The separate and combined effects of harmonic structure, phase, and FM on female preferences in the barking treefrog (Hyla gratiosa).

In natural advertisement calls of the barking treefrog, Hyla gratiosa, a small amount of incoherent frequency modulation (FM) is present. Incoherency in the FM of a call creates inharmonicity and phase changes between its frequency components. In this study, the combined and separate effects of the harmonic structure, phase spectrum, and FM of an advertisement call on female choice were tested. The harmonic structure of a call can have a direct effect on female preference; females showed a significant preference for static-inharmonic calls over the static-harmonic calls. Neither differences in phase or FM alone conferred a preference in two choice tests. However, when FM is present in both calls it does influence female preference for harmonic structure--namely harmonic calls become preferable to inharmonic calls. This reversal of female preference for inharmonicity in a call by the presence of FM suggests that call parameters may interact, and thereby effect mate choice.