the repertoire during the first weeks of life (Esser and Schmidt 1989; Esser
1994). Esser and Schmidt (1989) suggest that a female infant’s isolation
calls develop into maternal directive calls. Social modification is possible,
but these developmental changes could simply be due to maturation of the
vocal tract and increasing motor and respiratory control.
One piece of evidence that the fine structure of vocalizations is modified
by learning is the suggestion of dialects in maternal directive calls (Esser
and Schubert 1998). Captive adult females from two regions show differences in carrier frequency, modulation frequency, and the number of FM
peaks, which separates them into two partially overlapping multivariate
clusters in a multidimensional scaling analysis.The authors present no direct
data that dialects result from learning; rather, they draw parallels between
their findings and dialects in oscine birds to suggest learning. They also cite
the experimental evidence for social modification described below. Alternative explanations have not been ruled out. Groups from the two regions
are likely to have been genetically isolated, so genetic differences could contribute to the differences described.Variation among captive groups in body
size or age could also contribute to differences in the frequency measures.
Testing for the influence of these factors should be straightforward and
would greatly strengthen claims for learning-based dialects.
One experiment supports the importance of social modification in this
species’ vocalizations (Esser 1994). Four individually housed, hand-reared
bats were presented with one maternal directive call as a reference vocalization, and changes in the isolation calls of these pups over the first 100
days of life were monitored. The playback group was compared with a
control group of hand-reared pups that heard no maternal directive calls
(Fig. 4.1B). Both groups increased the amount of sinusoidal FM in their
calls during development; however, the playback group showed a larger
amount of FM at 100 days, coming close to the value of the maternal directive call. The rate of sinusoidal FM in the playback group’s calls showed
some convergence with the maternal directive call as compared with the
control group, although the extent of convergence was not great. No apparent convergence in fundamental frequency of the first FM peak was found.
Bandwidth of both control and playback groups overlapped the distribution of the reference call (Esser 1994). The observed convergence was
interpreted as evidence for vocal learning; clearly, social modification is
the relevant form. For P. discolor, vocal learning functions to increase vocal
similarity to promote mutual recognition and facilitate reunions.
There may be two opposing forces at work shaping isolation calls. Pups
that sound similar to their mother may facilitate recognition and reunion.
But pups that sound different from infant roost mates should also facilitate
recognition. The extent of convergence on the reference call that Esser
(1994) observed may have been constrained by the need for pups to distinguish themselves from other infants. This theme runs through other studies
on learned individual signatures—similarity to some individuals may be
favored simultaneously with increased distinction from other individuals.
158
J.W. Boughman and C.F. Moss
1994). Esser and Schmidt (1989) suggest that a female infant’s isolation
calls develop into maternal directive calls. Social modification is possible,
but these developmental changes could simply be due to maturation of the
vocal tract and increasing motor and respiratory control.
One piece of evidence that the fine structure of vocalizations is modified
by learning is the suggestion of dialects in maternal directive calls (Esser
and Schubert 1998). Captive adult females from two regions show differences in carrier frequency, modulation frequency, and the number of FM
peaks, which separates them into two partially overlapping multivariate
clusters in a multidimensional scaling analysis.The authors present no direct
data that dialects result from learning; rather, they draw parallels between
their findings and dialects in oscine birds to suggest learning. They also cite
the experimental evidence for social modification described below. Alternative explanations have not been ruled out. Groups from the two regions
are likely to have been genetically isolated, so genetic differences could contribute to the differences described.Variation among captive groups in body
size or age could also contribute to differences in the frequency measures.
Testing for the influence of these factors should be straightforward and
would greatly strengthen claims for learning-based dialects.
One experiment supports the importance of social modification in this
species’ vocalizations (Esser 1994). Four individually housed, hand-reared
bats were presented with one maternal directive call as a reference vocalization, and changes in the isolation calls of these pups over the first 100
days of life were monitored. The playback group was compared with a
control group of hand-reared pups that heard no maternal directive calls
(Fig. 4.1B). Both groups increased the amount of sinusoidal FM in their
calls during development; however, the playback group showed a larger
amount of FM at 100 days, coming close to the value of the maternal directive call. The rate of sinusoidal FM in the playback group’s calls showed
some convergence with the maternal directive call as compared with the
control group, although the extent of convergence was not great. No apparent convergence in fundamental frequency of the first FM peak was found.
Bandwidth of both control and playback groups overlapped the distribution of the reference call (Esser 1994). The observed convergence was
interpreted as evidence for vocal learning; clearly, social modification is
the relevant form. For P. discolor, vocal learning functions to increase vocal
similarity to promote mutual recognition and facilitate reunions.
There may be two opposing forces at work shaping isolation calls. Pups
that sound similar to their mother may facilitate recognition and reunion.
But pups that sound different from infant roost mates should also facilitate
recognition. The extent of convergence on the reference call that Esser
(1994) observed may have been constrained by the need for pups to distinguish themselves from other infants. This theme runs through other studies
on learned individual signatures—similarity to some individuals may be
favored simultaneously with increased distinction from other individuals.
158
J.W. Boughman and C.F. Moss