Animal Behavior Graduate Group and Department of Neurobiology, Physiology and Behavior, University of California Davis, Davis, CA 95616, USA.
J Exp Biol. 2011 Aug 15;214(Pt 16):2768-77. doi: 10.1242/jeb.057174.
Trade-offs between self-maintenance processes can affect life-history evolution. Integument replacement and the stress response both promote self-maintenance and affect survival in vertebrates. Relationships between the two processes have been studied most extensively in birds, where hormonal stress suppression is down regulated during molt in seasonal species, suggesting a resource-based trade-off between the two processes. The only species found to differ are the rock dove and Eurasian tree sparrow, at least one of which performs a very slow molt that may reduce resource demands during feather growth, permitting investment in the stress response. To test for the presence of a molt-stress response trade-off, we measured hormonal stress responsiveness during and outside molt in two additional species with extended molts, red crossbills (Loxia curvirostra) and zebra finches (Taeniopygia guttata). We found that both species maintain hormonal stress responsiveness during molt. Further, a comparative analysis of all available species revealed a strong relationship between molt duration and degree of hormonal suppression. Though our results support trade-off hypotheses, these data can also be explained by alternative hypotheses that have not been formally addressed in the literature. We found a strong relationship between stress suppression and seasonality of breeding and evidence suggesting that the degree of suppression may be either locally adaptable or plastic and responsive to local environmental conditions. We hypothesize that environmental unpredictability favors extended molt duration, which in turn allows for maintenance of the hormonal stress response, and discuss implications of a possible trade-off for the evolution of molt schedules.
自我维护过程之间的权衡取舍会影响到生活史的进化。蜕皮和应激反应都能促进自我维护，并影响脊椎动物的生存。这两个过程之间的关系在鸟类中研究得最为广泛，在季节性物种中，激素应激抑制在换羽期间被下调，这表明这两个过程之间存在基于资源的权衡取舍。唯一被发现不同的物种是岩鸽和欧洲树麻雀，其中至少有一种表现出非常缓慢的换羽，这可能会减少羽毛生长过程中的资源需求，从而允许对应激反应进行投资。为了检验蜕皮-应激反应权衡取舍的存在，我们在另外两种具有延长换羽期的物种中测量了换羽期和换羽期外的激素应激反应，这两种物种是红交嘴雀（Loxia curvirostra）和斑马雀（Taeniopygia guttata）。我们发现这两个物种在换羽期间都保持着激素应激反应。此外，对所有可用物种的比较分析显示，换羽持续时间和激素抑制程度之间存在很强的关系。尽管我们的结果支持权衡假设，但这些数据也可以用文献中尚未正式提出的替代假设来解释。我们发现应激抑制与繁殖的季节性之间存在很强的关系，并有证据表明，抑制的程度可能是局部适应性的或具有可塑性，并对当地环境条件有反应。我们假设环境的不可预测性有利于延长换羽期，这反过来又允许维持激素应激反应，并讨论了可能的权衡取舍对换羽时间表进化的影响。
