Why did manipulations of the interpulse interval have a detectable
effect on responses to grunts and shrill barks,but no effect on responses
to copulation screams? Closer inspection of the natural variation in
acoustic morphology provides a clue.Whereas grunts and shrill barks are
produced with no fewer than two pulses,copulation screams can be pro-
duced with a single pulse.Thus,although the number of pulses and inter-
pulse interval in a copulation scream may be relevant to male quality
(Hauser 1993b),such temporal features do not appear to be important
in terms of classifying the call as a rhesus copulation scream.
In summary,we appear to have identified at least one feature that
defines a rhesus monkey call,and showed that altering this feature causes
a shift in the direction of acoustic orientation.We interpret this orient-
ing bias as evidence that the left hemisphere is dominant with respect
to processing conspecific calls.This interpretation must be considered in
greater detail because there are possible noncortical mechanisms (biases
at the periphery) and at the cortical level,alternative pathways for
guiding the orienting bias (auditory,visual,cross-modal).Call Production
Neurophysiological studies of squirrel monkeys and several macaque
species revealed homologues to Broca’s and Wernicke’s areas (reviewed
in Jürgens 1990;Deacon 1992,1997).When the homologue to Broca’s
area was lesioned in these species,however,no detectable differences
in the acoustic morphology of the vocal repertoire were observed
(reviewed in Larson,Ortega,and DeRosier 1988;Hauser 1996).These
results led to the conclusion that in nonhuman primates,the locus of
control for production of species-typical vocalizations is the limbic
system.A problem with this interpretation is that both studies obtained
relatively crude measurements of preoperative and postoperative effects
on call structure (Kirzinger and Jürgens 1982).Specifically,spectro-
graphic differences in call structure were assessed qualitatively,rather
than quantitatively using detailed acoustic analyses.Given that damage
to Broca’s and Wernicke’s areas can lead to quite subtle linguistic effects
in humans,it is possible that comparably subtle effects would emerge
among nonhuman primates as well.Moreover,the potential effects of
these experimental lesions were measured only over a short period of
time;production and perception deficits may not reveal themselves
immediately after injury.In sum,the importance of higher cortical struc-
tures in nonhuman primate vocal production remains ambiguous.
Studies of cortical physiology aside,considerable interest has been
shown in the possibility that the fundamental units of human language
(phonemes,words) evolved from a homologous nonhuman primate
ancestor. For example, MacNeilage (1994) suggested that syllables95 Primate Vocalizations in Emotion and Thought