discoverers who failed to call,it is possible that this rhesus population
evolved a calling convention:members of a social group are expected to
call when food is discovered.In the absence of calling,the convention is
violated,and thus others respond with aggression.Whether this kind of
targeted aggression functions as a form of punishment remains to be
investigated in greater detail.Two,because peripheral males were never
recipients of targeted aggression,it is possible that this form of attack is
reserved for social group members for whom the possibility of future
interactions is high;peripheral males that one interacts with may or may
not join the group.Since aggression is costly for attacker and attackee,
there may be strong selection against attacking those with whom one is
unlikely to interact in the future.Again,much more work is needed
before we can properly evaluate these ideas.
Given our understanding of the contexts and apparent functions of
food-associated calls,we set up a playback experiment to determine how
they are classified (Hauser,in press).We borrowed a technique from
developmental psychologists interested in understanding the processes
underlying speech processing in prelinguistic infants.Specifically,a
habituation-discrimination procedure was used to determine whether
the primary factor guiding classification of rhesus food calls is its acoustic
morphology or referent.This procedure had been run in the field with
vervet monkey intergroup and alarm calls (Cheney and Seyfarth 1990),
and thus we had some confidence that it would work with rhesus
monkeys as well.Our experiments focused on three calls:warble,har-
monic arch,and grunt.All three are acoustically different;however,
warbles and harmonic arches are produced in the same general context
and thus may mean something quite different from grunts.
To set the stage,consider the following situation.You are at a restau-
rant and someone eating a dish of mashed potatoes repeatedly says,
“Yum,potatoes.”You turn and look after the first utterance,but then
stop responding.At some point,the customer says,“Yum,caviar”as the
second course arrives.You would certainly perk up and look back toward
the diner.In this case,did you look because you detected a mere acoustic
change or because you noticed a salient semantic change? My guess is
that the semantic change is largely responsible for your renewed inter-
est.If the diner continued to repeat “yum,caviar”for a while and then
switched to “yum,salmon eggs,”my guess is that you would not respond.
Although there is clearly a perceivable acoustic change,there is no
accompanying semantic change.
This hypothetical example is analogous to the situation confronted by
rhesus monkeys on Cayo Santiago.If a discoverer repeats the warble
over and over and then switches to a harmonic arch,will a listener’s inter-
est be revived or not? If a discoverer repeats the harmonic arch and84 Marc D.Hauser