Adaptive immune response and resting metabolism are unaffected by manipulation of flight intensity, but negatively related to each other

Activation of an immune response (IR) upon exposure to pathogens is crucial to ensure adequate organismal performance and is directly linked to survival. Fitness benefits of the response may be associated with costs in terms of increased energy expenditure and may compete for resources and compromise such fitness benefits. Trade-offs between immune function and other traits relevant for fitness are well documented, however, it remains unknown if such trade-offs are energetically mediated. We manipulated the flight activity of 70 zebra finches (Taeniopygia guttata) to investigate the energy reallocation to the immune system in rested and exercised birds. Four experimental groups exhibiting different flight intensity were used: trained, untrained, and birds that either stopped or started flight training after the immune challenge. If costs associated with the IR and flight activity compete for energy, we predicted the extent of inhibition of IR would be dependent on energy allocated to physical activity. Daytime resting metabolic rate was measured before and after the immune challenge, induced using sheep red blood cells (SRBC). Strength of the response was measured as the concentration of anti-SRBC antibodies 6 days post-challenge. We found no evidence for the predicted inhibition of the immune function between trained and untrained birds, as there was no difference in resting metabolic rate between experimental groups. However, resting metabolic rate following the challenge was negatively correlated with the IR. Surprisingly, individuals with relatively low resting metabolic rates following immune challenge were able to up-regulate their IR, indicating a trade-off in the use of the energy resource independent of flight activity levels. Our results suggest that energy allocation to mount the IR may represent a constraint that is possibly linked to the circadian pattern of the energy budget but appears to be independent of energetic challenges brought on by different levels of flight activity.

病原体暴露后触发的免疫应答（immune response, IR），对保障机体正常生理功能至关重要，且与生存直接相关。该免疫应答带来的适合度收益，往往伴随能量消耗增加所产生的成本，可能会争夺资源并削弱这类适合度收益。免疫功能与其他与适合度相关的性状之间的权衡（trade-off）现象已有大量文献记载，但目前尚不清楚这类权衡是否由能量分配介导。我们对70只斑胸草雀（Taeniopygia guttata）的飞行活动进行操控，以此探究休息与运动状态下鸟类体内向免疫系统的能量重分配情况。本研究设置了四种具有不同飞行强度的实验组：训练组、非训练组，以及分别在免疫刺激后停止或开始飞行训练的组别。若免疫应答与飞行活动相关的成本存在能量争夺，我们预测免疫应答的抑制程度将取决于分配给身体活动的能量多少。研究使用绵羊红细胞（sheep red blood cells, SRBC）诱导免疫刺激，并在刺激前后测量了日间静息代谢率。免疫应答强度通过刺激后6天的抗SRBC抗体浓度进行量化。结果显示，训练组与非训练组鸟类的免疫功能并未出现预期的抑制现象，各实验组间的静息代谢率并无显著差异。但免疫刺激后的静息代谢率与免疫应答呈负相关关系。令人意外的是，免疫刺激后静息代谢率相对较低的个体能够上调其免疫应答强度，这表明在能量资源的分配上存在权衡，且该权衡不受飞行活动水平的影响。本研究结果表明，启动免疫应答所需的能量分配可能存在一定限制，这或许与能量收支的昼夜节律模式相关，但似乎不受不同飞行强度带来的能量负荷影响。