Morphological variations in a widely distributed Eastern Asian passerine cannot be consistently explained by ecogeographic rules

Ecogeographic rules that describe quantitative relationships between morphologies and climate might help us predict how morphometrics of animals was shaped by local temperature or humidity. Although the ecogeographic rules had been widely tested in animals of Europe and North America, they had not been fully validated for species in regions that are less studied. Here, we investigate the morphometric variation of a widely distributed East Asian passerine, the vinous-throated parrotbill (Sinosuthora webbiana), to test whether its morphological variation conforms to the prediction of Bergmann's rule, Allen's rules, and Gloger's rule. We at first described the climatic niche of S. webbiana from occurrence records (n = 7838) and specimen records (n = 290). The results of analysis of covariance (ANCOVA) suggested that the plumage coloration of these parrotbills was darker in wetter/warmer environments following Gloger's rule. However, their appendage size (culmen length, beak volume, tarsi length) was larger in colder environments, the opposite of the predictions of Allen's rule. Similarly, their body size (wing length) was larger in warmer environments, the opposite of the predictions of Bergmann's rule. Such disconformity to both Bergmann's rule and Allen's rule suggests that the evolution of morphological variations is likely governed by multiple selection forces rather than dominated by thermoregulation. Our results suggest that these ecogeographic rules should be validated prior to forecasting biological responses to climate change especially for species in less-studied regions.

Automated summary

The study investigated the morphometric variation of a widely distributed East Asian passerine and found that its morphological variation does not conform to Bergmann's rule and Allen's rule, suggesting that morphological variations are likely governed by multiple selection forces rather than dominated by thermoregulation. Therefore, ecogeographic rules should be validated prior to forecasting biological responses to climate change, especially for species in less-studied regions.