Data from: Fractal geometry of a complex plumage trait reveals bird's quality

Animal coloration is key in natural and sexual selection, playing significant roles in intra- and interspecific communication because of its linkage to individual behaviour, genetics and physiology. Simple animal traits such as the area or the colour intensity of homogeneous patches have been profusely studied. More complex patterns are widespread in nature, but they escape our understanding because their variation is difficult to capture effectively by standard, simple measures. Here, we used fractal geometry to quantify inter-individual variation in the expression of a complex plumage trait, the heterogeneous black bib of the red-legged partridge (Alectoris rufa). We show that a higher bib fractal dimension (FD) predicted better individual body condition, as well as immune responsiveness, which is condition-dependent in our study species. Moreover, when food intake was experimentally reduced during moult as a means to reduce body condition, the bib's FD significantly decreased. Fractal geometry therefore provides new opportunities for the study of complex animal colour patterns and their roles in animal communication.

动物体色在自然选择与性选择中均占据核心地位，其与个体行为、遗传学及生理学密切关联，因此在种内与种间通讯中发挥关键作用。诸如均质色块的面积、色彩强度这类简单的动物性状，已被学界广泛研究。而自然界中广泛存在的复杂图案，却因难以通过标准简易的测量手段有效捕捉其变异特征，至今尚未得到充分解析。本研究采用分形几何（fractal geometry）量化了一种复杂羽色性状的个体间变异——即红腿石鸡（Alectoris rufa）的异质性黑色喉斑。研究结果表明，喉斑的分形维数（fractal dimension，FD）越高，个体的身体状况与免疫应答能力越强；而在本研究物种中，免疫应答能力恰好依赖于身体状况。此外，若在换羽期通过实验手段降低食物摄入以削弱个体身体状况，喉斑的FD会显著下降。因此，分形几何为复杂动物体色图案的研究及其在动物通讯中的功能探索提供了全新的研究路径。