Much current literature, covering a variety of sensory modes and animal groups, focuses on identifying cues that individuals use to assess one another and on the evolution of those cues. Such cues may be used during either interactions between animals of the same sex or interactions between animals of opposite sex. How animals perceive one another during these interactions has important implications for evolution of traits. Understanding how animals may use vocal cues has been hampered, however, by the difficulty of establishing cause and effect, rather than just correlation, between the vocal behavior and its possible consequences. By taking advantage of the vocal learning abilities of songbirds, Dr. Kroodsma will be able to experimentally "assign" different vocal behaviors to different individuals. With cues assigned at random and therefore irrespective of other male qualities, hypothesis testing can proceed. Initial studies will focus on song repertoire size in marsh wrens, and study of other vocal behaviors in other species will follow. Individuals with "assigned behaviors" become invaluable in other experiments that test for cause-and-effect relationships between behavior and brain characteristics; correlations have been discovered between song repertoire size and the volume of song control centers in the avian forebrain, for example, but until either the behavior (as in Dr. Kroodsma's experiments) or the brain is manipulated experimentally, one cannot determine the direction of causation.
