Patterns of Sexual Dimorphism in Avian Species Along Altitudinal Gradients

Abstract

Sexual dimorphism in avian species represents a key axis of evolutionary differentiation, yet its modulation by environmental gradients remains inadequately quantified. This study investigates patterns of sexual dimorphism across altitudinal gradients in 10 bird species inhabiting the Western Himalayas, integrating morphometric, ecological, and behavioral data. We conducted extensive field observations, biometric measurements, and vocal analyses across low (0–800 m), mid (801–2000 m), and high (>2000 m) elevation zones. Morphological traits—specifically wing length, body mass, and beak size—were compared between sexes using Sexual Dimorphism Indices (SDIs), ANOVA, and generalized linear models (GLMs) incorporating environmental predictors. Results demonstrated a consistent increase in dimorphism with elevation, with high-altitude populations exhibiting significantly larger trait disparities between males and females. SDI values were positively correlated with vegetation density and canopy cover, and negatively with ambient temperature. ANOVA confirmed significant altitudinal differences in wing length (F = 8.74, p = 0.021) and body mass (F = 6.95, p = 0.027). Figure-based visualizations reinforced these findings: bar plots and scatter plots highlighted male-biased dimorphism, while line plots traced trait divergence along altitudes. Notably, behavioral evidence suggested habitat partitioning by sex in some species, aligning with ecological niche specialization. These findings support the hypothesis that environmental harshness and resource structure intensify sexual trait differentiation. The study highlights elevation as a powerful ecological filter shaping the expression of dimorphic traits, with significant implications for species adaptation and conservation under climate change. This work advances our understanding of how spatial ecological heterogeneity influences evolutionary outcomes in avian sexual dimorphism.