The evolution of photoperiod response systems and seasonal GnRH plasticity in birds

Animals' lives are typically subdivided into distinct stages, some of which (e.g. breeding) contribute to fitness through enhancing current reproductive success, and some of which (e.g. molting and migration in birds; hibernation in mammals) contribute to fitness through enhancing survival and, therefore, future reproductive opportunities. There is often a trade-off between these two kinds of processes, either because they are temporally incompatible with one another (e.g. migration precludes simultaneous nesting in birds) or because they are energetically incompatible with one another (e.g. successful molting appears to be incompatible with simultaneous nesting in many birds). Consequently, adaptations facilitating appropriate timing and coordination of different life-cycle stages are arguably as important to fitness as are more obvious adaptations such as feeding morphologies and predator avoidance. Mechanisms that facilitate coordination of life-cycle events with the annual cycle of changes in the environment are therefore expected to evolve in response to selection imposed by different environmental challenges. This article focuses on how mechanisms affecting the timing of, and transitions between, life-cycle stages, particularly breeding, have evolved in birds. Through comparative analyses, we show that photorefractoriness and one neuroendocrine correlate of it-plasticity of the gonadotropin releasing hormone system-have evolved in ways that facilitate different degrees of flexibility in timing of the transition from breeding to molting in different environments. We argue that the nature of the mechanistic adaptations will affect the capacity for adaptive adjustments to changing environmental conditions both in the short term (plasticity inherent in individuals) and in the long term (evolutionary responses of populations to selection).