geographic variation arises due to local copying and cultural drift (or the
random fixation of culturally transmitted variants). Random copying errors
and innovations can be transmitted horizontally (between neighbors of
similar age), obliquely (from unrelated adult to juvenile), and vertically
(from parent to offspring). Thus, the closer two individuals are in space, the
more they sound alike. This same pattern can arise if dialects result from
genetic differences among populations, making it difficult to use pattern
alone to infer vocal learning. Either learned acquisition or social modification can give rise to dialects, and the calls can function as individual or group
signatures, but need not have a signature function.
The function of birdsong dialects has been debated for some time
(Catchpole and Slater 1995). The simplest hypothesis is that dialects themselves have no particular function but merely reflect the underlying process
of vocal learning described above (Andrew 1962; Wiens 1982). Others
have argued, sometimes forcefully, that dialects have evolved to preserve
local adaptation—the genetic adaptation hypothesis (e.g., Baker and
Cunningham 1985), to enhance sound transmission by matching local
habitat—the habitat matching hypothesis (e.g., Hansen 1979; Handford
1988), or to facilitate interactions between neighbors—the social adaptation hypothesis (e.g., Payne 1981; Rothstein and Fleischer 1987). The function of call dialects in birds is often assumed to be similar to that of song
dialects even though the function of the vocalizations themselves is quite
different. Dialect function has infrequently been discussed in relation to
mammalian dialects.
Assuming that dialects have the same function for song and calls may not
be warranted because calls and song function differently. Consequently, the
underlying processes might differ. Song in oscines functions to establish and
maintain territories and to attract females (e.g., Kroodsma and Byers 1991).
In temperate species, males sing primarily during the breeding season. A
similar function is assumed for hummingbirds. Many male hummingbirds
sing in assemblages, or “leks,” and couple this song with elaborate courtship
displays. Temperate oscine and hummingbird females rarely sing. In contrast, contact calls appear to function in various social interactions, are given
by both sexes throughout the year in birds, and are usually used by both
sexes in mammals. Their primary function is not to establish and defend
nesting sites, although they can be used in this way. This difference in function may be reflected in different patterns of dialect variation because selection will probably act differently. The prevalence of learned acquisition and
social modification may also differ between bird song and the calls of both
birds and mammals.
Calls need not be learned to vary geographically. Adaptive variation in
body size can produce geographic variation that correlates with habitat,
latitude, or altitude. Habitat matching itself can result from local adaptation without recourse to vocal learning. Restricted gene flow between
dialect areas is required to produce the genetic divergence and local adaptation responsible for both morphological and call variation. In most cases,
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J.W. Boughman and C.F. Moss
random fixation of culturally transmitted variants). Random copying errors
and innovations can be transmitted horizontally (between neighbors of
similar age), obliquely (from unrelated adult to juvenile), and vertically
(from parent to offspring). Thus, the closer two individuals are in space, the
more they sound alike. This same pattern can arise if dialects result from
genetic differences among populations, making it difficult to use pattern
alone to infer vocal learning. Either learned acquisition or social modification can give rise to dialects, and the calls can function as individual or group
signatures, but need not have a signature function.
The function of birdsong dialects has been debated for some time
(Catchpole and Slater 1995). The simplest hypothesis is that dialects themselves have no particular function but merely reflect the underlying process
of vocal learning described above (Andrew 1962; Wiens 1982). Others
have argued, sometimes forcefully, that dialects have evolved to preserve
local adaptation—the genetic adaptation hypothesis (e.g., Baker and
Cunningham 1985), to enhance sound transmission by matching local
habitat—the habitat matching hypothesis (e.g., Hansen 1979; Handford
1988), or to facilitate interactions between neighbors—the social adaptation hypothesis (e.g., Payne 1981; Rothstein and Fleischer 1987). The function of call dialects in birds is often assumed to be similar to that of song
dialects even though the function of the vocalizations themselves is quite
different. Dialect function has infrequently been discussed in relation to
mammalian dialects.
Assuming that dialects have the same function for song and calls may not
be warranted because calls and song function differently. Consequently, the
underlying processes might differ. Song in oscines functions to establish and
maintain territories and to attract females (e.g., Kroodsma and Byers 1991).
In temperate species, males sing primarily during the breeding season. A
similar function is assumed for hummingbirds. Many male hummingbirds
sing in assemblages, or “leks,” and couple this song with elaborate courtship
displays. Temperate oscine and hummingbird females rarely sing. In contrast, contact calls appear to function in various social interactions, are given
by both sexes throughout the year in birds, and are usually used by both
sexes in mammals. Their primary function is not to establish and defend
nesting sites, although they can be used in this way. This difference in function may be reflected in different patterns of dialect variation because selection will probably act differently. The prevalence of learned acquisition and
social modification may also differ between bird song and the calls of both
birds and mammals.
Calls need not be learned to vary geographically. Adaptive variation in
body size can produce geographic variation that correlates with habitat,
latitude, or altitude. Habitat matching itself can result from local adaptation without recourse to vocal learning. Restricted gene flow between
dialect areas is required to produce the genetic divergence and local adaptation responsible for both morphological and call variation. In most cases,
184
J.W. Boughman and C.F. Moss