A test of ecological and sexual selection hypotheses for geographical variation in coloration and morphology of golden-crowned kinglets (Regulus satrapa)

Abstract

Aim To date, studies on geographical variation have extensively investigated Bergmann’s rule, yet Gloger’s rule remains infrequently tested, and climatic predictors of variation in carotenoid coloration have not yet been studied. In addition, hypotheses based on sexual selection, which predict that sexual dimorphism should vary with population density and climatic conditions, have received little attention. Our goals were to characterize geographical variation in the coloration and morphology of golden-crowned kinglets, Regulus satrapa (Passeriformes, Regulidae), and to investigate possible ecological and sexual selection correlates of this variation. Location The entire species range of golden-crowned kinglets, comprising North and Central America. Methods We collected data from 511 museum specimens, dating from 1847 to 2006, encompassing all five subspecies of golden-crowned kinglets. We used reflectance spectrometry to quantify crown and mantle coloration, and measured wing-chord and tarsus length to approximate body size. We obtained geographical and climatic data from online databases, and population density estimates from the literature. Results There were significant subspecific and gender differences in crown coloration and morphology: male kinglets were generally larger and more colourful. Our data revealed mixed support for Bergmann’s rule: tarsus length decreased with increasing latitude, while patterns of variation in wing-chord and tarsus length showed conflicting results with temperature. Mantle coloration exhibited an opposite trend to that predicted by Gloger’s rule: upper parts became lighter with increasing relative humidity. Crown coloration was negatively correlated with actual evapotranspiration, suggesting that levels of primary productivity are not directly linked to carotenoid abundance. Sexual dimorphism and dichromatism generally increased with greater population density, lower latitudes and elevations, and warmer temperatures, supporting a previously observed pattern of variation in sexual dimorphism. Main conclusions Geographical variation in golden-crowned kinglets yielded mixed support for Bergmann’s rule and contradicted Gloger’s rule, suggesting that other mechanisms may be operating. Allen’s rule is likely to be a stronger factor influencing tarsus length. Differences in the degree of sexual dimorphism and dichromatism in varying climatic conditions suggest that the intensity of sexual selection differs between habitats. Further studies on geographical variation in sexual dimorphism in various taxa may reveal a previously unrecognized ecogeographical rule.