Data from: Causal link between insulin-like growth factor 1 and growth in nestlings of a wild passerine bird

Life history of animals is strongly linked with the variability in postnatal growth rates, because the ability to achieve necessary body mass early in life is directly connected with the ability to survive and reproduce later in life. Surprisingly, little is known about the physiological mechanisms connecting food availability with the variability of growth in wild animals.
We used a hormonal injection treatment in free-living nestlings of the pied flycatcher (Ficedula hypoleuca). In doing this, we aimed to see how their plasma insulin-like growth factor 1 (IGF-1) levels and its mediation of growth rate vary in response to administration of exogenous IGF-1 during the early period of postnatal development.
We showed that the levels of IGF-1 as well as growth rate and body size, in terms of body mass and tarsus length, were significantly higher in IGF-1-injected nestlings, compared with those in the control group. These differences became especially pronounced from day 7 post-hatch onwards during the 13-day nestling period. Additionally, the proportional investment into growth was more profoundly downregulated in IGF-1-injected nestlings at the end of the nestling period, which achieved fledging size earlier, compared with control nestlings.
Our results are the first to highlight IGF-1 as the causal link between growth conditions and variability of growth rate and body size in wild vertebrates. Interestingly, the change in IGF-1 levels also showed a plausible, distinct adaptive age- and physical maturity-dependent pattern in nestlings to match the benefits gained by them when investing in growth, with the cost endured during the early days of postnatal development to maximize their fitness. Therefore, this mechanism might be among those mediating the maintenance of genetic variability in growth rates in populations, presumably regardless of the presence of strong directional selection acting upon them.

动物的生活史与出生后生长速率的变异性密切相关，因为生命早期获得必要体质量的能力，直接关联到日后存活与繁殖的能力。令人意外的是，目前对于连接野外动物食物可获得性与生长变异性的生理机制，我们所知甚少。
我们对自由生活的斑姬鹟（Ficedula hypoleuca）幼鸟开展了激素注射处理，旨在探究在出生后发育早期阶段，其血浆胰岛素样生长因子1（insulin-like growth factor 1, IGF-1）水平及其对生长速率的介导作用，如何响应外源性IGF-1的给药干预。
研究结果显示，与对照组相比，注射IGF-1的幼鸟的IGF-1水平、生长速率以及以体质量和跗跖长度表征的身体尺寸均显著更高。在为期13天的育雏期内，这些差异自孵化后第7天起愈发显著。此外，育雏末期，注射IGF-1的幼鸟对生长的比例投入更为显著下调，其比对照组幼鸟更早达到离巢体型。
本研究首次证实，IGF-1是连接野生脊椎动物生长条件与生长速率、身体尺寸变异性的因果纽带。有趣的是，IGF-1水平的变化还呈现出一种合理且独特的、依赖于年龄和身体成熟度的适应性模式，以匹配幼鸟在投入生长时所获得的收益，并承受出生后早期发育阶段为最大化自身适合度所付出的代价。因此，该机制可能是介导种群内生长速率遗传变异维持的机制之一，且似乎不受作用于该性状的强定向选择存在与否的影响。