Sex-specific growth in the European Shag Stictocarbo aristotelis, a sexually dimorphic seabird

European Shag (Phalacrocorax aristotelis) Science Article 1


The European Shag Stictocarbo aristotelis is dimorphic in body size : males are 22% heavier than females . We used molecular techniques to sex of 25 male and 18 female chicks in colonies from the Cies Islands (NW Spain). Discriminant functions were then obtained based on morphometric variable s-(culmen, head, wing and tarsus length) measured throughout the growth of the sexed birds . Once chicks were 25 days of age the discriminant functions accurately classified over 95% of cases, and at 30 days of 100% of cases . Using these functions we retrospectively sexed another 30 males and 35 females to examine the growth of the two sexes. The growth of the different variables was fitted to a logistic model . Culmen growth was found to be similar in both sexes. The asymptotic head size was larger in males but had the same growth constant as in females . The wing, tarsus and body mass asymptotes were larger in males, but females had a higher growth rate . The first principal component (PC I), extracted from a principal component analysis of the morphometric variables, may be considered as a synoptic descriptor of body size . The differentiation in mass growth between males and females started when the birds were 15 days old. However, body size (PC1) was not distinguished until they were 30 days old. During the first growth stage females were similar in body size to males due to their faster growth rate . This would suggest that the smaller sex (the females) has the same competitive ability as the larger sex and that the hierarchy in the early stages of growth would be contingent upon hatching order and not sex.

Velando A ., J . Graves & J . Freire 2000, Ardea 88(2): 127-136

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