Ecogeographical patterns in owl plumage colouration: climate and vegetation cover predict global colour variation

Aim: Ecogeographical rules link animal colours, especially those produced
by melanin pigments, with variation in environmental conditions over wide
geographical scales. In particular, Gloger’s rule, coined in two versions
for endothermic animals, suggests that tegument darkness would increase at
high temperature, as well as in highly humid environments. On the other
hand, the thermal melanism hypothesis, predicts that darker colourations
should be more frequent in colder areas given their thermoregulation
benefits. Location: Global Time period: Contemporary Major taxa studied:
Strigiformes Methods: Here, we provide a global comparative test of these
contrasting expectations in all extant owls (n = 198 species), a group of
nocturnal birds displaying huge variability in the degree of melanin-based
plumage colouration and environmental specialization. Combining analyses
at both species and assemblage level, we assessed the climatic and
environmental variables explaining variation in plumage lightness and
redness across broad geographical gradients. Results: Darker and redder
owl phenotypes are more likely found near the equator. Species and
assemblage level analyses reveal that owls have more frequently darker and
redder plumages in warmer regions. In addition, owl species living in more
vegetated areas are darker, and owl assemblages show darker colours in
wetter areas. Main conclusions: Global patterns of colour variation in
owls do not fit expectations from the thermal melanism hypothesis but
supports Gloger´s rule. Our findings also stresses that several
alternative selective forces may explain climatic effects on colouration
over large geographical scales. Experimental work is urged to uncover the
possible mechanisms behind the detected associations between owl colour
and environmental variables.

研究目的：生态地理法则将动物体色（尤其是由黑色素介导生成的体色）与大地理尺度下的环境条件变异建立关联。其中，针对恒温动物提出两种版本的格洛杰法则（Gloger’s rule）指出，体表颜色深浅会随温度升高以及高湿度环境而加深。另一方面，热黑色素假说（thermal melanism hypothesis）则预测，鉴于体色加深所带来的体温调节益处，寒冷区域内体色更深的个体出现频率应更高。
研究区域：全球范围
时间跨度：当代
研究类群：鸮形目（Strigiformes）
研究方法：本研究针对现存所有鸮形目鸟类（共198种）开展全球比较分析，以检验这两种相悖的理论预测。鸮形目为夜行性鸟类类群，其基于黑色素的羽色深浅程度以及环境特化程度均存在显著变异。本研究结合物种水平与群落水平的分析方法，评估了能够解释大地理梯度下羽色明度与红度变异的气候及环境变量。
研究结果：体色更深且偏红的鸮类表型更大概率出现在赤道附近。物种水平与群落水平的分析均显示，在温度更高的区域，鸮类的羽色通常更深且更偏红色。此外，栖息于植被更茂密区域的鸮类体色更深，而群落水平的鸮类体色在湿度更高的区域也更深。
主要结论：鸮类体色变异的全球格局并不符合热黑色素假说的预测，但支持格洛杰法则。本研究结果同时表明，多种选择性压力或许能够解释大地理尺度下环境因素对体色的影响。未来亟需开展实验研究，以揭示鸮类体色与环境变量之间所观测到的关联背后的潜在机制。