed lemur,diana
monkey,pigtailed macaque,rhesus macaque,toque macaque),and one
bird,the domestic chicken (Dittus 1984;Gouzoules,Gouzoules,and
Marler 1984; Macedonia 1991; Evans, Evans, and Marler 1994;
Zuberbuhler,Noe,and Seyfarth 1997;reviewed in Hauser 1996).What
is lacking from these analyses is a more careful dissection of the acoustic
features associated with the caller’s affective state and those associated
with the object or event referred to.To address this gap,I turn to my own
research on rhesus monkeys and in particular,their food-associated calls.Natural Observations of Food-Associated Calls in Rhesus:Dissecting Content
For almost sixty years research has been conducted on a population of
semifree-ranging rhesus monkeys living on the island of Cayo Santiago,
Puerto Rico.As a result,we know a great deal about this population’s
demography,social behavior,mating system,and communicative signals.
In particular,when rhesus find food,they give one or more of five
acoustically distinctive vocalizations:warble,harmonic arch,chirp,coo,
and grunt.Although the monkeys are provisioned with chow,they forage
throughout the day on naturally available food items such as leaves,fruit,
flowers,grass,soil,and insects,many of which elicit calling.Beginning in
1988,I started a long-term study designed to reveal the sources of
acoustic variation in this restricted calling context.Research concen-
trated on the following questions:1.How does motivational state affect the production of food-associated
calls?
2.Does each call type refer to something like the kind of food or its
relative quality?79 Primate Vocalizations in Emotion and Thought