Character displacement from the receiver's perspective: species and mate recognition despite convergent signals in suboscine birds

Abstract

Many social animals use long-distance signals to attract mates and defend territories. They face the twin challenges of discriminating between species to identify conspecific mates, and between individuals to recognize collaborators and competitors. It is therefore often assumed that long-distance signals are under strong selection for species-specificity and individual distinctiveness, and that this will drive character displacement when closely related species meet, particularly in noisy environments. However, the occurrence of signal stereotypy and convergence in rainforest species seems to contradict these ideas, and raises the question of whether receivers in these systems can recognize species or individuals by long-distance signals alone. Here, we test for acoustically mediated recognition in two sympatric antbird species that are known to have convergent songs. We show that male songs are stereotyped yet individually distinctive, and we use playback experiments to demonstrate that females can discriminate not only between conspecific and heterospecific males, but between mates and strangers. These findings provide clear evidence that stereotypy and convergence in male signals can be accommodated by fine tuning of perceptual abilities in female receivers, suggesting that the evolutionary forces driving divergent character displacement in animal signals are weaker than is typically assumed.

Figure 1.

Scatter plots showing individually distinctive structure of male songs in (a) H. peruviana (n = 9), and (b) H. subflava (n = 9). Songs are plotted according to three discriminant functions (DF) derived from 20 acoustic parameters (DF3 is not labelled but is represented by depth). Each male is represented by one normal contour ellipsoid (coverage = 80%) and each point corresponds to one analysed song. In (a) DF1 is positively related to PC1 (which largely reflects note number, note duration, maximum frequency, peak frequency and bandwidth); DF2 is positively related to PC1 and negatively related to PC2 (song duration, overall pace and change in pace) and DF3 is negatively related to PC1 and PC4 (minimum frequency). In (b), DF1 is negatively related to PC1; DF2 is positively related to PC1 and PC2; DF3 is negatively related to PC4 (bandwidth, change in frequency) and positively related to PC5 (pace of segment 2). See electronic supplementary material for factor loadings (table S1) and description of acoustic characters (figure S1).

Figure 2.

Number of songs and calls given by captive female antbirds to playback of songs of their mate, a conspecific stranger and a heterospecific stranger. Data reveal a strong significant effect of playback treatment on female response in both (a) H. peruviana and (b) H. subflava. Females responded most strongly to playback of their mate's song than to playback of conspecific stranger song, and more strongly in response to conspecific stranger song than to heterospecific stranger song. Asterisks denote p-values from Wilcoxon-signed rank tests comparing female response to conspecific mate versus stranger, and to stranger versus heterospecific (*p < 0.05, **p < 0.001). Bars show mean ± s.e.; n = 9 females for all experiments.