Department of Zoology, DST-IRHPA Center for Excellence in Biological Rhythms Research, University of Lucknow, Lucknow 226 007, India.
Gen Comp Endocrinol. 2013 Sep 1;190:61-7. doi: 10.1016/j.ygcen.2013.04.013. Epub 2013 Apr 28.
Birds show adaptation in their physiology and behavior to daily and seasonal periodicities in the environment. A circannual clock system gates seasonal events in birds to happen at the most appropriate time of the year, since a mistiming will delay the event until the arrival of the favorable time next year. The circannual clocks, the self-sustained endogenous system, are expressed under aperiodic conditions with a period close to 12months. In stonechats, it has been shown that circannual rhythms in testicular and molt cycles persist for 12years; this suggests that circannual clocks are functional throughout the life of an individual. Circannual rhythms are synchronized with both the photoperiodic and non-photoperiodic cues, and a synchronized circannual rhythm provides information on annual timing of the physiological event. The integration of rigid circannual timekeeping with the cyclic environment ensures phenotypic plasticity that is required for successful survival of a species in its habitat. There can be however sex differences in the circannual rhythm characteristics and in the relationship between circannual rhythms and external environment. In few cases that have been investigated, females appear playing a larger role in defining the breeding season in the year in relation to the environment. There is no evidence for the involvement of circadian clock in the generation of circannual rhythms. Therefore, future researches need to focus on finding molecular gears that possibly form the neuroendocrine loop and are translated into a seasonal event. Here, we briefly review limited information that is available on circannual rhythms and their relationship with the external environment from a few bird species, inhabiting tropical and temperate environments.
鸟类在生理和行为上表现出对环境中日周期性和季节性周期性的适应。一个年周期时钟系统将鸟类的季节性事件安排在一年中最合适的时间发生，因为时机不当会导致事件延迟到明年有利时机到来。年周期时钟系统是一种自我维持的内源性系统，在非周期性条件下表现出接近 12 个月的周期。在石雀中，已经表明睾丸和换羽周期的年周期节律可以持续 12 年；这表明年周期时钟在个体的整个生命周期中都是有效的。年周期节律与光周期和非光周期线索同步，同步的年周期节律提供了关于生理事件年度时间安排的信息。刚性的年周期计时与周期性环境的整合确保了表型可塑性，这是物种在其栖息地成功生存所必需的。然而，年周期节律特征和年周期节律与外部环境之间的关系可能存在性别差异。在一些已经调查过的情况下，雌性似乎在确定与环境有关的当年繁殖季节方面发挥更大的作用。目前还没有证据表明昼夜节律钟参与了年周期节律的产生。因此，未来的研究需要集中于寻找可能形成神经内分泌环并转化为季节性事件的分子齿轮。在这里，我们简要回顾了来自热带和温带环境的几种鸟类的年周期节律及其与外部环境关系的有限信息。
