2.1. Mammals
2.1.1. Bats—Lesser Spear-Nosed Bat Pup Isolation Calls
Lesser spear-nosed bats (Phyllostomus discolor) roost in hollow trees in
large colonies of 50–300 bats. These colonies are composed of several
smaller groups of 4–14 females and a single male. Colonies move from roost
tree to roost tree as a unit and thus appear to be relatively stable, whereas
the composition of groups within colonies is dynamic (C. Kagarise, J.
Bradbury, and L. Emmons unpublished data). Adult females leave their
infants in the roost each night when they go out foraging and must find their
pup when they return. Finding their mothers has immediate fitness consequences for pups who depend on their mothers for sustenance, grooming,
thermoregulation, and protection. Identifying her own pup affects adult
female fitness as well. Females produce a single pup each year, so losing a
pup can seriously depress reproductive success. Misdirecting maternal care
is also costly because it decreases the amount of care a mother can provide
her own offspring. These costs should favor a mechanism for mothers to
identify offspring and for offspring to find mothers.
Like many bat species, when separated from their mothers, P. discolor
infants give sinusoidally frequency-modulated (FM), multiharmonic calls,
termed isolation calls (Fig. 4.1A). Mothers produce maternal directive calls
whose frequency structure is quite similar to infant isolation calls. Isolation
calls and maternal directive calls vary individually in the pattern of sinusoidal FM and duration (Esser and Schmidt 1989). Adults can discriminate
modulation frequencies that differ by as little as 2 or 3 Hz, more than sufficient to discriminate individuals whose modulation frequencies differ by as
much as 49–100 Hz (Esser and Lud 1997). Infants appear to recognize the
directive call of their own mother (Schubert and Esser 1997), and mothers
respond to their own pup’s calls. Reciprocal calling facilitates reunions
between pups and their mothers when pups fall or fly from the roost site or
when mothers return from foraging (Gould 1983). We focus here on data
concerning vocal learning of pup isolation and maternal directive calls.
Research on vocal learning in P. discolor includes observational information on normal development (Esser and Schmidt 1989), descriptive
work on geographic variation (Esser and Schubert 1998), and experimental tests of social modification (Esser 1994). In addition, psychophysical
work has described auditory thresholds in infants (Esser and Schmidt
1990) and adults (Esser and Daucher 1996), and detection of frequency
modulation (Esser and Kiefer 1996), which may be important for
individual discrimination of these FM calls.
Newborns produce isolation calls at birth, suggesting that learned acquisition is not necessary for normal production. However, pup isolation
calls do undergo progressive changes during development (Fig. 4.1A).
Calls given by newborns show little FM and the pattern of sinusoidal FM
increases with age. In addition, some calls given by newborns drop out of
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J.W. Boughman and C.F. Moss
2.1.1. Bats—Lesser Spear-Nosed Bat Pup Isolation Calls
Lesser spear-nosed bats (Phyllostomus discolor) roost in hollow trees in
large colonies of 50–300 bats. These colonies are composed of several
smaller groups of 4–14 females and a single male. Colonies move from roost
tree to roost tree as a unit and thus appear to be relatively stable, whereas
the composition of groups within colonies is dynamic (C. Kagarise, J.
Bradbury, and L. Emmons unpublished data). Adult females leave their
infants in the roost each night when they go out foraging and must find their
pup when they return. Finding their mothers has immediate fitness consequences for pups who depend on their mothers for sustenance, grooming,
thermoregulation, and protection. Identifying her own pup affects adult
female fitness as well. Females produce a single pup each year, so losing a
pup can seriously depress reproductive success. Misdirecting maternal care
is also costly because it decreases the amount of care a mother can provide
her own offspring. These costs should favor a mechanism for mothers to
identify offspring and for offspring to find mothers.
Like many bat species, when separated from their mothers, P. discolor
infants give sinusoidally frequency-modulated (FM), multiharmonic calls,
termed isolation calls (Fig. 4.1A). Mothers produce maternal directive calls
whose frequency structure is quite similar to infant isolation calls. Isolation
calls and maternal directive calls vary individually in the pattern of sinusoidal FM and duration (Esser and Schmidt 1989). Adults can discriminate
modulation frequencies that differ by as little as 2 or 3 Hz, more than sufficient to discriminate individuals whose modulation frequencies differ by as
much as 49–100 Hz (Esser and Lud 1997). Infants appear to recognize the
directive call of their own mother (Schubert and Esser 1997), and mothers
respond to their own pup’s calls. Reciprocal calling facilitates reunions
between pups and their mothers when pups fall or fly from the roost site or
when mothers return from foraging (Gould 1983). We focus here on data
concerning vocal learning of pup isolation and maternal directive calls.
Research on vocal learning in P. discolor includes observational information on normal development (Esser and Schmidt 1989), descriptive
work on geographic variation (Esser and Schubert 1998), and experimental tests of social modification (Esser 1994). In addition, psychophysical
work has described auditory thresholds in infants (Esser and Schmidt
1990) and adults (Esser and Daucher 1996), and detection of frequency
modulation (Esser and Kiefer 1996), which may be important for
individual discrimination of these FM calls.
Newborns produce isolation calls at birth, suggesting that learned acquisition is not necessary for normal production. However, pup isolation
calls do undergo progressive changes during development (Fig. 4.1A).
Calls given by newborns show little FM and the pattern of sinusoidal FM
increases with age. In addition, some calls given by newborns drop out of
156
J.W. Boughman and C.F. Moss