Aggressive behavior between pairs of female or male zebra finches (Poephila guttata) was measured before, during, and after presentation of either a female or a male finch (the stimulus) in an adjacent cage. Presentation of the stimuli led to marked increases in aggressive behavior for both sexes of subjects, such that presentation of a female stimulus caused an elevation in aggression in male pairs and presentation of a male stimulus caused an elevation in female pairs. These results are interpreted in the context of competition for mates in species, such as zebra finches, that have long-term monogamous pair bonds and biparental care.

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