Sonic Patterns I: The Noise - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 161

Frog songs are innate and not learned as in birds and primates: they are simpler
and stereotyped. Females select a male on the basis of song attributes and perfor-
mance (frequency or pitch, pulse rate, amplitude, call rate, or call length). Females
prefer calls that are costly, meaning they prefer calls that are louder, longer, and that
have a higher repetition rate. A male that produces a call with these properties is
indicating that he has enough energetic reserves and vigorous, healthy, with access
to abundant resources; in other words, a “winner.” In some species the females
prefer lower-frequency calls that are typical of large or older males with more
experience.
Masking represents an acoustic interference produced by background noise or by
the calls of other individuals. The noise produced by a chorus can reduce the female
choice for a male, but this interference can be reduced by selection of a more
efficient way of communication. When a noise masks their calls, frogs have two
possibilities to reduce this effect: to increase amplitude or to shift the frequency
pitch toward higher frequencies.
This second possibility is relative to the fact that noise occurs at the lowest
frequencies. Considering that frog calls are really costly energetically, the second
solution seems more favorable.
Frogs are important in ecosystems for they have a double position in the food
web. Frogs at a larval stage are herbivores and represent food for many predators.
At the adult stage, frogs are predators of many insects and pests. Their presence
around the world is at risk for reasons such as spontaneous diseases, soil reclama-
tion, climate changes, and increased numbers of roads that not only impact by
directly killing individuals during their migration to reproductive ponds but also
mask their choruses with increasing noise.
For instance, in birds adaptation to traffic noise can be more rapid than in frogs
because a large portion of the song repertoire of birds is learned from their parents,
neighbors, and other flock members; this probably does not occur in frogs, which
have an innate repertoire.
Parris et al. ( 2009 ) have investigated the effect of traffic noise in two species of
Australian frogs: the southern brown tree frog ( Litoria ewingii ) and the common
eastern froglet ( Crinia signifera ). The effects of traffic noise were different in these
two species. In L. ewingii calls in presence of traffic noise shifted to 4.1 Hz/dB of
traffic noise, with a total size of 123 Hz. In C. signifera , there is a call with higher
pitch, but the authors recommend more data to confirm this trend.
In evolutionary terms, the strategy to shift toward higher frequencies could
produce negative effects in mating strategies. In fact, females normally respond
better to lower-frequency calls of large or more experienced males.
Similar to male birds, calling at a higher frequency with background noise from
traffic seems a trade-off between communicating with females and reducing the risk
of predation for females that could be obliged to move more to localize males
whose calls are masked by noise.
In L. ewingii , the active distance at which calls can be received increases from
19 to 24 m, but really few occur there when compared with the distance of 74.5 m
predicted in absence of traffic. L. ewingii changes calls in response to traffic noise