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，因为生命早期达成必要体质量的能力，直接关联到后期存活与繁殖的能力。令人意外的是，目前对于连接食物可获得性与野生动物生长变异性的生理机制，我们仍知之甚少。
我们对自由栖息的斑姬鹟（Ficedula hypoleuca）雏鸟实施了激素注射处理实验。本研究旨在探究，在雏鸟出生后发育早期施加外源性胰岛素样生长因子1（insulin-like growth factor 1，IGF-1）后，其血浆IGF-1水平以及IGF-1对生长速率的介导作用会如何响应外源性IGF-1的给药而发生变化。
研究结果表明，与对照组雏鸟相比，注射IGF-1的个体其血浆IGF-1水平、生长速率以及以体质量和跗跖长度表征的身体体型均显著更高。在为期13天的育雏期内，上述差异自孵化后第7天起愈发显著。此外，在育雏期结束时，注射IGF-1的雏鸟对生长的比例性投入被更显著地下调，其较对照组雏鸟更早达到出飞体型。
本研究首次揭示，IGF-1是野生脊椎动物生长条件与生长速率、身体体型变异性之间的因果关联纽带。值得关注的是，雏鸟体内IGF-1水平的变化呈现出一种合理且清晰的、依赖于年龄与身体成熟度的适应性模式：该模式匹配雏鸟在投入生长时所获得的收益，同时以出生后早期发育阶段所承受的生长成本为代价，以最大化其适合度。因此，这一机制可能是介导种群内生长速率遗传变异性维持的机制之一，且大概率不受作用于该性状的强定向选择的影响。