Asymmetric frequency shift in advertisement calls of sympatric frogs

Character displacement is commonly observed when
species occur in secondary contact zones and traits related to resource
competition or reproduction diverge in sympatry. However, few studies have
considered the factors determining and delimiting the direction of character
evolution in this context. We studied displacement in advertisement calls in
two species of hylid frogs from allopatric and sympatric populations, both of
which call with similar frequencies but differ substantially in temporal
parameters. We found asymmetrical character displacement in sympatry, as only Scinax
madeira (but not S. fuscomarginatus) repeatedly showed displacement.
Instead of diverging in already existing differences in temporal characters, S.
madeirae showed character displacement for frequency-related characters. We
explored possible reasons for this specific pattern concerning the displaced
characters and tested if socio-functional constraints in specific call
parameters are responsible for the shift of only spectral parameters in that
species. Finally, we argue that the simultaneous action of ecological and
reproductive character displacement, or alternatively, a short-term behavioral
response for the same reason (avoidance of hybridization) could explain the
pattern. The present study identifies a set of new hypotheses that will
stimulate future research on mechanisms of mate recognition and behavioral
responses.