Departamento de Zoología, Universidad de Granada, Granada, Spain.
PLoS One. 2012;7(9):e44647. doi: 10.1371/journal.pone.0044647. Epub 2012 Sep 5.
Theoretical models aimed at explaining the evolution of honest, informative begging signals employed by nestling birds to solicit food from their parents, require that dishonest signalers incur a net viability cost in order to prevent runaway escalation of signal intensity over evolutionary time. Previous attempts to determine such a cost empirically have identified two candidate physiological costs associated with exaggerated begging: a growth and an immunological cost. However, they failed to take into account the fact that those costs are potentially offset by the fact that nestlings that invest more in begging are also likely to obtain more food. In this study, we test experimentally whether a 25% increase in ingested food compensates for growth and immunological costs of extra begging in southern shrike (Lanius meridionalis) nestlings. Three nestmates matched by size were given three treatments: low begging, high begging-same food intake, and high begging-extra food intake. We found that, while a higher food intake did effectively compensate for the growth cost, it failed to compensate for the immunological cost, measured as T-cell mediated immune response against an innocuous mitogen. Thus, we show for the first time that escalated begging has an associated physiological net cost likely to affect nestling survival negatively.
理论模型旨在解释雏鸟向其父母乞食时所使用的诚实、信息丰富的乞食信号的进化，这些模型要求不诚实的信号发出者在净生存能力上付出代价，以防止信号强度在进化过程中失控升级。之前试图从经验上确定这种代价的尝试已经确定了与夸大乞食相关的两个候选生理代价：生长代价和免疫代价。然而，它们没有考虑到这样一个事实，即那些代价可能被这样一个事实所抵消：即那些在乞食上投入更多的雏鸟也更有可能获得更多的食物。在这项研究中，我们通过实验测试了南方伯劳（Lanius meridionalis）雏鸟额外乞食的 25%食物摄入量增加是否可以补偿生长和免疫代价。通过大小匹配的三个巢伴，给予它们三种处理：低乞食、高乞食-相同食物摄入量和高乞食-额外食物摄入量。我们发现，虽然更高的食物摄入量确实有效地补偿了生长代价，但它未能补偿免疫代价，免疫代价是通过 T 细胞介导的针对无害有丝分裂原的免疫反应来衡量的。因此，我们首次表明，升级的乞食具有相关的生理净代价，可能会对雏鸟的生存产生负面影响。
