of the variation resulting from modulations in fundamental frequency
contour.For example,coos with one frequency contour pattern were
given during group progressions,whereas coos with a different contour
were given by estrous females.Experiments by Petersen et al.(1978)
showed that Japanese macaques, but not closely related species,
responded faster in a discrimination task when the call was played into
the right ear (left hemisphere) than when it was played into the left ear
(right hemisphere).Follow-up studies (Heffner and Heffner 1984,1990)
indicated that lesioning the left temporal lobe,but not the right,caused
subjects to lose the ability to discriminate coos on the basis of their char-
acteristic frequency contours;although this deficit was observed early on,
subjects with left hemisphere lesions recovered quite rapidly.In general,
these results have been taken as support for the view that,as well as
humans,monkeys also show a left hemisphere bias for processing
species-typical vocal signals.
The interpretation offered for Japanese macaque data has two poten-
tial problems.First,only one call type was used.Thus we do not yet
understand whether the perceptual bias extends to other calls within the
repertoire.Second,the claim that Japanese macaques show a pattern of
hemispheric bias that is comparable with that shown for humans pro-
cessing language hinges on the assumption that coos are languagelike,
that they convey,at some level,semantic information.And yet,studies
of this call type in both Japanese macaques (Owren et al.1992,1993) and
the closely related rhesus macaque (Hauser 1991,section 2) suggest that
the information conveyed is likely to be entirely emotive (currently no
evidence exists that the call conveys even functionally referential infor-
mation,sensuMarler,Evans,and Hauser 1992).
To address some of these concerns,a field study of rhesus macaques
was conducted (Hauser and Andersson 1994).Playback experiments
were carried out with a large number of adults and infants (age < 12 mo),
using most call types from the repertoire.A speaker was placed 180
degrees behind an individual,and a single exemplar of a call type was
played.The logic underlying the design was that if subjects preferentially
turned their right ear toward the speaker,they would bias the intensity
of input to the left hemisphere;if they turned the left ear,they would
bias input to the right hemisphere.Note that both ears receive acoustic
input,but interaural time and intensity differences are present due to the
orienting bias.For all conspecific calls played,adults consistently showed
right ear bias despite an overall lefthand motor preference for reaching
and manipulating objects in this population (Hauser et al.1991),with no
correlation between handedness and orienting bias in a subset of sub-
jects.In contrast,no ear bias was observed in infants for any call types.
Moreover,when the alarm call of a local bird (ruddy turnstone) was90 Marc D.Hauser