| In adult male songbirds, the brain regions that control singing behavior grow seasonally, providing a means to investigate the regulatory mechanisms and the functional consequences of adult neuroplasticity. Specifically, during the breeding season these regions are larger than outside the breeding season, due to increases in neuron number and size or decreases in neuron density. Numerous factors that change during the breeding season have been implicated in regulating the growth of these brain regions, most notably, testosterone (T), photoperiod and singing behavior. Additionally, other social or environmental cues interact with T and photoperiod, or act independently, to affect song control region growth. However, the contributions of each factor can be difficult to tease apart, because under natural conditions these factors change simultaneously. Many models that attempt to describe how these factors contribute to seasonal neuroplasticity usually center on direct effects of T on SCR growth and singing behavior, minimizing direct effects of other factors or interactions between these factors. Indeed, numerous experiments have demonstrated the powerful effect of T on certain aspects of neuroplasticity. However, direct evidence for the effect of T on new neuron incorporation during the breeding season is lacking. This dissertation takes a comparative approach to isolate the effects of T, photoperiod, singing and other social or environmental factors on song control region growth and new neuron incorporation.; This research has led to four major conclusions. First, T does not affect song control region growth equally under different photoperiodic or social conditions. Second, interactions between T, photoperiod and singing are likely necessary to stimulate maximal song control region growth. Third, when breeding is not coupled to changes in photoperiod, other environmental cues are associated with song control region growth. Finally, T affects neuron number, but not new neuron incorporation, under spring-like conditions. Together, these results indicate that current models describing the factors affecting song control region growth are incomplete and should take into account direct effects of environmental and social cues and interactions between these factors and circulating hormones on neuroplasticity. Although more complex, this approach provides a more complete and ultimately more reliable analysis of the contributions of various factors on regulation of seasonal neuroplasticity. |
