Sexual Selection and the Evolution of Complex Songs Among European Warblers of the Genus Acr Ocephal Us

Abstract An attempt has been made to explain why the songs of Acrocephalus warblers have become so complex during their evolution. A considerable body of evidence has implicated sexual selection, and two predictions from sexual selection theory have been tested. First, males with more complex songs should attract females for pairing before their rivals with simpler songs. This prediction has been confirmed, when a strong inverse correlation between syllable repertoire size (song complexity) and pairing date (female choice) was obtained in a population of sedge warblers. No significant correlation was obtained between male arrival date and pairing date. Assuming that the first (usually older) males back should select the best territories, then territory quality appeared to have little or no significant effect upon female choice. The second prediction tested was that due to increased sexual selection pressure, males of polygynous species should have evolved more comples songs than males of monogamous species. Of the six common European Acrocephalus species studied, four are monogamous and two polygynous. The prediction was not upheld, as the four monogamous species had long, complex and variable songs, and the two polygynous species had shorter, simpler and less variable songs. The explanation for this apparent paradox can be found in the different relationships between female choice, feeding ecology and parental investment in monogamous or polygynous Acrocephalus species. In polygynous species, a female must ensure the territory contains enough resources to enable her to feed the young and should base her choice primarily upon territory quality. In monogamous species, a female must select a male who will help her to feed the young, and should base her choice more directly upon male quality. The songs of polygynous species have therefore evolved primarily through indirect (intrasexual) sexual selection pressure, which has produced shorter, simpler and more stereotyped songs more suited to male-male interactions and a territorial function. The songs of monogamous species have evolved primarily through direct (intersexual) selection pressure, which has produced the long, complex and variable songs which females find more attractive.