Sexual Dimorphism in Continental Steamer-Ducks

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Abstract
Data from 133 specimens of 3 spp. of steamer-duck were used to assess sexual size dimorphism in the genus Tachyeres [T. pteneres, T. leucocephalus, T. patachonicus]. In these 3 spp., males on the average exceed females in all of the 15 external, 30 skeletal, and 2 muscular measurements analyzed, except number of bill lamellae. Sexual dimorphism is proportionately greatest in weights, intermediate in areal variables, and least in linear measurements. With few exceptions, magnitude of univariate sexual dimorphism is equal in the 3 spp. and among localities for T. patachonicus, although differences in magnitude of multivariate sexual dimorphism are indicated. Multivariate comparisons show that males are not only larger than females but also that, proportionately, males have larger core elements and smaller crania, proximal wing elements, and acetabular widths than females. The robust trunks and proportionately shorter wings of males may reflect structural refinements for combat; relatively large acetabular widths of females may be an accommodation for producing and laying large eggs. Sexual dimorphism of steamer-ducks may result from the combined effects of largely linear and environmentally stable habitats; strong selection in males for territorial defense of food supplies, mates, and young; and, possibly selection for smaller body size in females related to reproductive energetics. The greater number of bill lamellae in females than males probably allows the large, territorially active males to feed more on large food items; females are equipped to utilize smaller food items.