Sexual Monomorphism and Size-Related Shape Change in the Beak of the Greater Flamingo (Phoenicopterus roseus)

Background The avian beak is a functionally important structure shaped by ecological demands. In highly specialized filter-feeders such as flamingos, subtle variation in beak size and shape may influence feeding performance. However, the relative contributions of sexual dimorphism and allometry to beak shape variation remain unclear in Greater Flamingos (Phoenicopterus roseus).Objectives This study aims to evaluate sexual dimorphism and size-related shape variation in the beak of adult Greater Flamingos using two-dimensional geometric morphometrics.Methods Standardized dorsal and lateral beak photographs were obtained from 40 adults of known sex (15 females, 25 males). Fixed landmark configurations (7 dorsal and 12 lateral landmarks) were digitized and analyzed using landmark-based geometric morphometrics. Generalized Procrustes analysis was used to obtain Procrustes shape coordinates and centroid size, and principal component analysis was performed to summarize major axes of variation. Sex effects were tested using permutation-based Procrustes ANOVA, and allometry was assessed using Procrustes regression including sex and size & times; sex interaction terms.Results Sex was not a significant predictor of beak shape in either view (dorsal: R2 = 0.005, p = 0.971; lateral: R2 = 0.038, p = 0.120). Dorsal shape showed a significant allometric component (logCS: R2 = 0.123, p = 0.002), whereas lateral allometry was weaker and not significant at alpha = 0.05 (logCS: R2 = 0.047, p = 0.055). No sex-specific allometric trajectories were detected.Conclusions Adult Greater Flamingos exhibit sexual monomorphism in beak shape, and observed variation is partly explained by size-related shape change, particularly in the dorsal configuration.