Repeatable behaviors (i.e., animal personality) are pervasive in the animal kingdom and various mechanisms have been proposed to explain their existence. Genetic and nongenetic mechanisms, which can be equally important, predict correlations between behavior and body mass on different levels (e.g., genetic and environmental) of variation. We investigated multilevel relationships between body mass measured on weeks 1, 2, and 3 and three behavioral responses to handling, measured on week 3, which form a behavioral syndrome in wild blue tit nestlings. Using 7 years of data and quantitative genetic models, we find that all behaviors and body mass on week 3 are heritable (h² = 0.18–0.23) and genetically correlated, whereas earlier body masses are not heritable. We also find evidence for environmental correlations between body masses and behaviors. Interestingly, these environmental correlations have different signs for early and late body masses. Altogether, these findings indicate genetic integration between body mass and behavior and illustrate the impacts of early environmental factors and environmentally mediated growth trajectory on behaviors expressed later in life. This study, therefore, suggests that the relationship between personality and body mass in developing individuals is due to various underlying mechanisms, which can have opposing effects. Future research on the link between behavior and body mass would benefit from considering these multiple mechanisms simultaneously.

中文翻译：

---

蓝雀雏鸟行为与体重之间的多层次关系

可重复的行为（即动物的性格）在动物界很普遍，已经提出了各种机制来解释它们的存在。同等重要的遗传和非遗传机制可以预测行为和体重在不同水平（例如遗传和环境）上的相关性。我们调查了在第1、2和3周测得的体重与在第3周测得的对处理的三种行为反应之间的多级关系，这些反应在野生蓝山雀雏鸟中形成了行为综合症。使用7年的数据和定量遗传模型，我们发现第3周的所有行为和体重都是可遗传的（h ²= 0.18–0.23）并具有遗传相关性，而较早的体重是无法遗传的。我们还发现了体重与行为之间环境相关性的证据。有趣的是，这些环境相关性对于早期和晚期体重都有不同的征兆。总而言之，这些发现表明了体重与行为之间的遗传整合，并说明了早期环境因素和环境介导的生长轨迹对生命后期表达的行为的影响。因此，这项研究表明，发展中的个体的性格与体重之间的关系是由于各种潜在的机制所致，这些机制可能产生相反的影响。同时考虑这些多种机制，将有助于进一步研究行为与体重之间的联系。