Department of Biology, University of Ottawa, Ottawa, ON, Canada.
Centre for Integrative Ecology, Deakin University, Waurn Ponds, Victoria 3216, Australia.
Gen Comp Endocrinol. 2020 May 1;290:113403. doi: 10.1016/j.ygcen.2020.113403. Epub 2020 Jan 25.
Animals have well-documented individual differences in their behaviour, including in their response to stressful stimuli. The physiological bases for the repeatability of these traits has been the focus of much research in recent years, in an attempt to explain the mechanistic drivers for behavioral syndromes. Whilst a range of studies have demonstrated repeatable individual differences in physiological traits, little is known about potential trade-offs between reproductive investment and the physiological responses to subsequent stressors. We therefore sought to test the behavioral and physiological responses of male zebra finches (Taeniopygia guttata) to a novel environment, quantifying a series of repeated "temporal reaction norms" before and after reproduction. Given that reproductive investment is costly both in time and energy, it is likely to affect expression of behavioral and physiological traits. We hypothesised that reproductive investment would impact the consistency of these temporal reaction norms. Specifically, we predicted that individuals which invested more in reproduction would show altered rates of habituation to a stressful stimulus. Therefore, we quantified temporal reaction norm components (i.e., intercept and slope) of two behaviours and metabolic rate (MR) within and among individuals before and after a breeding season. We found that individuals consistently differed in how their locomotor and feeding activity increased upon introduction into a novel environment and also how their MR decreased after being handled and confined within the metabolic chamber. We also found that the slope of the feeding activity reaction norm was negatively correlated with stress-induced corticosterone levels at the within-individual level. Finally, in contrast to our prediction, we found that neither the intercept nor slope of the reaction norms were influenced by the reproductive effort (the number of fledglings produced) displayed by individual males. This suggests that the substantial individual variation in the expression of physiological and behavioural traits is not plastic with respect to the immediate consequences of reproductive investment. This study is the first quantification of metabolic rate reaction norms and their relationships with fitness, which represents an important first step towards understanding the evolutionary significance of instantaneous habituation to stressful and novel situations.
动物的行为存在着有充分文献记录的个体差异，包括对压力刺激的反应。近年来，人们对这些特征的可重复性的生理基础进行了大量研究，试图解释行为综合征的机械驱动因素。虽然许多研究已经证明了生理特征的可重复个体差异，但对于生殖投资与随后应对压力源的生理反应之间的潜在权衡知之甚少。因此，我们试图测试雄性斑胸草雀（Taeniopygia guttata）对新环境的行为和生理反应，在繁殖前后量化一系列重复的“时间反应规范”。由于生殖投资在时间和能量上都是昂贵的，它可能会影响行为和生理特征的表达。我们假设生殖投资会影响这些时间反应规范的一致性。具体来说，我们预测那些在繁殖上投入更多的个体，在适应压力刺激时会表现出不同的习惯化速度。因此，我们在繁殖季节前后，在个体内部和个体之间量化了两种行为（即活动和进食）和代谢率（MR）的时间反应规范成分（即截距和斜率）。我们发现，个体在引入新环境时，其运动和进食活动增加的方式以及在被处理和限制在代谢室内时 MR 下降的方式存在一致差异。我们还发现，个体内部水平上，进食活动反应规范的斜率与应激诱导的皮质酮水平呈负相关。最后，与我们的预测相反，我们发现，无论是反应规范的截距还是斜率都不受个体雄性显示的生殖努力（育雏数）的影响。这表明，生理和行为特征表达的个体差异在很大程度上不是生殖投资的直接后果的可塑性。这项研究是对代谢率反应规范及其与适应性关系的首次量化，这是理解对压力和新情况的即时适应的进化意义的重要第一步。
