Effects of High-Altitude Environments on Plumage Lightness (L-value) in Poultry: Melanin-Mediated Mechanisms and Multifactorial Interaction Effects.

High-altitude environments expose poultry to low temperature, hypoxia, and strong UV radiation, but how combined environmental gradients shape plumage lightness remains insufficiently resolved. Using Qiangshan cloud chickens from Wenchuan, we analyzed 150 dorsal plumage samples collected along 900-2800 m. Plumage lightness (L-value) was measured by colorimetry, and eumelanin content was quantified by alkaline dissolution spectrophotometry. Five environmental variables (UV index, altitude, mean annual temperature, annual precipitation, and forest coverage) were integrated from field monitoring, meteorological records, and remote sensing data. We combined correlation analysis, hierarchical regression, principal component analysis (PCA), path analysis, and interaction surface modeling to partition direct, indirect, and synergistic effects. L-value was negatively associated with altitude and melanin content, and the melanin-L-value relationship remained stable across altitude strata. PCA summarized environmental variation into PC1 (UV index, altitude, temperature) and PC2 (precipitation, forest coverage), jointly explaining 88.4% of L-value variance. Path analysis showed that melanin exerted a masking effect on the PC1 pathway while partially mediating the PC2 pathway (mediation proportion 76.2%). A significant PC1 × PC2 interaction indicated non-additive control of plumage lightness under combined environmental extremes. These results support a mechanism in which high-altitude stress promotes melanin deposition and darker plumage as a photoprotective and thermoregulatory adaptation, and they highlight the importance of environmental interactions in shaping poultry phenotypic responses.