Sonic Patterns II: The Animal Choruses - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 197

Boophis madascariensis , a frog endemic to Madagascar that has 28 distinct calls, as
reported by Narins et al. ( 2000 ).
7.4 Cooperative Versus Competitive Hypothesis
in Amphibian Choruses
It emerges from several studies that a cooperative hypothesis to explain the chorus
pattern could be presented and argued, at least in insects and anurans. Cooperation
means to produce a synchronized chorus with effects such as confusion of
predators, degrading the locability of signals, and the enhancement of female
detection because synchronization increases the acoustic peaks. This hypothesis
can be easily dismantled in favor of the competition hypothesis. In a competitive
scenario, leading calls have been observed to be more attractive for females. The
mechanism for a male to reduce the overlap is to wait when there is a gap in the
chorus, but this cannot always happen and a male could be silent for a long time. In
effect, a male interrupts calling just waiting for a temporal subset of a nearby male.
Brush and Narins ( 1989 ) have found in the Puerto Rican treefrog ( Eleutherodactylus
coqui ) that males were responding, avoiding the overlap, to two or three neighboring
males. This attention to avoid overlap can be found also in species that do not adopt
this strategy.
Calling is expensive in ectothermic species, and a trade-off must exist between
rate of calling and extension of the period of calling to reduce the risk for a male to
stop calling before the other males do because of exhaustion of the energy pool. In
many anurans, the calling effort increases with the increase of individual density,
but to compensate for a major requirement of energy the rate of calling is reduced,
as observed in Hyla versicolor by Grafe ( 1997 ).
Mate choice is of first importance in anurans, where females must be able to
distinguish the more energetic male in a noisy background. Several studies have
demonstrated that females select males that utter loud and conspicuous signals. The
leading signal seems the preferred and is considered a “precedence effect” (for
review, see Litovsky et al. 1999 ). This effect states that when two separate sounds
are presented with a brief delay in onset, the leading sound is the first to be
localized.
The habitat conditions are important to establish a profitable communication
network in anuran choruses. The distance between the sender and the receiver and
the height at which the call is emitted are factors that affect the efficiency of
acoustic transmission, although, as argued by Grafe ( 2005 ), “ The evidence for the
influence of transmission properties of the environment and network structure on
signal design and signaling behaviour in anurans is equivocal at best. ”
It is evident that females are attracted by leading and loud signals, and this has
been verified in the laboratory under conditions of few males and low background
noise: larger males with lower frequency capacity were preferred. In nature this can