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Evidence for reproductive character displacement (RCD) has accumulated
more slowly than for ecological character displacement, perhaps because
sampling scales and environmental covariates can obscure the role of RCD
in speciation. We examined an early example of RCD in an anuran
species group, the vocalizations of the sympatric cricket frogs Acris
crepitans and A. gryllus. With a relatively fine spatial
scale, we compared mixed-species choruses (syntopy), nearby locations
where A. gryllus is recently extirpated (historic sympatry), and
surrounding areas without secondary contact (allopatry). In each of
these areas, we evaluated variation in dominant frequency, click rate, and
mass of males. In addition, we determined the acoustic preferences
of syntopic females. Temperature influenced dominant frequency of
vocalizations in A. crepitans, but not in A. gryllus.
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Body size varied more and had a stronger influence on dominant frequency
in A. crepitans than in A. gryllus. Consequently, the
decrease in mass of A. crepitans in syntopy resulted in convergence
of body size and divergence of dominant frequencies of the two species.
In contrast, dominant frequency of A. crepitans did not
differ between historic sympatry and allopatry. Females of both
species used fine temporal structure to discriminate between conspecific
and heterospecific vocalizations and showed no preferences for dominant
frequency. Chorus noise limited the ability of A. gryllus
females to detect and discriminate vocalizations, so convergence in
mass might have resulted from RCD in dominant frequency to reduce
heterospecific acoustic interference. However, influences other
than RCD might have caused syntopic convergence in body size.
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