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Quispe Valdes, Rene (2016): Environmental and neuroendocrine control of seasonal phenotypes and song behavior of an equatorial songbird. Dissertation, LMU München: Faculty of Biology



At higher latitudes, as a result of the predictable and ubiquitous nature of the annual photoperiod, most temperate-zone bird species have converged in using day length as a primary environmental cue to time seasonal processes. Instead, in equatorial biomes the environmental fluctuations have usually a more heterogeneous degree of seasonality. Hence, the neural and endocrine mechanisms that mediate the occurrence of seasonal behaviors and life history stages might operate in diverse manners among equatorial songbirds. In the present thesis I investigated the regulatory mechanisms underlying the annual cycle and song behavior of the equatorial silver-beaked tanager, an endemic Amazon songbird with an entirely tropical phylogenetic background. Thereby, I sought to study proximate regulatory mechanisms that have likely evolved with small photoperiodic cycles. I show that males inhabiting the equatorial zone of the eastern Amazon, a highly productive region that surrounds the Amazon estuary, have marked seasonal schedules of molt and dawn-song behavior. This region imposes singular environmental conditions for birds, including unconstrained food availability over the year. Accordingly, male silver-beaked tanagers exhibited uniform annual levels of baseline corticosterone (Chapter 1). However, individual baseline corticosterone and testosterone levels are clearly associated with the molting status of males. On the other hand, males exhibited important seasonal changes in testis size and gonadal testosterone production, which denote a strong seasonal breeding pattern (Chapters 1 & 2). Particularly interesting is the neuroendocrine regulation of seasonal dawn-song, a behavior directly involved in the breeding territoriality of males, whose seasonal activation appears as decoupled from the seasonal peak of testosterone levels in plasma that occurs later in the breeding (Chapter 2). Further, although at equatorial latitudes environmental photic cues have a very small magnitude of variation over time, they do seem to influence the occurrence of dawn-song in male silver-beaked tanagers on a daily basis, as well as at a seasonal level. Besides this, males also integrate the occurrence of rainfall cycles as an environmental signal for the timing of seasonal phenotypes and dawn-song. Finally, in Chapter 4 I complemented my PhD research by proposing a new method of steroid hormone implantation to be used for the study of wild birds. Overall, the present thesis demonstrates a concerted orchestration of seasonal events in equatorial males. This phenology of silver-beaked tanagers is mediated by conserved neuroendocrine regulatory networks that function with a different temporal and relational pattern when compared with widely studied temperate-zone bird species