characteristics and context are involved in recognizing others (Snowdon
and Cleveland 1980).
Infant call repertoires differ from adult repertoires both in types of calls
and acoustic structure of adult-like calls (Elowson et al. 1998a). In addition,
correct usage of call types appears to be learned (Elowson et al. 1998a).
Several features of contact trills change with time, although stereotypy does
not increase so the cause is unclear. J-calls show even less age-related modification (Elowson et al. 1992). Individuals’ contact calls differ extensively
in acoustic structure and the pattern of ontogenetic changes. Such high individual variance makes finding ontogenetic patterns difficult. However, one
pattern that emerges is that infants go through a stage of vocal production
reminiscent of human infant babbling (Elowson et al. 1998b). Similarities
to human babbling include repetition of call types and use of a subset of
the adult repertoire in modified infant form. Babbling infants are responded
to by caregivers more than their silent counterparts, suggesting that babbling is important in the normal development of social relationships. Data
on development indicate vocal flexibility exists and show that calls are not
always produced in adult form by juveniles. Vocal learning may or may not
play a role; vocal tract maturation and developing social relationships
may be responsible for such changes. Descriptions of development cannot
directly test the importance of vocal learning. Considering the current lack
of evidence for most primates, alternative explanations seem likely.
The potential for social modification of contact calls was studied with
family groups from two different captive colonies (Elowson and Snowdon
1994). Animals were housed in family groups initially in two separate rooms
during a quarantine period. Contact calls were recorded and analyzed from
this initial time period. Subsequently, family groups from both captive
colonies were housed in the same room in acoustic but not visual or physical contact. Contact calls recorded after the animals were introduced to
each other (postquarantine) were compared with those from the initial
period (Fig. 4.8B). Peak frequency and bandwidth rose in a parallel fashion
in most animals from both captive colonies, duration showed some evidence
of convergence within groups, and trill rate showed normal developmental
changes (Elowson and Snowdon 1994). Thus, this experiment demonstrated
that vocalizations were not fixed, and the changes observed appear to be
in response to a changed social context. However, neither acoustic convergence nor divergence occurred, so social modification was not demonstrated. It seems likely that the animals were stressed by the presence of
unfamiliar conspecifics and may have responded by increasing the frequency of calls. Vocalizations often rise in frequency when animals are
stressed, and calls that are used in agonistic contexts often are of high frequency (Morton 1975, 1982). The lack of convergence in call characters
is not surprising. There is actually little reason to predict convergence
between animals from the two colonies because they were not social companions, had very little social interaction, and neither individuals within
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J.W. Boughman and C.F. Moss
and Cleveland 1980).
Infant call repertoires differ from adult repertoires both in types of calls
and acoustic structure of adult-like calls (Elowson et al. 1998a). In addition,
correct usage of call types appears to be learned (Elowson et al. 1998a).
Several features of contact trills change with time, although stereotypy does
not increase so the cause is unclear. J-calls show even less age-related modification (Elowson et al. 1992). Individuals’ contact calls differ extensively
in acoustic structure and the pattern of ontogenetic changes. Such high individual variance makes finding ontogenetic patterns difficult. However, one
pattern that emerges is that infants go through a stage of vocal production
reminiscent of human infant babbling (Elowson et al. 1998b). Similarities
to human babbling include repetition of call types and use of a subset of
the adult repertoire in modified infant form. Babbling infants are responded
to by caregivers more than their silent counterparts, suggesting that babbling is important in the normal development of social relationships. Data
on development indicate vocal flexibility exists and show that calls are not
always produced in adult form by juveniles. Vocal learning may or may not
play a role; vocal tract maturation and developing social relationships
may be responsible for such changes. Descriptions of development cannot
directly test the importance of vocal learning. Considering the current lack
of evidence for most primates, alternative explanations seem likely.
The potential for social modification of contact calls was studied with
family groups from two different captive colonies (Elowson and Snowdon
1994). Animals were housed in family groups initially in two separate rooms
during a quarantine period. Contact calls were recorded and analyzed from
this initial time period. Subsequently, family groups from both captive
colonies were housed in the same room in acoustic but not visual or physical contact. Contact calls recorded after the animals were introduced to
each other (postquarantine) were compared with those from the initial
period (Fig. 4.8B). Peak frequency and bandwidth rose in a parallel fashion
in most animals from both captive colonies, duration showed some evidence
of convergence within groups, and trill rate showed normal developmental
changes (Elowson and Snowdon 1994). Thus, this experiment demonstrated
that vocalizations were not fixed, and the changes observed appear to be
in response to a changed social context. However, neither acoustic convergence nor divergence occurred, so social modification was not demonstrated. It seems likely that the animals were stressed by the presence of
unfamiliar conspecifics and may have responded by increasing the frequency of calls. Vocalizations often rise in frequency when animals are
stressed, and calls that are used in agonistic contexts often are of high frequency (Morton 1975, 1982). The lack of convergence in call characters
is not surprising. There is actually little reason to predict convergence
between animals from the two colonies because they were not social companions, had very little social interaction, and neither individuals within
182
J.W. Boughman and C.F. Moss