When it comes to picking a mate, it's important to make the right choice. And the choice is even tougher when nearby closely related species are sending out similar messages. Females have to identify not only an attractive male,but also a male of the same species. Carl Gerhardt from the University of Missouri explains that the frequency spectra of closely related Cope's gray treefrogs and gray treefrogs are very similar, with peaks around 1 kHz and 2 kHz. However the croaks differ in one crucial detail. Cope's gray treefrogs trill at 35–70 pulses s–1 while gray treefrogs trill at 10–35 pulses s–1. Curious to know which frequency is the best carrier for crucial mate choice messages, Gerhardt offered fertile female frogs a choice between two simplified croaks; the first trill matched to the real male's time pattern and tuned to one of the frequency peaks in the male's croak and the second at the same pitch, but with a modified time pattern. Then he monitored the females to see which call they preferred and hopped towards (p. 2609).

Gerhardt found that both species more often correctly identified the trill that resembled the real male's trill when it was at a deep pitch than when it was high pitched. The Cope's gray tree frogs were also more accurate than the gray treefrogs, correctly identifying real male-like croaks over a wider volume range at both pitches more often than the gray treefrogs. So croak frequency does affect both species' responses to the croak pulse rate, and Gerhardt suspects that the differences `may reflect the different ways in which females of the two species assess trains of pulses, and could have broad implications for understanding the underlying auditory mechanisms'.

Gerhardt, H. C. (
). Phonotactic selectivity in two cryptic species of gray treefrogs: effects of differences in pulse rate, carrier frequency and playback level.
J. Exp. Biol.