Song evolution in Maluridae: influences of natural and sexual selection on acoustic structure

Many factors may influence the evolution of acoustic signals, including sexual selection, morphological constraints and environmental variation. These factors can play simultaneous and interacting roles in determining signal phenotypes. Here, we assess the evolution of song features in the Maluridae, a passerine family with significant variation among taxa in levels of sperm competition, morphological features and breeding habitats ranging from arid grasslands in Australia to tropical rainforests in New Guinea. We used phylogenetic comparative methods and a robust molecular phylogeny to compare song characteristics with a variety of other measures, including testes mass, body-size and latitude. Several aspects of the temporal and frequency structure of song were associated with relative testes mass, suggesting that sexual selection may influence some song characteristics in this family. The lowest frequencies of song were strongly predicted by body-size, indicating that morphological constraints have also likely influenced acoustic phenotypes. Song versatility, reflecting the diversity of note types in a song, was positively correlated with latitude, suggesting that complexity may increase in association with more temperate or variable environments. Variation in song structure across the family appears to reflect a complex interaction between natural and sexual selection.