VARIATION IN THE CODING OF SPECIES IDENTITY IN THE ADVERTISEMENT CALLS OF LITORIA VERREAUXI (ANURA: HYLIDAE)

Variation within and among males was documented in two properties of the advertisement call of Litoria verreauxi; comparisons were also made between populations of L. verreauxi that were allopatric and sympatric with L. ewingi. Pulse rate, a property of known importance in mate recognition, was stereotyped. However, a character upon which the full expression of pulse rate depends, the depth of amplitude modulation (AM) or pulsing, was significantly lower and more variable in populations of L. verreauxi that coexisted with L. ewingi than in allopatric populations of L. verreauxi. Furthermore, there were significant changes in the depth of AM in the calls of sympatric males recorded from one night to another within the breeding season. The within-male variation (between recordings) in this character within sympatric populations was so great that statistically significant differences between males could not be detected. Thus, even if gravid females in sympatry choose a male on the basis of the degree to which his calls are pulsed, the evolutionary response to such selection would probably be very limited. The change that appears to be occurring in the temporal code for species identity vis-à-vis L. ewingi in sympatry (a change from a quantitative difference [pulse rate] to a qualitative one [pulsed versus unpulsed calls]) is probably a passive consequence of the shift to higher pulse rates.