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Many animals communicate in environments with high levels of background
noise. Although it is a fundamental prediction of signal detection theory
that noise should reduce both detection and discrimination of signals,
little is known about these effects in animal communication.
Female treefrogs, Hyla ebraccata, in Costa Rica choose mates in large
noisy multispecies choruses. We tested gravid females for preferences
between computer-synthesized calls with carrier frequencies of 3240 and
2960 Hz (values near the mode and the fifth percentile of the population,
respectively) in four levels of background noise from a natural chorus.
In the absence of noise (signal/noise ratio >25 dB), females preferred the
lower frequency. With moderate signal/noise ratios (6 and 9 dB), they did
not discriminate between these
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frequencies. With low signal/noise ratios (3 dB), females preferred the
frequency near the mode for the population. Similar experiments had
previously demonstrated that females can detect the presence of a male's
calls with signal/noise ratios of 3 dB or greater.
Thus moderate levels of natural background sound reduced a female's
ability to discriminate between males' calls even when she could detect
them. In high levels of background sound, females abandoned discrimination
for low-frequency calls and reverted to the task of detecting signals with
modal properties for the population.
These results justify recent theoretical analyses of the importance of
receivers' errors in the evolution of communication.
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