Climate is a strong predictor of near-infrared reflectance but a poor predictor of colour in butterflies

Colour variation across climatic gradients is a common ecogeographical pattern; yet there is long-standing contention over underlying causes, particularly selection for thermal benefits. We tested the evolutionary association between climate gradients and reflectance of near-infrared (NIR) wavelengths, which influence heat gain but are not visible to animals. We measured ultraviolet, visible and NIR reflectance from calibrated images of 372 butterfly specimens from 60 populations (49 species, 5 families) spanning the Australian continent. Consistent with selection for thermal benefits, the association between climate and reflectance was stronger (slope and effect size) for NIR than UVA-visible wavelengths. Furthermore, climate predicted reflectance of the thorax and basal wing, which are critical to thermoregulation; but did not predict reflectance of the entire wing, which has a variable role in thermoregulation depending on basking behaviour. These results provide evidence that selection for thermal benefits has shaped the reflectance properties of butterflies.

沿气候梯度的体色变异是一类常见的生态地理格局，但学界对其背后的核心成因长期存在争议，尤其是围绕基于热适应性优势的选择机制所展开的争论。本研究针对气候梯度与近红外（near-infrared, NIR）波长反射率之间的演化关联开展检验——近红外波长会影响生物的热量获取效率，但无法被动物视觉系统感知。我们对横跨澳大利亚大陆的60个种群（隶属于5科、共49种）的372件蝴蝶标本的校准图像进行了紫外、可见光及近红外反射率测量。结果与热适应性选择的预期相符：近红外波段下，气候与反射率的关联强度（斜率与效应量）显著强于紫外A（UVA）-可见光波段。此外，气候可有效预测胸部与翅基部的反射率——这两个部位对蝴蝶体温调节至关重要；但无法预测全翅的反射率，而全翅的体温调节功能依赖于晒翅行为，存在显著变异性。本研究结果证实，热适应性选择塑造了蝴蝶的反射率特征。