Urbanization's hidden influence: Linking landscape alterations and feather coloration with pigeon's cholesterol...

Urbanization significantly impacts synanthropic birds, influencing their physiology and appearance. For instance, urban environments are associated with higher plasma cholesterol levels in birds due to human-derived food sources. Additionally, landscape changes create environmental pressures, which favor more melanic plumages. Current evidence suggests that urbanization may influence bird cholesterol through mechanisms beyond diet, possibly via the pleiotropic effects of the melanocortin system. In this study, we examine how cholesterolemia varies in urban pigeons (Columba livia), a species with polymorphic plumage, across different levels of urbanization and degrees of melanism. To investigate this, we analyzed pigeons along urbanization gradients and sampled birds from various locations in Santiago, a South American metropolis. Plumage color was characterized using image analysis, and the expression of the MITF and MC1R genes in the skin was measured. Blood samples were analyzed for biochemical parameters, genetic sex determination, and nitrogen isotopic signatures (δ¹⁵N) in red blood cells. We found correlations between human-induced spatial heterogeneity indexes (e.g., Impervious Surface, Global Human Modification, and Land Surface Temperature) and plumage melanism. However, no direct relationship was found between these landscape features and cholesterol levels, although there were links to glucose and triglycerides. Interestingly, plasma cholesterol levels strongly correlated with plumage melanism features independent of δ¹⁵N, suggesting a non-trophic origin of elevated cholesterol. Furthermore, we identified and quantified the indirect effects of urbanization on cholesterolemia using structural equation modeling. This evidence highlights the interplay between urban stressors and bird melanism, underscoring the importance of pleiotropic phenomena in socio-eco-evolutionary dynamics of urban ecosystems.